GB2131638A - Microwave frequency generator - Google Patents

Abstract

A microwave frequency generator comprises first and second voltage- tuned microwave oscillators (VCOA, VCOB), and switch means (SWA, SWB), operable to connect the output of either oscillator to an output connection (OP1, OP2). Frequency control means responsive to a digital frequency signal are operable to set the frequency of oscillation of each oscillator to a required value whilst the output of the other oscillator is connected to the output connection, thereby allowing each oscillator time to stabilise before its output is selected. <IMAGE>

Description

SPECIFICATION Microwave frequency generator This invention relates to microwave frequency generators, and in particular to such generators for use as local oscillators in radar systems.

Radar receivers use local oscillators to provide a signal which is mixed with a received signal to shiftthe frequency of the latterto an intermediate frequency.

When the frequency of the transmitted radar signal is constant then the local oscillator signal will also be constant, and design problems are slight. However, many types of modern radar use frequency agile systems in which the frequency ofthe transmitted signal is caused to vary over a range, often in an apparently random manner. This may be done to avoid interference problems due to unwanted return signals known as "clutter". Frequency agile systems require the local oscillatorfrequencyto follow the transmitted frequency and to remain relatively stable whilst return signals or echos are received. In some instances, such as when using pulse compression techniques, it is importantthatthe oscillatorfrequency shall not drift whilst echos are being received, and this has led to difficulties.It is possible to provide fast switching frequency synthesisers to provide stable but quickly-changed outputs, but these synthesisers are large and expensive. Simplerforms of generators, such as the voltage-tu ned oscillator, tend to be insufficiently stable to be suitableforuse with pulse compression radars.

It is an object of the present invention to provide a microwave frequency generator which does not suffer from the above disadvantages.

According to the present invention there is provided a microwave frequency generatorwhich includes first and second voltage-tuned microwave oscillators, switch means operable to connect the output of either one ofthe oscillators to an output connections, switch control meansoperabletoselectthe output of each oscillator in turn, and frequency control means operable to set the frequency of oscillation of each oscillatorto a required value whilst the output ofthe other oscillator is selected by the switch means.

The invention will now be described with reference to the accompanying drawings, in which Figure lisa schematic block diagram of one form of generator; Figure 2 shows waveforms produced during the operation ofthegeneratorof Figure 1; and Figure 3 is a block diagram of an alternative form of part of the generator of Figure 1.

In the diagram of Figures 1 and 3 microwave energy paths are shown in thick lines, and signal or data paths in thinner lines.

Referring now to Figure 1, the generator includes two voltage-controlled microwave oscillators VCOA and VCOB, which may for example bevaractor-tuned transistor oscillators. The microwave enery output of oscillator VCOA is connected by way of a switch SWA, such as a PIN diode, to one input of a 3dB directional coupler DC. Similarly the output of the other oscillator VCOB is connected by way of a second switch SWB to the other input of the coupler DC. The two outputs of the coupler are connected to output connectors OP1 and OP2 respectively.

The components so far detailed make up the microwave section ofthe generator. The remaining components as described belowform the control section of the generator. The two oscillators have identical control arrangements, and onlythatfor oscillatorVCOAwill be described in detail.

Ambit parallel digital inputrepresentingthere- quired frequency of oscillation is applied to a register RAwhen an "update" signal UA is applied to the register The contents of the register are applied to a digital-to-analogue converter DAA, which in turn applies a control voltagethrough a control amplifier Auto the control input oscillatorVCOA. the same arrangements apply to the other oscillatorVCOB. A switch control signal SWS controls the operation of switches SWA and SWB.

The operation of the generator will now be de scribed with reference to Figure 2. It is assumed that oscillatorVCOA is operating at a frequency fl and oscillatorVCOB is tuned to a frequencyf2 and ready to operate. This time will be referred toast0. As shown at a) in Figure 2 the frequency code FC applied to the two registers is F3, which defines the next frequency of operation of oscillators VCOA, namelyf3. Switch SA is closed and switch SA opens so that an output at frequency fl from oscillatorVCOA is applied via the coupler DCto provide two equal signals atthetwo outputs OP1 and OP2.

AttimetOthe update signal UA is appled briefly as show at b) to cause the frequency code F3to be applied to register RA. Atthe sametimethe switch signal SWS shown at d) opens switch SWA and closes switch SWB. The output of the generatortherefore changes from fl,to f2,to which frequency oscillator VCOB has already been tuned, as shown atf).

Oscil lator VCOA changes frequency from fl,to f3 over a time interval as shown ate).

Once register RA has been loaded with the code F3 defining the nextfrequencyforVCOA, the code may be changed to F4, defining the nextfrequencyfor VCOB.

Timet2, denotes the time at which VCOA has re-tuned to frequency f3. However, due to the post-tuning drift associated with voltage controlled oscillators, the oscillator is not ready for use, and the output of VCOB is used until time t2. At this time the output of VCOA has become stable, and the switch signal SWS opens switch SWB and closes switch SWA, so that the output of VCOA is now applied to the output connectors OP1 and OP2. The update signal UB is applied, causing the frequency code F4to be applied to register RB, and allowing oscillatorVCOB to startto re-tune to frequency f4. The procedure continues with each change of frequency.

It will be seen that each oscillator is allowed a considerabletimeforthefrequencyto stabilise, namely to time (t2-t0) in the case of VCOB. This is far longerthan the time available if only a single oscillator is used, since in that case the time is only that available between the end of the radar "listening" time and the nexttransmitted pulse. This longertime makes it possible to use the less stable but less costly voltage-controlled oscillator.

A number of modifications may be applied to the arrangement described above, and these are shown in Figure 1. In most cases voltage-controlled oscillators requirestabilisation againsttemperaturechanges.A temperature sensorTS is carried by the oscillator, and a temperatu re -- co m pensating cuircuit applies a compensating voltageto the control amplifierAAto compensateforthe change.

If a constantfrequency output is require, then a reference voltage RV source may be used to apply a controlvoltagetothecontrol amplifierAA.

It is possible to pick off a small proportion of each microwave output by way of a 20dB coupler DC2 through a threshold circuitTH to give a fault signal FS for either oscillator.

Figure 3 shows an alternative form of control forthe oscillator. Here again, identical control circuits are usedforeach oscillator. No registers are used in this arrangement, and the digital frequency is applied directlyto a single digital-to-analogue converter DA.

The control voltage obtained from the converter is applied to a separate sample and hold circuitfor each oscillator. As shown in Figure 3, each sample and hold circuit consists of a sampling switch SSA (or SSB), a sampling capacitor CA (or CB) and buffer amplifier.

The switch SSA is closed by the update signal UA, and causes the capacitor CAto charge up to the voltage supplied by the converter DA. When the switch SSA is opened at the end of the UAsignal,thevoltage stored on the capacitorCA is applied through the buffer amplifier to the tuning input of the oscillator VCOA.

The other sample and hold cicuit operates in the same way underthe control of update signal UB.

In the embodiments described above the changeoverfrom onefrequencyto another is very rapid, being on lythe operating time of the microwave switches SA and SB.

Ifthe reflective switches are used for SA and SB then isolatorswill haveto be used between each switch and the associated oscillatorto preventthe reflection of energy back into the oscillator.

Claims (8)

1. Amicrowavefrequencygeneratorwhich includes first and second voltage-tuned microwave oscillators, switch means operable to connectthe output of either one of the oscillators to an output connection, switch control means operable to select the output of each oscillator in turn, and frequency control means operable to setthe frequency of oscillation of each oscillatorto a required value whilst the output of the other oscillator is selected by the switch means.

2. A generator as claimed in Claim 1 in which the frequency control means comprise, for each oscillator, circuit means responsive to a digital signal indicative of the required frequency of oscillation to produce a frequency control voltage for application to the voltage-controlled oscillator.

3. Ageneratorasclaimed inclaim2inwhichthe circuit means comprise, for each oscillator, a register operabletostorethedigitalsignal and a digital-toanalogue converter operable to convertthestored digital signal into the frequency control voltage.

4. A generator as claimed in Claim 2 in which the circuit means comprise, for each oscillator, a digital to-analogueconverteroperableto convertthe digital signal into thefrequency control voltage and storage means operable to store said voltage whilst applying it to the oscillator.

5. A generator as claimed in Claim 4 in which the storage means compriseacapacitorconnected to the output ofthe said converterfor a predetermined time.

6. A generator as claimed in any one of Claims 1 to 5 which includes a directional coupler having each of its two inputs connected to a separate one of the two oscillators and having its two outputs connected to two output connections to provide two output signals ofthe same frequency and amplitude.

7. A generator as claimed in anyone of Claims 1 to 6which includestemperaturestabilisation means connected to each oscillator and operable to combine a compensating voltage with the frequency control voltage applied to the oscillator.

8. A microwave frequency generator substantially as herein described with reference to the accompanying drawings.