GB2160077A - Electric signal frequency changer - Google Patents

Abstract

In display apparatus, e.g. an oscilloscope, the frequency of a repetitive electric signal Fx is changed by means including means to sample a first signal at varying intervals with respect to a reference point in the signal and means to form the samples obtained thereby into a second signal F2 having a waveshape similar to that of the first signal. Trigger circuit 2 derives trigger signals from the input signal Fx which are then rate reduced by divider 6. The trailing edge of these rate reduced pulses is used as a trigger reference for pulse width modulator 7. Scanner circuit 8 produces a slow speed ramp under the control of time base circuit 9. The ramp controls the width of pulses from the pulse width modulator 11. The sampling means 3 takes samples at times controlled by the trailing edges of the pulses from modulator 11. Therefore the system samples the input waveform Fx at slightly different points every few cycles to produce the low frequency waveform F2. <IMAGE>

Description

SPECIFICATION Electric signal frequency changer This invention relates to a method and apparatus for use in changing the frequency of an electric signal.

In a particular embodiment to be described, a comparatively low frequency signal output is derived from a comparatively high frequency signal input. The output signal follows the form of the input signal and enables processing of the output signal to be effected at a lower frequency and within a narrower bandwidth than would otherwise be the case, thereby permitting the signal to noise ratio to be maximised and a higher level of amplification to be obtained.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which Figure 1 is a block schematic circuit diagram and Figure 2 shows at A to F the waveforms of signals occurring at various points in the circuit of Figure 1.

Figure 1 shows a circuit which is particularly suitable for processing a radio frequency signal in such a way that it may be more easily displayed on an oscilloscope. The circuit of Figure 1 includes an input terminal 1 which is connected to the inputs of both a trigger circuit 2 and a sampling head 3. The output of the sampling head 3 is connected to a buffer amplifier/4 and the output of the buffer amplifier/4 is connected to the output terminal 5 of the circuit. The output of the trigger circuit 2 is connected to a divider circuit 6 and the output of the divider circuit 6 is connected to one input of a pulse width modulator 7. To the other input of the pulse width modulator 7 there is connected the output from a scanner 8.The input to the scanner 8 is connected to one output of a time base generator 9 and the other output 10 of the time base generator 9 is available for connection to the X deflection circuit of a cathode ray tube, which is not shown.

The output from the pulse width modulator 7 is applied via a shaper 11 to the sampling head 3.

The output at the terminal 5 from the buffer amplifier/4 is available for connection to the Y deflection circuit of the cathode ray tube mentioned above.

The operation of the circuit which has been described above may be better understood by reference also to the waveforms shown in Figure 2.

At A in Figure 2 there is illustrated an input signal fx connected to the input terminal 1. The frequency of the signal fx in the particular example is 200 MHz, although the use of the circuit at higher frequencies, for example in the gigahertz range, is possible. The back edge of the signal from the divider 6 is used as a trigger reference, as indicated at 15 in Figure 2. In the particular example, the divider 6 divides by 200.

At the same time, the time base generator 9 controls the scanner circuit 8 which produces a slow speed ramp, as indicated at D. The period of the ramp signal indicated at D can be controlled from 1 msec.(upwards) to infinity.

Considering the waveform shown at C, which represents the output of the pulse width modulator 7, it will be noted that the waveform at C incorporates a portion K subsequent to the trigger point 15, which represents the constant propagation delay in the system, and a portion marked At,; At2; At3 etc., which is voltage dependent variable. The pulse width modulator 7 incorporates a monostable multivibrator circuit which is triggered by the output from the divider 6.

The duration At,; At2; At3 of the signal from the monostable multivibrator circuit is a function of a bias voltage and of a resistance-capacitance (RC) network to which the bias voltage is applied. The negative going ramp voltage shown at D in Figure 2 is added to the bias voltage thereby successively delaying the times which are taken by the RC network to reach the voltage at which the monostable circuit is caused to turn off. Thus with the ramp voltage at AV1, the trigger or turn-off point is reached at a period At, after K. With the ramp voltage at Av2, the trigger or turn-off point 2 is reached at a period At2 after K, and so on. The periods At,; At2 At3 are thus increased gradually.

As shown at E in Figure 2, a succession of positive going pulses, derived from the collapsing back edges of the pulses generated by the multivibrator circuit in the pulse width modulator 7, is produced at the output of the pulse shaper 11 and applied to a field effect transistor in the sampling head 3, in order to enable the sampling head 3 and sample the signal entering the sampling head 3 from the terminal 1.

It will be understood from the foregoing description that the intervals between the samples are increased successively by periods determined by the slope of the ramp signal from the scanner 8.

At F1 in Figure 2 there is indicated a pulse output from the sampling head 3 in which the pulses are not only shown to be spaced by successively increasing intervals, but in which the amplitude of the pulses increases according to the increasing slope of the portion of the input waveform that is being sampled.

The pulse signal shown at F1 is then applied to the buffer amplifier/4 which incorporates an (RC) circuit having a time constant such that the level of the signal is maintained between samples while following the amplitude changes of the pulses shown at F1, thereby enabling a reformed envelope as shown at F2 in Figure 2 to be obtained from the output of the buffer amplifier/4.

It will be noted that, in the particular example given, the signal applied at terminal 1 has a period of nano seconds. The period of the signal obtained from the divider 6 has a period of 1 micro second, the period of the ramp signal obtained from the scanner 8 has a period which is measured in milliseconds and the reformed wave has a period which is measured in milli-seconds.

The result of this reduction in time scale is to enable a large number of samples to be effectively integrated to form an envelope, as shown at F2 in Figure 2, which represents the shape of the origi nal high speed signal but which is at a much lower frequency. This we refer to as a "time-dilated" signal.

To display the reformed envelope the signal on terminal 5 is connected to the Y deflection circuit and the time base signal from the time base generator 9 is connected to the X deflection circuit of a cathode ray tube.

In the particular embodiment described, the slow speed ramp shown at D can be controlled to have a period of from 1 ms upwards. At a period of 1 ms and with an input signal having a frequency fx of 200 MHz, it is possible to produce at terminal 5 1000 samples per second with intervals of llfx x (DIV RATIO) t At + KCLS between the samples.

It will be appreciated that, although the invention has been described, by way of example, with reference to a specific embodiment, it is possible to make variations and modifications within the scope of the invention defined in the claims as granted.

It will, for example, be understood that the particular embodiment described is designed for use in producing a display on a cathode ray tube and that variations of the circuit may be made when it is employed in changing a frequency for other purposes. Such a variation may consist of the omission of the time base generator.

An embodiment has been described in which the intervals between the times at which the samples are made is varied by amounts Atl; At2; and Ats, which increase with time by very small amounts in a linear manner, according to the slope of the ramp voltage shown at D Fig. 2. Of course, it is not necessary that this ramp voltage should be linear.

It is possible to vary the intervals between which the samples are taken in a non- linear, for example, logarithmic, manner. It is also possible to vary the intervals in such a way that they reduce in value with time by small amounts, for example as a result of using a positive-going ramp voltage.

Claims (7)

1. Apparatus for use in changing the frequency of a repetitive electric signal, including means to sample a first signal at varying intervals and means to form the samples obtained thereby into a second signal having a waveshape similar to that of the first signal.

2. Apparatus as claimed in claim 1 including a divider circuit, an input for coupling the first signal to the divider, a scanner circuit, a pulse width modulator, the output of the scanner circuit and the output of the divider being coupled to the pulse width modulator, a sampling head, the output of the pulse width modulator and the input for the first signal being connected to the sampling head and a buffer connected to the output of the sampling head, the buffer providing an output signal which follows the amplitude changes of the samples.

3. Apparatus as claimed in either claim 1 or claim 2 in which the sampling intervals increase with time.

4. Apparatus as claimed in claim 2 in which an RC circuit in the buffer has a time constant such that the amplitude changes of the sample signals are followed.

5. A method of changing the frequency of a repetitive electric signal in which the signal is sampled at successive intervals of time which are varied and in which the samples obtained thereby are formed into a second signal having a waveshape similar to that of the first signal.

6. Apparatus as claimed in claim 1 arranged to operate substantially as described herein with reference to the accompanying drawings.

7. A method of changing the frequency of a repetitive electric signal substantially as described herein with reference to the accompanying drawings.