GB2234355A - Swept frequency signal analyser - Google Patents

Abstract

Swept frequency signal analysing apparatus for displaying signals received by radio frequency receiver chain (12,14, 16,...) as a function of frequency produces a swept frequency display by feeding to a mixer (16) in the receiver chain a local oscillator signal which is derived both from a digital frequency synthesiser in a periodically open and closed phase locked loop (66) and from an analogue ramp generator (44). The local oscillator signal is generated by a voltage controlled oscilator (VCO) (58) which is periodically coupled in the synthesiser loop and at other times receives a stored voltage corresponding to the voltage fed to the VCO when the loop was last locked. Superimposed upon the stored voltage is a ramp signal for causing the oscillator frequency to execute repeatedly a frequency sweep. The apparatus also includes circuitry (128, 132) for generating a cursor deflection in the display during the flyback period pf the display, the circuitry including an arrangement for moving the cursor deflection when a display shift control is operated. <IMAGE>

Description

SWEPT FREOUENCY SIGNAL ANALYSING APPARATUS This invention relates to swept frequency signal analysing apparatus comprising local oscillator means operable to generate a local oscillator signal for feeding to a mixer of a signal receiver chain, the signal having a frequency which varies so as to execute repeatedly a selected ramp characteristic, and a display device for displaying an output signal from the receiver chain as a function of frequency.

Such apparatus forms the basis of the conventional spectrum analyser, an instrument for observing and monitoring signals occurring over a band of frequencies, and for studying the characteristics of such signals. A common disadvantage associated with conventional spectrum analysers is frequency instability or drift arising from the difficulty in stabilising an oscillator which executes a swept frequency characteristic. To the user, this difficulty manifests itself as drift of the position of a fixed frequency signal indication in the display, particularly when a narrow sweep frequency range together with a particular centre frequency is selected.

It is an object of this invention to provide signal analysis apparatus which is improved in this respect in a comparatively simple manner.

According to a first aspect of this invention, swept frequency signal analysing apparatus comprises a frequency synthesiser, a phase locked loop associated with the frequency synthesiser, local oscillator means connectable in the phase locked loop and operable to generate a local oscillator signal for feeding to a mixer of a signal receiver chain, the oscillator signal having a frequency which varies with time so as to execute repeatedly a selected frequency sweep characteristic, and a display device for displaying an output signal from the receiver chain as a function of frequency, wherein the loop is arranged to be periodically opened and closed during operation of the oscillator means. Preferably, the oscillator means comprise a voltage controlled oscillator which may be coupled in a phase locked loop forming part of the synthesiser.The oscillator may be arranged to receive a ramp signal during a first, sweep period and a synthesiser control voltage during a second, flyback period, the synthesiser loop being closed during at least part of the flyback period. This dual voltage control of the voltage controlled oscillator may be achieved by arranging for the frequency of the oscillator to be dependent on at least two voltage inputs, one of which is the synthesiser control voltage during the flyback period and a stored representation of the control voltage during the sweep period, and the other of which is a voltage reference during the flyback period and a ramp voltage during the sweep period. Generally, the voltage controlled oscillator frequency is a function of the difference between the two input voltages.

In a preferred embodiment of the apparatus, the synthesiser control voltage is derived from a phase or frequency comparator stage of the synthesiser via a series switch device for interrupting the loop during the sweep period and an analogue voltage storage stage comprising at least a storage capacitor which can be periodically coupled to a line for carrying a voltage representative of the comparator stage output. A buffer amplifier is connected to the comparator stage output and drives a second amplifier having an input connectable to the storage capacitor and a feedback loop containing a switched shunt capacitance.Series switches are preferably connected both to the input and the output of the buffer amplifier to isolate the output and interrupt the synthesiser loop during the sweep period, while switching devices coupled to a storage capacitor and the shunt capacitance are preferably controlled so as to be non-conductive during a synthesiser settling interval following the closing of the synthesiser loop.

In this way, provided that the synthesiser settling time is relatively short compared with the sweep period, i.e., the synthesiser is capable of achieving a locked loop condition during a fraction of the sweep period, the comparator can, in effect, be sampled once during each sweep cycle so that the local oscillator means, as well as executing the ramp characteristic for sweeping across a required frequency range, is stabilised by a frequency synthesiser.

The apparatus may include a single ramp voltage generator for driving both an X-coordinate input of the display device and the second voltage input of the voltage controlled oscillator. In order to provide the facility for altering the magnitude of the swept frequency range and for shifting its position, the ramp generator is coupled to the voltage controlled oscillator via an intermediate stage with means for adjusting gain and voltage level. This allows any selected received signal or signals identified within a broad frequency range to be conveniently examined in greater detail.

It will be appreciated that shifting the swept frequency range prevents the use of a frequency reference scale applied to the viewing screen of the display device. For this reason, the preferred embodiment of the invention includes a cursor generator operable to generate a cursor in the display indicative, for instance, of the centre frequency of the swept frequency range.Thus, the invention includes, according to a second aspect thereof, swept frequency signal analysing apparatus comprising: a frequency synthesiser, a phase locked loop associated with the frequency synthesiser, local oscillator means connectable in the phase locked loop and operable to generate a local oscillator signal for feeding to a mixer of a signal receiver chain, the oscillator signal having a frequency which varies with time so as to execute repeatedly a selected frequency sweep characteristic, a display device for displaying an output signal from the receiver chain as a function of frequency in a two-dimensional dual coordinate system, the device having first and second coordinate inputs, the first co-ordinate input being connected to receive the output signal from the receiver chain, a sweep signal generator coupled to, and arranged to feed a periodic sweep signal to, the second co-ordinate input, and a cursor generator for generating a cursor mark on the display device, and having first and second parts for feeding cursor signals respectively to the first and second co-ordinate inputs of the display device, the first part being arranged to generate a display deflection to produce the mark, and the second part being associated with the sweep signal generator and arranged to cause the position of the mark to correspond to a selected frequency. The Xcoordinate of the display trace may then be fixed at the required centre frequency position for an interval during which, preferably, an oscillating voltage is applied to the Y-coordinate input to create a vertical cursor bar.

The position of the cursor bar, generated during the flyback period, may be made to track the display of the sweep period when the shift control (and where appropriate the range control) is operated by supplying the X-coordinate input of the display device from a feedback loop which includes the shift and range controls and which operates during the flyback period. The feedback loop comprises an amplifier the output of which forms the input to the intermediate stage containing the shift and range controls during the flyback period, one input of the amplifier being coupled to the output of the intermediate stage and the other input being coupled to the second input of the voltage controlled oscillator (the local oscillator means) to receive the voltage reference which is applied to the voltage controlled oscillator during the flyback period.The amplifier is a differential amplifier which, it will be appreciated, produces at the input of the intermediate stage a voltage appropriate to produce at the output of the intermediate stage a voltage equal to the voltage reference. As a result, the position of the cursor bar, as determined by the voltage applied to the X-coordinate input of the display device accurately represents the selected centre frequency while the shift and range controls are operated.

A further refinement included in the preferred embodiment is an attenuator in the intermediate stage coupled to means for controlling the centre frequency set by the frequency synthesiser. The attenuator is positioned after the shift and range controls and applies a variable attenuation to the ramp voltage fed to the voltage controlled oscillator (and to the voltage fed to the cursor generator loop), so that the settings of the shift and range controls have an effect which is at least approximately proportional to the set centre frequency.As a consequence, the visual effect on the display arising from operation of these controls is largely the same regardless of the centre frequency setting It should be noted that the adjusting means of the intermediate stage, including, where fitted, the attenuator, also use as their voltage reference the reference voltage fed to the voltage control oscillator so that the variation of the ramp voltage applied to the voltage controlled oscillator is a variation of its value with respect to the reference voltage. This means that a common voltage reference for the second input of the voltage controlled oscillator is maintained for the sweep period and the flyback period. In this way, it is possible to arrange that the point on the display trace representing the centre frequency remains at the same position regardless of frequency setting and the position of the range control.

Since the ramp generator acts as the source both for the Xcoordinate input of the display device and for the ramp voltage applied to the voltage controlled oscillator during the sweep period, and since the cursor generator includes a feedback loop around the intermediate stage, the linearity of the ramp and accuracy of the voltage switching points of the ramp generator are not critical. This allows a large ramp voltage range to be achieved, i.e. substantially the full difference between the voltage rails of the ramp generator, in a comparatively simple manner using two operational amplifier elements. The first element is connected as an integrator with a capacitor coupled between its output and its inverting input and a series resistor connected to the inverting input.The opposite end of the series resistance is connected to the output of the second operational amplifier element which has its inverting input coupled to the inverting input of the first operational amplifier element. Positive feedback is provided on the second operational amplifier element by means of a resistance connected between the output and the noninverting input, while the non-inverting input of the first operational amplifier element is held at the reference voltage level.In operation, a ramp voltage is obtained at the output of the first operation amplifier element which continues until it reaches one or other of the supply rail voltages at which point the voltage at the inverting input ceases to be equal to the reference voltage, causing the second operational amplifier element to change state, thereby beginning a reverse discharging or charging period resulting in a ramp voltage in the opposite direction at the output of the first operational amplifier element. By coupling a second resistor and diode in series with the series resistance coupled to the inverting input of the first operational amplifier, the slope of one ramp can be made different from the slope of the other ramp. The output of the second operational amplifier element can be used as a switching signal source.

According to a third aspect of the invention, there is provided a method of displaying signals from a radio frequency receiver chain as a function of frequency, comprising: generating a local oscillator signal, the frequency of which varies with time, so as to execute repeatedly a selected frequency sweep characteristic, and feeding a signal obtained from the receiver chain downstream of the mixer to a display device, wherein the local oscillator signal is generated by means of a frequency synthesiser and a phase locked loop associated with the synthesiser, the local oscillator signal being obtained from local oscillator means connectable in the loop, and wherein the loop is periodically opened and closed.

According to a fourth aspect of the invention, there is also provided a method of displaying signals from a radio frequency receiver chain as a function of frequency, comprising: generating a local oscillator signal, the frequency of which varies with time, so as to execute repeatedly a selected frequency sweep characteristic, the local oscillator signal being generated by means of a frequency synthesiser, a phase locked loop associated with the synthesiser, and local oscillator means connectable in the loop, feeding a signal obtained from the receiver chain downstream of the mixer to a first co-ordinate input of a two-dimensional display device, generating a periodic sweep signal and feeding it to a second co-ordinate input of the display device, generating first and second cursor signals, applying the first cursor signal periodically to the first co-ordinate input to cause a cursor deflection in the display, and applying the second cursor signal to the second co-ordinate input, the second cursor signal being such as to define the position of the cursor deflection corresponding to a selected frequency.

The invention will now be described by way of example with reference to the drawings in which: Figure 1 is a block diagram of swept frequency analysing apparatus in accordance with the invention; Figure 2A is a representation of a typical display trace, and Figures 2B, 2C, and 2D are graphs of the display Yinput, the display X-input, and the local oscillator frequency respectively plotted against time; Figure 3 is a more detailed block diagram of a synthesiser controlled local oscillator; Figure 4 is a simplified circuit diagram of the local oscillator of Figure 3; and Figure 5 is a circuit diagram of a ramp generator and cursor generator.

Referring to Figure 1, the preferred embodiment of the invention has a receiver chain with an input 10 which may, for example, be connected to the first intermediate frequency (IF) stage of a communications receiver at a point upstream of any narrow band filtering. The received signal is amplified in an input amplifier 12 and fed through a low pass filter 14 to a first mixer 16 where it is mixed, generally, with a local oscillator signal of a higher frequency. If the input frequency range is 0.1-29.99 MHz, then typically the frequency of the local oscillator signal may vary between 120.1 and 149.99 MHz, yielding a second intermediate frequency at the output of the mixer 16 of 120 MHz.The local oscillator signal applied to the mixer 16 is a swept frequency signal having a sweep period during which the frequency rises approximately linearly from the lower to the upper limit of a preselected frequency range centred on a preselected centre frequency.

The frequency converted signal then passes to the first IF stage of the apparatus in accordance with the invention, comprising two amplifier/filter blocks 18,20, and thence to a second mixer 22 supplied by a second local oscillator 24.

This second mixer serves to convert the signal to a second intermediate frequency of the apparatus which may be 21.4 MHz, the signal then being amplified and filtered in blocks 26, 28 and 30 before being received by a detector stage 32, the output of which is coupled to the Y-amplifier 34 of a display device such as a cathode ray tube (not shown). A shunt switching device 36 is connected across the coupling between the detector stage 32 and the Y-amplifier 34. Gain adjustment of the receiver chain is provided for in the first intermediate frequency stage 18,20 of the apparatus via gain signal input 38. If required, a roughly logarithmic response may be obtained in the second intermediate frequency stage 26,28,30 using an A.G.C.

detector 40 and A.G.C. 42 by means of a log/lin control 43.

The switching device 36 is operated so as to be nonconductive during the sweep period with the result that the Y-deflection of the display device represents, for example, the amplitude of any signals received at the input 10 as the local oscillator signal fed to first mixer 16 is caused to sweep across its frequency range.

The X deflection of the display device (not shown) during the sweep period is derived from a ramp generator 44 via a switching device 46, the necessary voltage to drive the cathode ray tube being produced by X-amplifier 48. Since the ramp generator during the sweep period produces an increasing substantially linear ramp voltage, the trace produced in the cathode ray tube may typically be of the form shown in Figure 2A in which three signals received at different frequencies are shown as peaks 50,52, and 54. It will be understood that during the sweep period the input voltage to the Y-amplifier 36 has the form shown during the first part of Figure 2B, while the input to the X-amplifier follows a ramp as shown in Figure 2C.

Referring again to Figure 1, the means by which the local oscillator signal fed to mixer 16 is generated will now be described. This signal, filtered in band-pass filter 56, is obtained from a voltage controlled oscillator 58 forming part of a frequency synthesiser loop 60 in accordance with the invention. The voltage control oscillator 58 has two voltage inputs 58A and 58B, the first of which 58A receives a synthesiser control voltage from a sampling and integrator stage 62 which, once every sweep cycle (during a flyback period), samples the output of a synthesiser logic module 64 and, during the sweep period stores that output, the synthesiser loop being broken during the sweep period. The loop is completed by a line 66 for carrying a radio frequency signal containing the local oscillator frequency from the voltage control oscillator 58 to the synthesiser logic 64.The other input 58B of the voltage controlled oscillator receives a ramp voltage during the sweep period and a reference voltage during the flyback period. The frequency of the voltage controlled oscillator is a function of the difference between the voltages supplied to its two inputs 58A and 58B. As a consequence, the frequency of the voltage controlled oscillator is synthesiser stabilised, yet can be varied over a wide sweep range.

The frequency synthesiser portion of the local oscillator will now be described in more detail with references to Figure 3 and 4.

The synthesiser is conventional insofar as it has a synthesiser logic chip 64A supplied with a reference frequency signal from a crystal reference oscillator 64B and a signal representative of the frequency of the voltage controlled oscillator (VCO) 58 provided by a pre-scaler 64C.

It is also conventional insofar as the logic chip 64A contains a phase and/or frequency comparator the outputs 64D of which serves as the source for circuitry supplying variable capacitance (varicap) diodes 58C forming part of the oscillator 58. The logic chip 64A includes frequency counters acting as dividers programmable via two groups of inputs 68, 70, here driven from an EPROM device 72 which stores look-up tables for selecting the appropriate division ratios depending on the required centre frequency set on frequency selection thumb wheel switches 74. In this example, the logic chip is type MC145152P supplied by Motorola, Inc., and the prescaler is a known dual modulus pre-scaler type SP8793 available from the Plessey Company Ltd.As is well known, this configuration allows a wide range of frequency settings at frequencies in the order of lOOMHz while maintaining a sufficiently high phase comparison frequency for short loop lock-up times.

By connecting the VCO 58 as part of the synthesiser phase locked loop only during the flyback period, it can be made to execute its frequency sweep during the sweep period.

However, during this period, the voltage applied to the first input 58A of the VCO is stabilised by storing the control voltage resulting from the output of the synthesiser logic comparator in a capacitive analogue voltage storage stage 76.

The output from the phase comparator of the synthesiser logic chip 64A takes the form of a series of pulses at the comparison frequency and is produced on a pair of lines 64D.

The difference in phase between the divider output of the logic chip and the output of the reference divider appears as a difference in pulse widths which is converted to a d.c.

voltage by feeding the pulses via a low pass filter 78 to a differential amplifier 80. Switching devices 82 and 84 are connected between the phase comparator outputs and the filter 78 to prevent voltage variations of the phase comparator outputs during the sweep period from affecting the charge stored on the capacitors of the filter. The switches are implemented by a pair of CMOS transmission gates type CD4066 available from RCA. The d.c. voltage obtained from the amplifier 80 passes via another switching device 86 (another CMOS transmission gate) to the noninverting input of another differential amplifier (operational amplifier) 88.The series combination of a 0.47 microfarad capacitor 90 and a 680 ohm resistor 92 coupled across this inverting input act as a low pass filter, whilst another capacitor/resistor pair comprising 2.2 microfarad capacitor 94 and 56 kilohm resistor 96 is connected across the non-inverting input by a fourth transmission gate 98 which is conductive at all times except during the synthesiser settling period. The gain of the operational amplifier 88 is determined by a feedback loop including a 5.6 kilohm resistor 100 and a 4.7 kilohm resistor 102 coupled to its inverting input and a large capacitance (approximately 235 microfarads) formed by the series connected pair of 470 microfarad electrolytic capacitors 104, these latter being connected in parallel with the resistor 100 by a fifth transmission gate 106 which is also non-conductive during the synthesiser settling period. The main voltage storage element is the capacitor 94. The function of capacitors 104 is to reduce the A.C.

gain of the operational amplifier 88 to one, thereby to filter out internal noise of the amplifier 88, the switch 106 being present to prevent the charge on the capacitors being affected by the synthesiser settling transients and to reduce synthesiser jitter. Switch device 98 which is coupled between capacitor 94 and resistor 96 on the line coupled to the non-inverting input of operational amplifier 88 is provided for a similar reason. In this way, the output of operational amplifier 88 is caused to carry a control voltage for the voltage controlled oscillator, this voltage forming the synthesiser locking voltage during the flyback period and being stored by the analogue voltage storage stage represented by the operational amplifier 88, capacitor 94 and associated components.This provides a stable control voltage to input 58A of the VCO 58 during the sweep period, the voltage being applied via a low pass filter 108 to the varicap diodes 58C of the VCO 58.

Control of the switch devices 82,84,86,98 and 106 is performed by applying appropriate pulses to a timing input 110 from the ramp generator 44 (Figure 1). Devices 82,84 and 86 receive pulses corresponding to the flyback periods so that during each flyback period the loop formed by the VCO 58, the synthesiser logic 64, and the filtering and amplifying stages 78,80,88 is closed. An inverter and delay device 112 is connected between the timing input 110 and the switch devices 98 and 106 to cause these devices to be conductive at all times except for a short period after the other switch devices 82,84 and 86 become conductive at the beginning of the flyback period.

As will be seen from Figure 4, the second input 58B of the voltage controlled oscillator is coupled to the varicap diodes 58C such that the oscillator frequency is a function of the difference between the voltage applied to input 58B and the control voltage applied to input 58A. It will be recalled that, during the sweep period, input 58B (hereinafter referred to as the modulation input) receives a ramp voltage, and in the flyback period receives a constant reference voltage.Since the ramp voltage change is only a fraction of the total voltage across diodes 58C, the change in capacitance is a substantially linear function of the ramp voltage and the frequency of the VCO 58 follows an approximately linear ramp characteristic during the sweep period as shown in Figure 2D, while during the flyback period the frequency, after the synthesiser settling interval, stabilises on a constant frequency which is, in practice, a frequency corresponding to the centre frequency set on the frequency select switches 74, for reasons which will become clear from the following description. The synthesiser settling interval is visible as a transient 113 at the beginning of the flyback period in Figure 2D.

Referring back to Figure 1, the voltage applied to the modulation input 58B, as mentioned above, is derived from a ramp generator 44 during the sweep period and from a reference voltage applied to reference voltage input 114 during the flyback period. The application of these voltages is controlled by further transmission gates 116 and 118 coupled to the input of a modulation amplifier 120. It will be understood that switch device 116 is conductive during the flyback period, and that switch device 118 is conductive in the opposite sense, i.e., during the sweep period.

In order to be able to view either a large range of frequencies or a very narrow band, and, when required, to investigate a particular signal in detail, the ramp signal applied to the modulation input of the VCO 58 is variable both in amplitude and level by means of range and shift controls associated with an amplifier 122 coupled between the ramp generator 44 and switch device 118. These controls allow the ramp voltage amplitude and level to be adjusted continuously between predetermined limits. It will understood that the ramp signal for determining the Xdeflection of the display device is obtained from the ramp generator 44 upstream of amplifier 122 in order for the range and shift controls to be effective.Thus, the effect of operating the range control is to vary the slope of the frequency-versus-time characteristic of the sweep period shown in Figure 2D, while the effect of operating the shift control is to raise or lower the sweep period ramp characteristic with respect to a selected centre frequency.

In addition, a switched resistive attenuator is coupled between the amplifier 122 and switch device 118 to modify the ramp voltage applied to the modulation input of the VCO 58 according to a frequency setting selected on the thumb wheel switches 74. This attenuator is indicated by reference numeral 124. Its effect is to reduce the swept frequency range as the set centre frequency is lowered, and also to attenuate the effect of the shift and range controls associated with amplifier 122. The attenuator 124 is arranged so that, to the user, the effect of the shift and range controls remains subjectively constant when the centre frequency is altered.

It should be noted that the operation of switch device 46 between the ramp generator 44 and the X-amplifier 48 of the display device is conductive only during the sweep period.

At other times, the X-amplifier receives a voltage via switch device 126, which is conductive only during the flyback period, from a differential amplifier 128 taking its inputs from the output of the attenuator 124 and the modulation input of the VCO 58. This amplifier 128 forms part of a feedback loop for a cursor generator which will be described below.

The timing pulses for all of the switch devices described above with reference to Figures 1,3, and 4 are obtained from a timing pulse generator 130 coupled to the ramp generator 44.

Still referring to Figure 1, the cursor generator also includes an oscillator 132 arranged to be coupled via a resistor 133 to the Y-amplifier 34 of the display device, which is itself gated by a pulse from a positioning pulse generator 135, the purpose of which will become apparent.

A d.c. restoring circuit 136 is provided to add a pulse 137 (see Figure 2D) to the voltage applied to the modulation amplifier 120 at the beginning of the flyback period, with the object of maintaining the average d.c. voltage applied to the modulation amplifier input at the reference voltage.

This has the effect of reducing the amplitude of the locking transient 113 of the synthesiser.

At this point it is appropriate to summarise the operation of the switch devices 46,126,116,118 and 36. During the sweep period switch devices 46 and 118 are conductive, the others being non-conductive. This means that a signal path exists from the ramp generator through switch device 46 to the amplifier 122 and thence through the attenuator 124 and via switch device 118 and the modulation amplifier 120 to the modulation input 58B of the VCO 58. Also, the detector 32 of the receiver chain is connected to the Y-input of the display device switch 36 being non-conductive. During the flyback period the conduction states are reversed, so that switches 126,116 and 36 are conductive. Consequently, the reference voltage applied to input 114 appears at the input of the modulation amplifier 120 so as to apply a reference voltage at the modulation input 58B of the VCO 58.This reference voltage applied to input 58B is also fed back to one input of the differential amplifier 128. At the same time the X-input of the display device is supplied with a voltage from the output of the differential amplifier 128.

This same output voltage passes through amplifier 122, thereby being subject to the position of the range and shift controls, and through attenuator 124, thereby producing a voltage at the other input of amplifier 128 which is dependent on both the positions of the range and shift controls and on the attenuation produced by the attenuator 124. The operation of the feedback loop containing differential amplifier 128 is such that the voltages at the two inputs of that amplifier are constrained to be substantially equal so that the voltage fed to the X-input of the display device is equal to the voltage present at that X-input necessary to produce a voltage at the input 58B of the VCO 58 equal to the reference voltage derived via the modulation amplifier 120 from the reference source 114.

Thus, the X-deflection in the display device during the flyback period is exactly the same as the X-deflection at the instant during the sweep period when the VCO frequency passes through the frequency corresponding to the centre frequency selected on the thumb wheel switches 74. It will now be appreciated that by enabling the oscillator 132 an oscillation burst 138 is applied to the Y-input of the display device via resistor 133 during the flyback period so that a vertical cursor bar can be produced in the display coinciding with that point on the sweep period trace corresponding to the centre frequency.

The oscillation burst is shown in Figure 2B and is produced by the oscillator 132 shown in Figure 1. A delay/logic stage 131 controls the signal output of the oscillator 132 to delay the burst with respect to the beginning of the flyback period to avoid the disturbances caused by the settling action of the differential amplifier 128. A positioning pulse circuit 135 establishes the necessary d.c.

level presented to the Y amplifier 34 for achieving the required height of the cursor bar on the display and performs a blanking operation during the cursor settling period.

The cursor generator circuitry will now be described in further detail with reference to Figure 5.

Beginning with the signal path operative during the sweep period, the ramp generator 44 is visible on the left hand side of Figure 5 as a circuit containing two operational amplifiers 139 and 140. It will be noted that the reference voltage present at the non inverting input of amplifier 139 is the reference voltage available at reference source 114 buffered by a voltage follower 142. The first operational amplifier 139 has a feedback capacitor 144 connected between its output and its inverting input, and a series resistance 146 coupling the inverting input to the output of the second operational amplifier 140. In parallel with resistance 146 is the series combination of a resistor 148 and a diode 150 with its cathode connected to the inverting input of operational amplifier 139.Amplifier 139 thus acts as an integrator, integrating the voltage at the output of second operational amplifier 140. The connection of the inputs of amplifier 140 is such that its inverting input is connected to the inverting input of amplifier 139, while its non inverting input is coupled by resistor 152 back to the output, thereby applying positive feedback to amplifier 140.

Operation of this circuit is such that second amplifier 140 is forced to be either at the level of the positive supply rail or that of the negative supply rail (the supply rails are not shown in the drawings). The operation of the integrator formed by amplifier 139 is thus to produce a linear slope at its output, the voltage rising or decreasing, as the case may be, until the voltage reaches the level of one of the supply rails. At this point, the voltage on the inverting input of the amplifier 140 becomes different from that on its non inverting input with the result that amplifier 140 changes state so that the output voltage of amplifier 139 begins to ramp in the opposite direction. The action of resistor 148 and diode 150 is to increase the slope of the ramp during the flyback period compared with that during the sweep period.Resistor 146 is variable to allow adjustment of the sweep rate.

The timing pulse generator 130 comprises a series of NAND gates 154 to buffer the output of operational amplifier 140 for controlling the switch devices 46,126, etc., via outputs A and B as indicated.

During the sweep period, switch device 46 is conductive and switch device 126 is non-conductive. As a result, the increasing ramp voltage available at the output of amplifier 139 is applied to one end of a voltage divider, the other end of which is coupled to the voltage reference (output of voltage follower 142). The voltage divider is variable by means of variable resistor 156 to drive amplifier 122 having a predetermined gain defined by its feedback resistor 158 and series resistor 160. The non-inverting input of amplifier 122 is coupled to the reference voltage source 114. As already described, the output of this amplifier is coupled through the attenuator 124 and switch device 118 to the modulation amplifier 120.As will be seen from Figure 5, the attenuator comprises a series of switch devices formed by three transmission gate chips 162 (type CD 4066 mentioned above) which are coupled to a variable potential divider comprising a plurality of series resistances 164 of predetermined values and resistor 165 connected to the voltage reference source 114. The transmission gates form a matrix controlled by 10 of the 14 data lines coupling the frequency select thumb wheel switches to the synthesiser logic chip (lines 68 and 70 in Figure 4). The operation of the range potentiometer 156 is to alter the amplitude of the ramp voltage derived from the ramp generator with respect to the voltage reference obtained from reference source 114.

Alteration of the level of this voltage is achieved by means of a shift control comprising a potentiometer 166 coupled between the taps of two voltage dividers 168 and 170. The first of these dividers 168 serves to make available a voltage which is higher than the reference voltage by means of an amplifier 172, while the second voltage provides a voltage below the reference voltage by being connected between the zero volts supply rail 174 and the reference source 144. The wiper of potentiometer 166 is applied via a series resistance 176 to the inverting input of amplifier 122 so that the voltage on the wiper is summed with the voltage on the wiper of the range potentiometer 156. The effect of the shift control 166 is to shift the position of the centre frequency in the display.

During the flyback period switches 46 and 118 are Upen circuit, while switch devices 126 and 116 are conductive.

Consequently, the output 178 to the X-amplifier of the display device is connected to the output of amplifier 128 rather than to the output of the ramp generator 44 and the feedback loop containing amplifier 128, switch 126, amplifier 122 and attenuator 124 is operative. As already explained, the effect of amplifier 128 is to produce at the output 178 to the X-amplifier a voltage which is the same as the voltage present at that output at the instant during the sweep period when the voltage applied to the input of the modulation amplifier 120 equals the value of the reference voltage available at reference source 114, thereby to control the X-input of the display device to maintain the cursor bar generated by the cursor generator at the same horizontal position in the display as the centre frequency, when the range and shift controls and the attenuator are operated.

When the shift control is operated the effect on the X-input versus time characteristic shown in Figure 2C -is to shift the horizontal portion of the characteristic corresponding to the flyback period up or down with respect to the ramp portion of the characteristic corresponding to the sweep period. Use of the reference voltage obtained from source 114 as the reference for all of the circuitry between the ramp generator and the modulation amplifier, including the attenuator enables high accuracy to be achieved while the ramp generator itself need only be approximately linear.

In summary, the apparatus disclosed above displays signals received by a radio frequency receiver chain as a function of frequency. It produces a swept frequency display by feeding to a mixer in the receiver chain a local oscillator signal which is derived both from a digital frequency synthesiser and from an analogue ramp generator. The local oscillator signal is generated by a voltage controlled oscillator (VCO) which is periodically coupled in the synthesiser loop and at other times receives a stored voltage corresponding to the voltage fed to the VCO when the loop was last locked. Superimposed upon the stored voltage is a ramp signal for causing the oscillator frequency to execute repeatedly a frequency sweep. The apparatus also includes circuitry for generating a cursor deflection in the display during the flyback period of the display, the circuitry including an arrangement for moving the cursor deflection when a display shift control is operated.

Claims (23)

1. Swept frequency signal analysing apparatus comprising: a frequency synthesiser, a phase locked loop associated with the frequency synthesiser, local oscillator means connectable in the phase locked loop and operable to generate a local oscillator signal for feeding to a mixer of a signal receiver chain, the oscillator signal having a frequency which varies with time so as to execute repeatedly a selected frequency sweep characteristic, and a display device for displaying an output signal from the receiver chain as a function of frequency, wherein the loop is arranged to be periodically opened and closed during operation of the oscillator means.

2. Apparatus according to claim 1, wherein: the phase locked loop is part of the synthesiser, the synthesiser includes a comparator stage for comparing a first signal representative of the local oscillator signal with a reference signal having a reference frequency thereby to produce a loop locking signal at an output of the comparator stage, the local oscillator means has control input circuitry and is arranged such the frequency of the local oscillator signal is defined by at least one control signal applied to the control input circuitry, switch means coupled in series in a signal path between the comparator stage output and the control input circuitry, timing circuitry coupled to the switch means and operable to control the switch means so that the loop is open during a sweep period of the display device and closed during at least part of a flyback period of the display device, and a storage capacitor associated with the said signal path between the switch means and the input circuitry for storing, when the loop is open, a voltage representative of the loop locking signal obtained from the comparator stage when the loop is closed.

3. Apparatus according to claim 2, including a sweep signal generator arranged to generate a ramp signal for application to the input circuitry of the local oscillator means during the sweep period.

4. Apparatus according to claim 3, wherein the control input circuitry of the local oscillator means is arranged such that it receives at least first and second control signals, the first control signal being derived from the loop locking signal during at least part of the flyback period and from the said loop lock representative voltage of the capacitor during the sweep period, the second control signal being obtained from the sweep signal generator which includes a ramp generator and a reference source operable in combination such that the second control signal takes the form of a ramp signal during the sweep period and a constant level signal during at least part of the flyback period.

5. Apparatus according to claim 4, wherein the local oscillator means is arranged such that the frequency of the local oscillator signal is a function of the difference between the levels of the first and second control signals.

6. Apparatus according to any of claims 2 to 5, wherein the signal path between the switch means and the input circuitry of the local oscillator means includes an amplifier having (a) an input connectable to the storage capacitor, and (b) a feedback loop containing a switched capacitance.

7. Apparatus according to claim 6, wherein the storage capacitor is shunt connected across the said amplifier input via an isolating switch, and the switched capacitance is connected in series in the feedback loop with an interrupting switch, the isolating and interrupting switches being connected to the timing circuitry so as to be substantially non-conductive during at least part of the flyback period.

8. Apparatus according to claim 7, wherein the timing circuitry and the isolating and interrupting switches are arranged such that the switches are substantially non-conductive only during a synthesiser settling period which occupies only a part of the flyback period.

9. Apparatus according to any preceding claim, further comprising a cursor generator for causing a cursor mark to be displayed by the display device.

10. Apparatus according to claim 1, including: a ramp generator coupled via an intermediate stage to the local oscillator means for causing the local oscillator frequency to vary progressively during a sweep period of the display device, the intermediate stage including means for modifying the level of a ramp signal obtained from the ramp generator, a reference voltage line connectable to the local oscillator means during flyback period, means for feeding the ramp signal, prior to its modification in the intermediate stage, to a sweep input of the display device during the sweep period, and a cursor generator having (a) a first part coupled to another input of the display device for generating a cursor mark at a position defined by a signal applied to the sweep input of the device, the position corresponding to a selected frequency, and (b) a second part in the form of a feedback circuit having inputs coupled to the output of the intermediate stage and to the reference voltage line, and an output connectable during the flyback period to the input of the intermediate stage and the sweep input of the display device, the feedback circuit being operable to cause the level of the signal applied to the said sweep input to adopt a level which, when received by the intermediate stage, causes the level of the signal at the output of the intermediate stage to be substantially equal to the voltage level obtained from the reference voltage line.

11. Swept frequency signal display apparatus comprising: a frequency synthesiser, a phase locked loop associated with the frequency synthesiser, local oscillator means connectable in the phase locked loop and operable to generate a local oscillator signal for feeding to a mixer of a signal receiver chain, the oscillator signal having a frequency which varies with time so as to execute repeatedly a selected frequency sweep characteristic, a display device for displaying an output signal from the receiver chain as a function of frequency in a two-dimensional dual co-ordinate system, the device having first and second co-ordinate inputs, the first co-ordinate input being connected to receive the output signal from the receiver chain, a sweep signal generator coupled to, and arranged to feed a periodic sweep signal to, the second co ordinate input, and a cursor generator for generating a cursor mark in the display device, and having first and second parts for feeding cursor signals respectively to the first and second co-ordinate inputs of the display device, the first part being arranged to generate a display deflection to produce the mark, and the second part being associated with the sweep signal generator and arranged to cause the position of the mark to correspond to a selected frequency.

12. Apparatus according to claim 11, wherein the sweep signal generator is arranged such that the sweep signal has a sweep period and a flyback period, and wherein the cursor generator is operable to feed the cursor signals to the display device during the flyback period.

13. Apparatus according to claim 12, wherein the sweep signal generator includes: a ramp generator and a reference level source operable in combination such that the said sweep signal fed to the second co-ordinate input of the display device takes the form of a ramp signal during the sweep period and a constant level signal during at least part of the flyback period, and an intermediate stage having an input coupled to receive the said sweep signal and an output coupled to a control input of the local oscillator means for causing the local oscillator frequency to execute the said sweep characteristic, the intermediate stage including means for altering the level of the ramp signal in a manner such that the position of a selected frequency in the display of the said receiver chain output signal is alterable.

14. Apparatus according to claim 13, wherein the sweep signal generator includes: ramp generator switch means coupled to the output of the ramp generator for disconnecting the ramp generator from the intermediate stage and from the said second co-ordinate input during the flyback period, and output switch means coupled to the said control input of the local oscillator means and to a reference voltage line, the output switch means being operable as a changeover switch to connect the said control input to the output of the intermediate stage during the sweep period and to the reference voltage line during the flyback period, and wherein the cursor generator second part comprises a flyback circuit having:: an amplifier with inputs connectable to the output of the intermediate stage and the reference voltage line respectively, and an output for generating a feedback signal which is a function of the difference between signal levels at the amplifier inputs, and feedback switch means for applying the feedback signal to the input of the intermediate stage and the second co-ordinate input, the amplifier being arranged such that the signal applied to the second co-ordinate input adopts a level which, when received by the intermediate stage, causes the level of the signal at the output of the intermediate stage to be substantially equal to the voltage level obtained from the reference voltage level.

15. Apparatus according to claim 14, wherein the cursor generator first part comprises circuitry for applying a oscillating cursor mark signal to the first co ordinate input of the display.

16. A method of displaying signals from a radio frequency receiver chain as a function of frequency, comprising: generating a local oscillator signal the frequency of which varies with time so as to execute repeatedly a selected frequency sweep characteristic, and feeding a signal obtained from the receiver chain downstream of the mixer to a display device, wherein the local oscillator signal is generated by means of a frequency synthesiser and a phase locked loop associated with the synthesiser, the local oscillator signal being obtained from local oscillator means connectable in the loop, and wherein the loop is periodically opened and closed.

17. A method according to claim 16, wherein: the selected frequency sweep characteristic is executed during repeated sweep periods which alternate with display flyback periods, the loop being open during the sweep period and closed during at least part of the flyback period, the synthesiser compares a first signal representative of the local oscillator signal with reference frequency signal to produce a loop locking signal which is fed to control input circuitry of the local oscillator means during the flyback period, control input circuitry also being supplied with a reference signal level when it receives the loop locking signal, storing a signal level corresponding to the level of the loop locking signal and applying the stored signal level to the said control input circuitry during the sweep period, the control input circuitry simultaneously receiving a sweep signal for causing the frequency of the local oscillator signal to vary progressively during the sweep period.

18. A method according to claim 17, wherein the said signal level corresponding to the level of the loop locking signal is generated by applying a voltage corresponding to the loop locking signal to a storage capacitor when the loop is closed and arranging for the capacitor to be coupled to the control input circuitry of the local oscillator means while the loop is open

19.A method of displaying signals from a radio frequency receiver chain as a function of frequency, comprising: generating a local oscillator signal the frequency of which varies with time so as to execute repeatedly a selected frequency sweep characteristic, the local oscillator signal being generated by means of a frequency synthesiser, a phase locked loop associated with the synthesiser, and local oscillator means connectable in the loop, feeding a signal obtained from the receiver chain downstream of the mixer to a first co-ordinate input of a two-dimensional display device, generating a periodic sweep signal and feeding it to a second co-ordinate input of the display device, generating first and second cursor signals, applying the first cursor signal periodically to the first co-ordinate input to cause a cursor deflection in the display, and applying the second cursor signal to the second co-ordinate input, the second cursor signal being such as to define the position of the cursor deflection corresponding to a selected frequency.

20. A method according to claim 19, wherein: the sweep signal has a sweep period in which its level varies progressively and a flyback period in at least part of which the sweep signal is a signal of a constant level, and the first and second cursor signals are applied to the display device during the flyback period.

21. A method according to claim 20, wherein: the sweep signal is fed in modified form during the sweep period to a control input of the local oscillator means via an intermediate stage to cause the local oscillator frequency to execute the said sweep characteristic, the modification of the sweep signal being performed by the intermediate stage to alter the level of the sweep signal in a manner such that the position of a selected frequency in the display of the signal obtained from the receiver chain is alterable, supplying a reference voltage to the local oscillator means during the flyback period, applying the second cursor signal to the intermediate stage during the flyback period, generating a feedback signal dependent upon the difference between a reference signal level applied to the local oscillator means and an output signal obtained from the intermediate stage during the flyback period, and feeding the feedback signal back to the input of the intermediate stage and to the second co-ordinate input of the display device in order that the signal applied to the second co-ordinate input adopts a level which, when modified by the intermediate stage, causes the level of the signal at the output of the intermediate stage to be substantially equal to the said reference voltage.

22. Swept frequency signal analysing apparatus constructed and arranged substantially as herein described and shown in the drawings.

23. A method of displaying signals from a radio frequency receiver chain, the method being substantially as herein described with reference to the drawings.