GB1576761A - Microwave frequency discriminators - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MICROWAVE FREQUENCY DISCRIMINATORS (71) We, AEI SEMICONDUCTORS LIMITED, a British Company, of Carholme Road, Lincoln, LNI ISG, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is performed, to be particularly described in and by the following statement:- This invention relates to microwave frequency discriminators and microwave oscillator arrangements including the same.

As is well known a problem which occurs in connection with pulsed Gunn oscillators is a variation of frequency across the output pulse, which variation is known as "chirp". The causes of chirp are not known with certainty, but it appears to be associated with variations of certain parameters of the Gunn diode semiconductor material.

Chirp can be modified by the r.f. load so that if the oscillator is tuned (for example, by a variable capacitance diode) the chirp will vary as the oscillator is tuned by the diode. The chirp may be increasing or decreasing in nature or may reverse during the pulse.

Partial compensation for chirp can be achieved by applying a shaped waveform to the tuning element, but at best this is only satisfactory if the chirp is constant.

One object of the present invention is to provide an improved thin film frequency discriminator which is suitable for including in a feedback circuit in a pulsed Gunn oscillator arrangement to provide a feedback signal which may be used to stabilise the frequency of the oscillator and so reduce the effects of chirp.

According to one aspect of this invention, a thin film microwave frequency discriminator comprises a substrate and on said substrate a conductive track providing an r.f. input path capacitively coupled to the input ends of two conductive tracks forming resonators of slightly different electrical lengths both substantially equal to no/2, the output ends of which are capacitively coupled to respective diodes of opposite polarity and thence to further conductive tracks which couple into one conductive track providing an output r.f. path, where n is an integer and A is at least approximately the median guide wavelength in the operating frequency band of said frequency discriminator.

Normally, said further conductive tracks and said conductive track into which they are coupled are continuous.

Normally said diodes are provided with earth returns constituted by conductive tracks of length equal to no/4.

Preferably in each case n is equal to unity.

Preferably said two conductive tracks forming resonators of length substantially equal to nA/2 lie on adjacent sides of an imaginary square and said further conductive tracks lie on the remaining sides of said imaginary square.

Means may be provided for reactively loading said two conductive tracks forming resonators of length substantially equal to no/2, whereby the effective value for A may be varied.

Preferably said loading means comprises two varactor diodes, each capacitively coupled to a respective end of one of the two conductive tracks forming resonators of length substantially to no/2.

Normally each of said varactor diodes is arranged to be grounded on one side by a conductive track of length substantially equal to n4 and means are provided for applying d.c. bias at one end of a conductive track of length substantially equal to no/2, the other end of which is connected to the other side of said varactor diode.

Typically said substrate is of fused silica or alumina ceramic and said conductive tracks are of gold.

According to another aspect of this invention, a pulsed Gunn oscillator arrangement includes a discriminator as described above in a feedback path from the output of a Gunn oscillator and a bias responsive variable capacitance device is arranged to vary the carrier frequency of said oscillator whereby variation of frequency across each output pulse of said arrangement tends to be reduced.

The invention is illustrated in and further described with reference to the drawings accompanying the provisional specification in which, Figure I is a semi-schematic plan view of one thin film frequency discriminator in accordance with the present invention, Figure 2 is an explanatory graph, Figure 3 is a semi-schematic plan view of another thin film frequency discriminator in accordance with the present invention, Figure 4 is a block schematic diagram showing a Gunn oscillator arrangement in accordance with the present invention.

In the Figures, like references are used for like parts.

Referring to Figure 1, the thin film discriminator illustrated thereby consists essentially of a pattern etched in a thin gold layer on a substrate 1 of fused silica approximately + mm thick.

The conductive pattern provides a conductive track 2, which acts as an r.f.

input path. The conductive track 2 is capactively coupled to the input ends 3 and 4 of two conductive tracks 5 and 6 which are respectively of lengths L, and L2, one of length slightly less than A/2 and the other of length slightly greater than A/2. The extent to which the lengths of the resonators are chosen to differ from A/2 will be chosen for optimum operation in dependence upon the frequency range of interest but typically L, and L2 will be of lengths respectively 3o/ less and 3/o greater than A/2. The output ends 7 and 8 of the conductive tracks 5 and 6 are capacitively coupled to respective rectifier diodes 9 and 10. As shown the rectifier diodes 9 and 10 are oppositely poled.

Rectifier diodes 9 and 10 are connected respectively to one and the other of two further conductive tracks 11 and 12, which continue into a single conductive track 13 which forms an r.f. output path.

As shown, the current returns for the rectifier diodes 9 and 10 are provided by conductive tracks 14 and 15 respectively, each of length equal to A/4, which are connected at their ends remote from the diodes 9 and 10 to points of earth potential 16 and 17 respectively.

Figure 2 is a graph showing the output voltage appearing on output conductive track 13, against frequency. If the output appearing on conductive track 13 is utilised to control the tuning of a Gunn diode oscillator (as will be described later with reference to Figure 4) of nominal frequency fQ=V/A the discriminator will have a locking range as shown from fl to f2, these latter being the frequencies at which the output falls to the noise level, where V is the velocity of propagation of a signal on the substrate.

The widths and lengths of the conductive tracks 5 and 6 forming the resonators of length substantially equal to A/2 determine the sharpness of discrimination, or in other words the slope of the curve shown in Figure 2 at f0.

Referring to Figure 3, the frequency discriminator illustrated therein is similar to the frequency discriminator shown in Figure 1 except for the addition of means for reactively loading the A/2 resonators formed by the conductive tracks 5 and 6. As shown these last mentioned means comprises varactor diodes 18 (in the case of conductive track 5) and 19 (in the case of conductive track 6). Ground returns for the varactor diodes 18 and 19 are again provided by conductive tracks 20 and 21 respectively, which extend to points of earth potential 22 and 23 respectively. D.c.

bias sources 24 and 25 are connected, via conductive tracks 26 and 27 respectively each of length equal to A/2, to apply d.c.

bias respectively to the varactor diodes 18 and 19.

If the d.c. bias sources 24 and 25 are provided to be adjustable, the loading upon the resonators 5 and 6 may be varied and hence the median frequency f0 of the frequency band over which the discriminator operates.

Whilst as shown in Figure 3 separate d.c.

bias sources 24 and 25 are shown, it is possible to utilise a single d.c. bias source if the varactor diodes 18 and 19 are well matched.

Referring to Figure 4, the block 28 represents a Gunn oscillator, the output of which is connected to an output path 29 for utilisation and to the input conductive track 2 of a thin film frequency discriminator 30 which is as described with reference to Figure 1 or Figure 3 of the drawings accompanying the provisional specification.

The output conductive track 13 of the discriminator 30 is connected via an amplifier 31 to provide varactor bias for a tuning varactor diode forming part of the Gunn oscillator 28. An input 32 to the oscillator 28 represents the input for normal Gunn bias.

The effect of connecting the discriminator 30 in a tuning feedback circuit to the Gunn oscillator is to frequency stabilise the r.f. output of the Gunn oscillator. Provided the rectifier diodes 9 and 10 are matched, the frequency of the stabilised oscillator will tend not to vary with power output.

The gain of the amplifier 31 is provided to be as high as possible consistent with stability and the amplifier 31 should be of relatively large bandwidth. A suitable example of integrated circuit amplifier which may provide the amplifier 31 is a National Semiconductor reference LM318.

WHAT WE CLAIM IS: 1. A thin film microwave frequency discriminator comprising a substrate and on said substrate a conductive track providing an r.f. input path capacitively coupled to the input ends of two conductive tracks forming resonators of slightly different electrical lengths both substantially equal to nN2, the output ends of which are capacitively coupled to respective diodes of opposite polarity and thence to further conductive tracks which couple into one conductive track providing an output r.f. path, where n is an integer and A is at least approximately the median guide wavelength in the operating frequency band of said frequency discriminator.

2. A discriminator as claimed in claim I and wherein said further conductive tracks and said conductive track into which they are coupled are continuous.

3. A discriminator as claimed in claim 1 or 2 and wherein said diodes are provided with earth returns constituted by conductive tracks of length equal to no/4.

4. A discriminator as claimed in any of the above claims and wherein in each case n is equal to unity.

5. A discriminator as claimed in any of the above claims and wherein said two conductive tracks forming resonators of length substantially equal to nN2 lie on adjacent sides of an imaginary square and said further conductive tracks lie on the remaining sides of said imaginary square.

6. A discriminator as claimed in any of the above claims and wherein means are provided for reactively loading said two conductive tracks forming resonators of lengths substantially equal to no/2, whereby the effective value for A may be varied.

7. A discriminator as claimed in claim 6 and wherein said loading means comprises two varactor diodes, each capacitively coupled to a respective end of one of the two conductive tracks forming resonators of length substantially to no/2.

8. A discriminator as claimed in claim 7 and wherein each of said varactor diodes is arranged to be grounded on one side by a conductive track of length substantially equal to n4 and means are provided for applying d.c. bias at one end of a conductive track of length substantially equal to nA/2, the other end of which is connected to the other side of said varactor diode.

9. A discriminator as claimed in any of the above claims and wherein said substrate is of fused silica or alumina ceramic and said conductive tracks are of gold.

10. A thin film frequency discriminator substantially as herein described with reference to Figure 1 of the drawings accompanying the provisional specification.

11. A thin film frequency discriminator substantially as herein described with reference to Figure 3 of the drawings accompanying the provisional specification.

12. A pulsed Gunn oscillator arrangement including a discriminator as claimed in any of the above claims arranged in a feedback path from the output of a Gunn oscillator and a bias responsive variable capacitance device is arranged to vary the carrier frequency of said oscillator whereby variation of frequency across each output pulse of said arrangement tends to be reduced.

13. A Gunn oscillator arrangement substantially as herein described with reference to Figure 4 of the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. example of integrated circuit amplifier which may provide the amplifier 31 is a National Semiconductor reference LM318. WHAT WE CLAIM IS:

1. A thin film microwave frequency discriminator comprising a substrate and on said substrate a conductive track providing an r.f. input path capacitively coupled to the input ends of two conductive tracks forming resonators of slightly different electrical lengths both substantially equal to nN2, the output ends of which are capacitively coupled to respective diodes of opposite polarity and thence to further conductive tracks which couple into one conductive track providing an output r.f. path, where n is an integer and A is at least approximately the median guide wavelength in the operating frequency band of said frequency discriminator.

2. A discriminator as claimed in claim I and wherein said further conductive tracks and said conductive track into which they are coupled are continuous.

3. A discriminator as claimed in claim 1 or 2 and wherein said diodes are provided with earth returns constituted by conductive tracks of length equal to no/4.

4. A discriminator as claimed in any of the above claims and wherein in each case n is equal to unity.

5. A discriminator as claimed in any of the above claims and wherein said two conductive tracks forming resonators of length substantially equal to nN2 lie on adjacent sides of an imaginary square and said further conductive tracks lie on the remaining sides of said imaginary square.

6. A discriminator as claimed in any of the above claims and wherein means are provided for reactively loading said two conductive tracks forming resonators of lengths substantially equal to no/2, whereby the effective value for A may be varied.

7. A discriminator as claimed in claim 6 and wherein said loading means comprises two varactor diodes, each capacitively coupled to a respective end of one of the two conductive tracks forming resonators of length substantially to no/2.

8. A discriminator as claimed in claim 7 and wherein each of said varactor diodes is arranged to be grounded on one side by a conductive track of length substantially equal to n4 and means are provided for applying d.c. bias at one end of a conductive track of length substantially equal to nA/2, the other end of which is connected to the other side of said varactor diode.

9. A discriminator as claimed in any of the above claims and wherein said substrate is of fused silica or alumina ceramic and said conductive tracks are of gold.

10. A thin film frequency discriminator substantially as herein described with reference to Figure 1 of the drawings accompanying the provisional specification.

11. A thin film frequency discriminator substantially as herein described with reference to Figure 3 of the drawings accompanying the provisional specification.

12. A pulsed Gunn oscillator arrangement including a discriminator as claimed in any of the above claims arranged in a feedback path from the output of a Gunn oscillator and a bias responsive variable capacitance device is arranged to vary the carrier frequency of said oscillator whereby variation of frequency across each output pulse of said arrangement tends to be reduced.

13. A Gunn oscillator arrangement substantially as herein described with reference to Figure 4 of the drawings accompanying the provisional specification.