GB2263369A - Frequency discriminator - Google Patents

Abstract

The frequency discriminator comprises a mixer (10) and a phase shifting circuit (15). An input signal is supplied to first and second input ports (11, 12) of the mixer (10). The phase shifting circuit (15) shifts the phase of the input signal supplied to the second input port (12) by an amount related to the frequency of the input signal. The phase-shifting circuit comprises a resistively driven, parallel resonant circuit (15', 15") connected across the second input port (12) of the mixer (10). The arrangement avoids spurious responses outside the pass band of the tuned circuit (15) by arranging a current to flow into the second part (12) via the tuned circuit so that the discriminator does not fall to zero even at frequencies well beyond the pass band. The mixer (10) may be a Gilbert cell mixer using differential pairs of transistors. <IMAGE>

Description

FREOUENCY DISCRIMINATOR This invention relates to frequency discriminators, especially, though not exclusively, frequency discriminators for use in automatic frequency control (AFC) circuits such as are used, for example, to regulate the local oscillator (LO) of the super heterodyne downconversion stage in a microwave receiver.

Microwave receivers usually employ super heterodyne downconversation early in the signal processing path whereby to convert the received microwave signals into a more convenient intermediate frequency (IF) signal - typically, an IF of 60 MHz is used. The received microwave signals are combined with an LO signal in a mixer, the frequency of the LO signal being so adjusted that the differencefrequency produced at the output of the mixer is the required IF.

The LO frequency can be adjusted manually by an operator, a procedure requiring some skill and practice. However, the LO frequency is subject to drift, due to ambient temperature changes, for example, and so it is desirable to provide an AFC instead of, or in addition to, the manual adjustment.

An AFC consists of a closed loop incorporating a frequency discriminator, for comparing the actual IF with the required IF, and some form of servo feedback network for correcting tuning errors detected as a result of the comparison. If the actual IF is correct, the output from the frequency discriminator will be zero (i.e. no error signal is generated), whereas if the actual IF is greater or less than the required value, the frequency disciminator will generate a respective error signal comprising a positive or a negative d.c. level.

Ideally, the frequency discriminator should be capable of operating over a frequency range which is sufficiently wide as to correct for a gross mis-adjustment by an operator. Furthermore, because the afore-mentioned comparison of the IF signal is usually carried out during the transmission periods of an associated transmitter, the frequency discriminator should not respond to harmonics of the IF signal caused by overloading of the receiver front end during such transmission periods.

Many frequency discriminators comprise the combination of a mixer and a frequency-dependent phase-shifting circuit.

As shown in Figure 1 of the accompanying drawings, the input signal I/P to be monitored (e.g. the afore-mentioned IF signal) is supplied directly to a first input port of mixer 1 and is supplied to a second input port via the phase-shifting circuit 2. The output O/P from the mixer will be at the difference-frequency of the two input signals, and because the input frequencies are exactly the same, the output O/P will be a d.c. level having a magnitude and sign related to the relative phases of the two inputs.

If the input signal I/P has the correct frequency, o say, no adjustment is needed. In this circumstance, the phase-shifting circuit 2 is so designed as to shift the phase of the input signal through 900 such that the output from the mixer will be zero. Alternatively, if the input frequency is respectively smaller or larger than the required frequency, the phase-shifting circuit shifts the phase by a greater or lesser amount, giving the required negative or positive d.c. level at the mixer output.

A typical frequency discriminator, commonly used in AFC and FM demodulation applications, is the so-called "quadrature" discriminator, shown in Figure 2a. Here, the frequency-dependent phase-shifting circuit 20 is a reactively-fed, parallel resonant circuit connected to the second input port of the mixer. The input signal I/P is supplied to the parallel resonant circuit 21 via a high value reactance 22 (such as provided by a small value capacitor or a large value inductor).

The values of the inductor L and the capacitor C which form circuit 21 are so chosen that resonance occurs at the required input frequency. o. At resonance, circuit 21 is resistive, and the high value reactance 22 subjects the 0 input signal I/P to a 90 phase shift, giving zero output from the mixer. At frequencies to either side of resonance, but still within the passband of the resonant circuit 21, the frequency-to-phase characteristic of circuit 20 has the form shown in Figure 2(b), and for sinusoidal input signals this characteristic has substantially the same shape as the frequency-to-voltage characteristic of the discriminator. The shape of the characteristics is determined by the Q-factor of the resonant circuit 21.

At frequencies outside the passband of the resonant circuit 21, the phase shifting circuit 20 progressively attenuates the input signal, and this has the undesirable effect of "folding over" the discriminator response characteristic, as shown in Figure 2(c). At frequencies far removed from the required resonant frequency (O), the mixer output falls off to zero and this could lead to an incorrect tuning condition. Furthermore, harmonics present in the input signal result in spurious responses, e.g. S1, S2' at sub-harmonics of the required frequency and, again, these could give rise to tuning errors.

Other known frequency discriminators suffer from similar problems.

It is, therefore, an object of the present invention to provide a frequency discriminator which at least alleviates the afore-mentioned short-comings of known frequency discriminators. More specifically, it is an object of the invention to provide a frequency discriminator which is suitable for use in an AFC and has a substantially unambiguous frequency-to-voltage characteristic, free from spurious responses, even at large offset frequencies.

According to the invention, there is provided a frequency discriminator comprising a mixer having first and second input ports, first and second signal paths for directing an input signal respectively to the first and second input ports, and a phase-shifting circuit for shifting the phase of the input signal directed to the second input port by an amount related to the frequency of the input signal, wherein the phase-shifting circuit comprises a parallel resonant circuit so arranged as to provide a current input to said second input port even at input signal frequencies outside the passband of the parallel resonant circuit.

If the input signal frequency is within the passband of the parallel resonant circuit, current will circulate around the resonant circuit creating a differential input current across the second input port.

At input frequencies outside the passband of the parallel resonant circuit, current will still be supplied to the second input port, via the inductor only of the resonant circuit at relatively low input frequencies, and via the capacitor only of the resonant circuit at relatively high input frequencies. Accordingly, the frequency-to-voltage characteristic of the frequency discriminator will be free from spurious responses and zero-crossings.

A frequency discriminator in accordance with the invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of a known frequency discriminator; Figure 2a illustrates an example of a known frequency discriminator; Figure 2b shows the frequency-to-phase characteristic of the frequency discriminator shown in Figure 2a; Figure 2c shows the response characteristic of the frequency discriminator . of Figure 2a over a relatively wide frequency range; Figure 3a shows a frequency discriminator according to the present invention; Figure 3b shows the frequency-to-voltage characteristic of the frequency discriminator shown in Figure 3a; and Figure 3c is a detailed illustration of an embodiment of the frequency discriminator shown in Figure 3a.

Referring now to Figure 3a, the frequency discriminator comprises a balanced mixer 10 having a first input port 11 (the in-phase port) and a second input port 12 (the quadrature-phase port).

An input signal I/P is routed directly to the first input port 11 over a first input path 13 and is routed to the second input port 12 over a second input path 14 incorporating a phase-shifting circuit 15. As will be described, the phase-shifting circuit is effective to shift the phase of the input signal I/P by an amount related to the frequency Co thereof.

The mixer 10 generates an output signal O/P at the difference-frequency of the two input signals applied to the respective input ports 11 and 12. As the two input signals have exactly the same frequency, the difference frequency is zero and so the output signal O/P is a d.c.

level having a size and polarity related to the relative phases of the input signals. If the input signals are in phase quadrature, the d.c. level will be zero, whereas a smaller phase difference results in a positive d.c. level and a larger phase difference results in a negative d.c.

level. Provided the input signal I/P has a sinusoidal waveform, the frequency-to-voltage characteristic of the frequency discriminator will be similar to the frequencyto-phase characteristic of the phase-shifting circuit 15.

The phase-shifting circuit 15 comprises a parallel resonant circuit 15' which is driven resistively via a series-connected resistor 15". The capacitor C and the inductor L of circuit 15' are connected across the second input port 12, as shown in Figure 3a, and the second input port has low input impedance Zin which completes the in resonant circuit. Provided the frequency of the input signal I/P is within the resonant circuit passband (the frequency range ACo in Figure 3b), current will pb circulate via the capacitor and inductor creating a differential, "push-pull" current input across the input port 12.

At the resonant frequency coO (i.e. the required input 0 frequency) of circuit 15', the differential input current is in phase quadrature with respect to the input voltage I/P, giving a zero d.c. output level. Higher frequencies within the passband give rise to lower phase differences, and positive d.c. output levels, whereas lower frequencies within the passband give rise to higher phase differences, and negative d.c. ouput levels.

At frequencies outside the resonant circuit passband, the resonant circuit 15' will still provide a current path for the input signal I/P, via the inductor only at relatively low frequencies (giving a negative d.c. output level) and via the capacitor only a relatively high frequencies (giving a positive d.c. level). At these extreme frequencies, the current drive to the second input port 12 becomes single-ended, as compared with the differential, "push-pull" current drive attained at frequencies within the passband A(%br and in such circumstances, the effective mixer transconductance is halved, giving a lower d.c. output level. The solid line curve in Figure 3b represents the actual overall frequency response of the discriminator, whereas the broken line curve represents the ideal response.

It will be apparent that the response has no spurious zero-crossings, the sole zero-crossing occuring at the required input frequency, i.e. the resonant frequency cho0. Accordingly, when integrated, the d.c. output level O/P will always drive an associated AFC circuit towards the required frequency cho0, without any ambiguity, irrespective of the starting frequency. This represents a significant improvement over hitherto known frequency discriminators. As already explained, the frequency responses of known frequency discriminators tend to exhibit one or more spurious zero-crossings which could give rise to an erroneous tuning condition, particularly if the starting frequency is far removed from the required frequency, Coo Figure 3c of the drawings illustrates how the frequency discriminator could be implemented using a standard Gilbert cell mixer. With this configuration, the input signal is fed in-phase to one switching terminal 31 of the mixer, the other switching terminal 32 being decoupled.

The phase-shifted input signal, supplied via phaseshifting circuit 15, is fed to the two emitter terminals 33,34 of the mixer, and the base terminals 35,36 of the mixer are decoupled to create a relatively low impedance at the emitter terminals.

Claims (7)

1. A frequency discriminator comprising a mixer having first and second input ports, first and second signal paths for directing an input signal respectively to the first and second input ports, and a phase-shifting circuit for shifting the phase of the input signal directed to the second input port by an amount related to the frequency of the input signal, wherein the phase-shifting circuit comprises a parallel resonant circuit so arranged as to provide a current input to said second input port even at input signal frequencies outside the passband of the parallel resonant circuit.

2. A frequency discriminator as claimed in claim 1, wherein the parallel resonant circuit is resistively coupled to the first signal path.

3. A frequency discriminator as claimed in claim 1 or claim 2, wherein the parallel resonant circuit comprises the parallel arrangement of a capacitor and an inductor.

4. A frequency discriminator as claimed in claim 3, wherein the capacitor and inductor are connected across respective input terminals of the second input port.

5. A frequency discriminator as claimed in any one of claims 1 to 4, wherein said mixer is a Gilbert cell mixer and said parallel resonant circuit is connected across respective emitter terminals of the Gilbert cell mixer.

6. An automatic frequency control circuit incorporating a frequency discriminator as claimed in any one of claims 1 to 5.

7. A frequency discriminator substantially as herein described with reference to Figures 3(a) to 3(c) of the accompanying drawings.