Self detection performance of an FET active patch antenna under injection locking and its application for PM signal detection

S. Sancheti V.F. Fusco

Indexing terms: Antennas (active), Detection

Abstract: The self detection capabilities of an active patch antenna for Doppler detection is experimentally investigated under injection locking conditions. The results show a significant improvement in detection performance under certain operating conditions. Further, the pos- sibility of using an injection locked FET active patch antenna for PM(PSK) signal detection is investigated.

1 Introduction

The industrial and commercial applications of short range radar systems is an area of interest particularly for automobile sensing applications. Short range radar systems have been analysed in detail by several authors [l, 21 for their Doppler detection action. One such short range radar system using a microstrip patch element and GaAs MESFET device [3] was shown to have self detec- tion capabilities [4]. Here the primary reason for detec- tion of the signal was identified to be RF power variations across the device terminals inducing low fre- quency bias variations [SI. Further, these results showed that the detection process was weakly dependent on RF frequency variations. This evidence prompted the invest- igation of self detection performance under injection locking conditions, reported in this work. It is shown that Doppler detection is possible under injection locking conditions and can be used to improve the sensitivity of a self oscillating detector. This injection locking condition ensures the removal of RF variation [SI.

The possibility of using direct PM signal generation combined with self oscillating detection for the FET active patch antenna is also investigated; detection is achieved using a homodyne process. The received signal is directly homodyned with the local oscillator (detector) signal to produce the baseband (detected) signal. The results show that, in principle, there exists the possibility of direct generation and detection of PSK/PM signal using active antenna modules. The system described in

0 IEE, 1994 Paper 1114H (Ell) , first received 23rd August and in revised form 17th December 1993 The authors are with the High Frequency Electronics Laboratory, Department of Electrical and Electronic Engineering, The Queen’s Uni- versity of Belfast, Ashby Building, Stranmillis Road, Belfast BT9 5AH, N. Ireland, United Kingdom

IEE Proc.-Microw. Antennas Propag., Vol. 141. No. 4, August 1994

this paper has potential for application in short range data links, vehicle tolling, etc.

2

A detailed analysis of the self detection performance of an FET active antenna module [SI showed that the detected signal appears as a low frequency bias variation across the bias resistance of the circuit. This variation has prin- cipal components of 6VE,/6A and 6VdSo as given in eqn. 1 .

Self detection under injection locking

where V, is bias voltage, A is RF current magnitude, w is angular frequency and K is defined as a constant. 0, and 0“ are defined as phase angles of the complex quantities 6Z/60 and 6Z/6A, respectively.

Under injection locking conditions both RF current and frequency are complex functions of locking power, locking frequency, operating bias and angle of intersec- tion between device and circuit line [SI. Once frequency locking is achieved, 6VB/6w reduces to zero and remains so even under load pulling conditions induced by a moving target or a variable load. However, signal detec- tion is still possible, resulting from to the 6VB/6A com- ponent, which is known to be dominant even when frequency locking is not considered.

Using the practical set up of Fig. 1, a set of results were obtained for the Doppler detected signal. Fig. 2 shows the (S + N)/N of the detected signal together with the RF power variation as a function of locking fre- quency. The detected signal increases/decreases linearly with the change in RF power at low power levels. However at a certain power level (- 6.0 dB relative to free running), the detected signal becomes smaller than the free running level. This change is a function of (6VE/6A) AA. At low power levels, the increase in detec- tion is caused by an increase of (6VE/6A)AA and vice

The authors would like to acknowledge the finan- cial support provided by the Commonwealth Scholarship Commission, Association of Com-

253

versa, but at high power levels this effect is compensated for by poor back biasing (rectification). It is thought that at high RF device current levels, the nonlinearity

DC

t Q bias drive signal

analysis

locking source

Fig. 1 Sensitivity measurement set up

Fig. 2 Sensitiuity measurements Free running frequency 8.986 GHz Free running (S + N)/N 18.0 dB o power 0 detected signal

responsible for rectification changes to a weak nonlin- earity, resulting in poor detection.

An important observation from this graph is, that at a certain locking frequency and injected power, within the locking range, up to 6.0 dB improvement in detected signal compared with the nominal free running value can be obtained. This effect may be used to enhance the detection range of the system and to reduce FM noise in order to obtain better signal to noise ratio. In this way, both the range and sensitivity of the system when used as a short range Doppler detector can be improved.

3

Homodyning techniques have been used for optical PSK signal detection [7]. An optical homodyne receiver using an injection locked laser was discussed by Lidoyne and Gallion [8]. They showed that injection locking provides synchronisation of the local oscillator frequency to the received signal, hence homodyning is performed. Further, the paper states that PSK modulation can also be achieved by using an injection locked laser at the trans- mitter. Hobson [9] has shown that circuit controlled variations of the frequency of a CW X band Gunn diode oscillator causes sympathetic variations of bias current. Hobson and Thomas [IO] investigated FM demodu- lation using this principle and showed that the receiving Gunn diode oscillator when frequency locked to the incoming signal reproduces modulation waveform in the form of a bias current variation. Also the FM signal was

PM signal generation and detection

254

obtained from a bias circuit modulated Gunn oscillator. In the following sections, a direct PM signal implementa- tion is presented for the first time for an active FET patch antenna operated in one of two modes: as a direct PM modulator, and as a self oscillating detector direct PM demodulator.

Using the techniques reported by Martin and Hobson [Il , 123 to phase modulate Gunn oscillator and by Fusco and Drew [13] to generate a phase modulated signal using an FET active patch antenna, a PM signal was generated by superimposing a low frequency AC signal onto the active antenna DC bias under frequency locked condition. The bias versus RF frequency and power characteristics of transmitter module were used to select the actual DC bias point and the peak to peak voltage of AC modulation signal. It is observed that by properly selecting the active antenna bias, associated amplitude modulation can be eliminated leaving only phase modulation. The phase of the transmitter is con- trolled by the peak to peak voltage of a low frequency square wave applied to the transmitter DC bias. The module when operated under proper biasing conditions, injection locking and free running frequencies, can be used to generate a biphase signal (BPSK) having 180" phase difference between two states with minimal ampli- tude modulation superimposed on the carrier.

It is is well known that Doppler detection of a moving target is a function of the relative phase of the received reflected signal at the oscillator port. This is shown by eqn. 1. In the case of a moving target, due to reflec- tion from the target, the phase continuously changes, causing changes in the RF current at the device port. This results in DC bias changes which can be conveni- ently detected by audio frequency methods. The same effect can also be generated at the active antenna port by an ordinary phase modulated transmitter, provided that the received power at the detector is small, but suficient enough, to injection lock the oscillator. Under locking conditions, RF current variations are easily obtained by controlling the phase of locking signal. Therefore, the possibility exists for direct PM detection. The direct detec- tion of a PM signal can be explained using the principles of injection locking or homodyne mixing [14].

3.1 injection locking Using the basic theory of injection locking, Kurokawa [6], the injection locking vector is given by (E/A,)B+ where E is the injected signal voltage, A, is free running RF current and 4 is the phase difference between E and A,. In the case where a PM signal is used for injection locking, the phase of the injection vector is constantly changed at a predetermined rate. In turn, this produces a low frequency RF power variation at the detector module. However, as the injection frequency is held con- stant, the injection vector intersection with Z(w) is fixed, allowing only RF current variations as shown in Fig. 3. Once again, the rectification of RF current variations (back biasing) will produce detectable low frequency variations in the DC bias circuit. Further, Kurokawa [6] and Mackey [IS] have shown that in the presence of a phase modulated injection signal, the phase of the oscil- lator follows the locking signal phase (with a constant phase difference) provided the modulation frequency range is smaller than the locking range. As these condi- tions are adhered to in the experimental set up used in this work, the phase of the receiver/detector must follow the phase of transmitted signal. Thus phase changes are manifested as bias changes in the receiver, allowing direct

IEE Proc.-Microw. Antenna5 Propag., Vol. 141, No. 4, August 1994

detection of the transmitted signal. Further, as the detec- tion is obtained only under the injection locked condi- tion, the possibility of false detection resulting from

free running operating point

I ReZ

Fig. 3 tation

Illustration of detection process using complex plane represen-

change of reflection coefficient under influence of low fre- quency bias changes is eliminated.

3.2 Homodyne mixing It is known that self oscillating detectors have mixing properties [16, 171. If the received signal, E,, is given by

(2) and the local oscillator signal, E, (under injection locking), by

(3)

E , = ~ ~ p r + + s ( t ) ~

E - ~ ~ ~ j ( a l f 9 ~ ) L -

where EI.* and & L are magnitude and phase of the received and local oscillator signals respectively. Mixing both signals produce an output which is a function of cos [&-(t) - 4,]. Thus under locking conditions a DC output signal proportional to the cosine of phase differ- ence between transmitter and receiver phase at the recei- ver port is produced. This change is transferred to the circuit bias point, because of the circuit and device mechanisms explained above. Finally, measurement of circuit bias variation will reproduce the modulating signal characteristics.

4 Experimental verification

A number of experiments were performed to ascertain the proper detection of PM signal by varying the phase of the transmitter or detector relative to each other. The experimental set up shown in Fig. 4 was used. The set up is such that the stability of the active antennas is enhanced by mutual radiation induced injection locking to the phase reference oscillator. The effect of a change in the peak to peak voltage of the transmitter bias is seen in Fig. 5, in terms of the detected signal voltage measured at the wave analyser in Fig. 4. The variation of the detected output waveform represents the change of transmitter phase for unchanged receiver operating conditions. A similar effect was observed by changing the phase of the receiver by varying its DC bias slowly, while holding the transmitter phase constant. The results of this experiment

I E E Proc.-Microw. Antennas Propag., Vol. 141, No. 4, August 1994

are plotted in Fig. 6 in terms of detected signal at the wave analyser in Fig. 4. Once again, the detected signal variations are the result of changes of the relative phase

analysis

bias

~

Rx. patch Tx. patch

Fig. 4 PM communication link

0.6 1 2 3 4 transmit antenna bias, peak to peak V

Fig. 5 DC bias 3.5 volts, locking frequency 9.052 GHr 0 detected signal

Detected signal as a function of transmitter modulation

40 k

U ' 336 3.46 3.65 3.78 3.87 3.99 4.21 434

receive antenna bias. d c V Fig. 6 Locking frequency 9.052 GHz 0 detected signal

Detected signal as function of receiuer bias

between transmitter and receiver. The results in Figs. 5 and 6 illustrate the direct PM detection capability of the active antenna when operated as a self oscillating detec- tor.

5 Conclusions

The self oscillating detection performance of a FET active antenna has been demonstrated under frequency locking conditions. It is shown that system performance can be enhanced by proper selection of the active antenna operating conditions and injection locking fre- quency. Up to 6 dB improvement in detection sensitivity was observed. Improvement in signal to noise ratio is

255

. - . l l

also to be expected, caused by reduced FM noise as a result of injection locking.

A PM signal generation and detection scheme using simple FET active patch antennas for transmit and receive functions has been demonstrated. An explanation, of the self detection mechanism, has been given, based on the theories of injection locking and mixing. The results have been verified experimentally.

6 References

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1 1 MARTIN, B., and HOBSON, G.S.: ‘High speed phase and ampli- tude modulation of Gunn oscillators’, Electron. Left.. 1970, 6, (8), pp. 244-246

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14 KING, R.J.: ’Microwave homodyne systems’ (Peter Peregrinus Ltd., London, UK, 1978)

15 MACKEY, R.C.: ‘Injection locking of klystron oscillators’, IRE Trans., Microwave Theory and Techniques, 1962, IO, pp. 228-235

16 NAGANO, S., and AKAIWA, Y.: ’Behaviour of Gunn diode oscil- lator with a moving reflector as a self excited mixer and load varia- tion detector’, IEEE Trans., 1971, MTT-19, (12). pp. 906-910

17 GUPTA, M.S., LOMAX, R.J., and HADDAD, G.I.: ‘Noise wn- sideration in self mixing IMPA’IT diode oscillators for short range Doppler radar approximations’. IEEE Trans., 1974, MIT-22, (1). pp. 37-43

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