GB2097208A - Am/Fm receivers; driving signal- strength/tuning meters - Google Patents

Abstract

Apparatus is disclosed comprising a limiting amplifier 20 receiving an RF input signal 21, 22 which goes to one input pair of a double- balanced modulator 25, the other input pair of which receives the output of the limiting amplifier. The modulator produces a differential output voltage on lines 28, 29 whose magnitude depends on a characteristic of the input signal. A differential amplifier 30 (Fig. 1) or 36 (Fig. 3) converts the differential voltage to a single-ended voltage. Arrangements are described for both AM and FM. The output voltage for AM indicates signal level and may be used for AGC and a signal strength meter; the output voltage for FM indicates timing deviation and may be used for AFC and a centre-zero tuning meter. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electronic apparatus and methods This invention relates to electronic apparatus and methods.

In known arrangements for obtaining a control voltage using an AM or FM detector, such voltage has tended to have substantial drift in magnitude with changes in the temperature of the equipment and the present invention has been made from a consideration of this.

According to one aspect of this invention electronic apparatus comprises means responsive to an RF input signal to provide a differential output voltage dependent on an input signal characteristic, and means for converting said differential voltage to a single-ended voltage with respect to a datum voltage.

The characteristic may be the magnitude of the input signal.

The responsive means may comprise first means for receiving said input signal and providing an output signal of uniform amplitude, second means for receiving the input signal and said output signal and providing said differential output voltage.

The first receiving means may comprise a limiting amplifier.

The second receiving means may comprise a double-balanced modulator.

The responsive means may comprise an AM detector including a limiting amplifier and a double-balanced modulator, the limiting amplifier output and the input signal being respectively applied to inputs of the modulator, the arrangement being such that the modulator output voltage is proportional to the magnitude of the input signal.

In the case where the input signal may be frequency modulated, the characteristic may be deviation from a desired or datum frequency.

The responsive means may comprise an FM detector including a limiting amplifier and a double-balanced modulator, a frequency-responsive circuit, the input signal being applied to the limiting amplifier and to the circuit, the outputs of the limiting amplifier and the circuit being respectively applied to the inputs of the double-balanced modulator, the modulator output voltage being proportional to the magnitude of the deviation in frequency of the input signal from a datum.

The responsive means may comprise a detector product reference SL624C by Plessey Semiconductors Limited of Swindon, England.

The converting means may comprise a differential amplifier arranged to receive said differential voltage.

According to another aspect of the invention a method comprises obtaining a differential output voltage, dependent on an input signal characteristic, from an RF input signal and converting said differential voltage to a single-ended voltage with respect to a datum voltage.

The invention may be performed in various ways and two specific embodiments with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a block diagram of one embodiment; Figure 2 is a circuit of a double-balanced modulator; Figure 3 shows a differential amplifier; Figure 4 is a circuit of a detector; Figure 5 shows the detector in AM mode; Figure 6 shows a differential amplifier for use with the arrangement of Fig. 7; and Figure 7 shows the detector in FM mode.

It is desirable for a radio receiver to provide an output which provides an indication of the level of the input signal when the receiver is detecting an amplitude modulated input signal, and which indicates a deviation from current (center) tuning when the receiver is detecting a frequency modulated input signal.

The output when detecting AM could normaily be used to derive AGC (automatic gain control) and drive a signal strength display meter. The output when detecting FM would normally drive a centre-zero tuning error meter and an automatic frequency correction device.

The present arrangement provides such outputs in a repeatable and temperature stable manner.

Referring to Fig. 1, an audio frequency (AF) amplitude modulated radio frequency (RF) input signal S IN of frequency F is applied on line 21, 22 to the input terminals of a limiting amplifier 20.

The output signal of the limiting amplifier between lines 26, 27 is fed to one pair of input terminals of a double-balanced modulator 25. The limiting amplifier removes any amplitude modulation from the signal S IN so that the output from the limiting amplifier is a signal of uniform amplitude at the carrier frequency F, limited in amplitude and usually a square wave. The signal S IN is also applied on lines 23, 24 to the other pair of input terminals of the double-balanced modulator 25. This amplitude limiting applies both to AM and FM.

An example of a double-balanced modulator is shown in Fig. 2.

The limiting amplifier and double-balanced modulator may be arranged in a synchronous detector of an amplitude modulated carrier signal. With this arrangement the output of the double-balanced modulator between lines 28, 29 is two signals, one of which is at radio frequency and the other of which would conventionally be an audio signal S OUT. The radio frequency signal is normally removed by a capacitor. If there is no input signal S, the voltage on each of lines 28, 29 is steady, the voltage being the same on each line. If there is an input signal, both the positive and negative cycles of the signal produce a voltage difference between lines 28, 29, the difference always being of the same polarity.

The magnitude of voltage difference between lines 28, 29 varies with the amplitude of the signal S. If the input signal S is modulated at an audio frequency, the magnitude of the difference voltage varies in proportion to the modulated signal. The audio frequency variation of this difference voltage is S OUT.

The operation of the detector is known to the skilled man and it is believed further details are not required.

In the present case, as shown in Fig. 1, the output signal on lines 28, 29 is fed to means 30 for converting the difference voltage to a single ended voltage X with respect to a datum voltage.

One example of such means is shown in Fig. 3 in the form of a differential amplifier 31 having resistors 32-35 of equal magnitude and an operational amplifier 36. The voltage on lines 28, 29 is applied to lines 40, 41.

When there is no input signal S, the standing voltage on lines 28, 29 is the same, say N, but as reference voltage 42 is zero the voltage at 37 is reduced to half N, and the operational amplifier, therefore, reduces the voltage at 38 to half N, so that the single ended output voltage X is zero, corresponding to the zero difference voltage between lines 28, 29.

If an input signal S is present so that the voltage on line 28 is N + R and on line 29 is N - R, a difference of 2R, where N is the standing bias voltage when no signal is present and R is that caused by the signal, the voltage at 37 will be (N + R)/2 and, therefore, the voltage at 38 will be (N + R)/2 also because of operational amplifier 36, so that the voltage at X is 2R, linearly proportional only to the difference voltage between lines 28, 29. Resistors 32, 33 are equal, and resistors 34, 35 are equal, but if the ratio between resistors 32, 33 and resistors 34, 35 is changed, the ratio of the voltage at X to the difference voltage between lines 28, 29 can be varied.

The differential amplifier is a form of selfbalancing bridge.

Fig. 4 shows a convenient example of limiting amplifier and double-balanced modulator in the circuit of a multi-mode integrated circuit radio detector 39 available from Plessey Semiconductors Limited of Swindon, England, under reference SL624C which can detect or demodulate AM, acting as a synchronous detector, and FM, acting for example as a quadrature detector, depending on particular combinations of external components. The device 39 has a double-balanced modulator M and a five stage limiting amplifier LA, and includes a voltage controlled audio gain system including amplifier AA with gain control system GCS, see Figs 4, 5. The outputs 26, 27 from the limiting amplifiers are passed to one pair of modulator inputs and also appear on pins 6, 7.The device 39 also includes the audio amplifier AA connected directly to the modulator output, and a separated audio amplifier AB.

Pin 1 is an AF amplifier input; pins 3, 4 the limiting amplifier inputs; pin 5 is earth: pins 6, 7 the limiting amplifier outputs; pins 8, 9 conventionally the RF input signal inputs; pins 10, 11 the modulator outputs; pin 1 2 an AF output; pin 1 3 the gain control input; pin 14 gives a bias reference; and pins 15, 1 6 are audio amplifier output options.

The output of the double balanced modulator having two signals respectively of frequencies f, and f2 applied to it is two frequencies F1 = If 1 + f,l and F2 = If - f,l. In normal use, F, would be several MHz and F2 an audio signal.

In one example, Fig. 5, with the device 39 connected for detecting AM, capacitors 43 are 100 pF, capacitors 44 are 0.1 juF, resistor 45 is 4.7K, resistors 46, 47 are 10K, 15K respectively and resistor 48 is up to 47K. The input signal S to be detected is applied at terminals 8, 9. A detected audio signal output is available at terminal 16.

The input signal S to be detected is applied to the limiting amplifier and to the modulator inputs, as above. Because any audio modulation of the input signal is removed from the input signal by the limiting amplifier, both the carrier signal and the whole input signal S are applied to the modulator which, therefore, demodulates the AF signal which appears at pin 1 2. Pins 10, 11 are connected by a capacitor T as shown to remove the unwanted F, signal.

The output on lines 28, 29 at pins 10, 11 is a differential output voltage in proportion to the magnitude of the input signal S.

At zero input signal, the differential output voltage is nominally zero and both pins sit at the common voltage of approx. 8.4 volts at 20 centigrade. As the input signal increases from zero the voltage on pin 11 moves proportionally more positive, that on pin 10 proportionally more negative, with respect to the standing bias voltage, as described above.

In the present case this differential voltage is converted into a single ended voltage with respect to a selected datum level, whilst not loading the pins unduly and without responding to the common mode voltage on the pins which changes with temperature. This is achieved by use of a form of differential amplifier. An example is shown in Fig. 3 where lines 40, 41 are respectively connected to pins 11, 10. The resistors are all 100K ohms. In this case the datum or reference level has been made chassis 42, the differential voltage between pins 10 and 11 being transiated into a single ended voltage with respect to chassis. The voltage at the output point X is nominally zero with respect to the datum when there is no signal input S, point X becoming proportionally more positive as the input signal S increases. The operational amplifier is type 324.

The voltage output component between pins 10, 11 is measured by reference to a datum level, for example a zero voltage rail (ground, earth or chassis).

Embodiments of the invention may also be used where the input signal S is frequency modulated at an audio rate.

One example is shown in Figs. 6 and 7, again using the detector 39 but with different external components.

The input signal S Fig. 7 is fed to the limiting amplifier LA and then, via a frequency-sensitive network 60, to the modulator M to provide a frequency sensitive output signal in known manner.

The differential voltage on pins 1 0 and 11 is in this case zero when the input signal is correctly tuned to the centre of the FM detector characteristic. In the event of the input signal S not being correctly centred, the differential voltage on pins 10 and 11 will swing one way or the other depending on the input signal being either high or low in frequency.

Fig. 6 shows a form of differential amplifier which will provide the desired output voltage.

Lines 50, 51 are respectively connected to pins 11, 10. The supply rail for example 1 2V is indicated at 49. Resistors 52, 53 are 1 K ohms, resistors 54-57 are 100K. The output voltage Y of the differential amplifier is expected to swing both positive and negative about its standing value, the datum level at E.

In the present case its reference at E is arranged to be half the supply voltage by connecting two equal resistors 52, 53 across the supply. The output voltage at point Y is thus nominally half the supply when the input signal S is correctly tuned, swinging positive or negative from this level as the signal S deviates either side of the correct tuning.

A centre-zero tuning meter 58 (and/or frequency correction systems) can thus be driven by utilising the single ended output voltage at point Y as shown in Fig. 6 where resistors R, provide the datum voltage level at point L equal to the voltage at reference level E.

The differential amplifier shown in Fig. 6 is identical in operation to that shown in Fig. 3; the datum level cannot sensibly be zero, and is made half supply voltage, when driving a centre-zero meter.

Claims (11)

1. Electronic apparatus comprising means responsive to an RF input signal to provide a differential output voltage dependent on an input signal characteristic, and means for converting said differential voltage to a singleended voltage with respect to a datum voltage.

2. Electronic apparatus as claimed in Claim 1, in which the responsive means is responsive to the magnitude of the input signal.

3. Electronic apparatus as claimed in Claim 1 or Claim 2, in which the responsive means comprises first means for receiving said input signal and providing an output signal of uniform amplitude, second means for receiving the input signal and said output signal and providing said differential output voltage.

4. Electronic apparatus as claimed in Claim 3, in which the first receiving means comprises a limiting amplifier.

5. Electronic apparatus as claimed in Claim 3, or Claim 4, in which the second receiving means comprises a double-balanced modulator.

6. Electronic apparatus as claimed in Claim 1 or Claim 2 or Claim 3, in which the responsive means comprises an AM detector including a limiting amplifier and a doublebalanced modulator, the limiting amplifier output and the input signal being respectively applied to inputs of the modulator, the arrangement being such that the modulator output voltage is proportional to the magnitude od the input signal.

7. Electronic apparatus as claimed in Claim 1, in which the responsive means is responsive to the frequency of the input signal.

8. Electronic apparatus as claimed in Claim 1 or Claim 7, in which the responsive means comprises an FM detector including a limiting amplifier and a double-balanced modulator, a frequency-responsive circuit, the input signal being applied to the limiting amplifier and to the circuit, the outputs of the limiting amplifier and the circuit being respectively applied to the inputs of the doublebalanced modulator, the modulator output voltage being proportional to the magnitude of the deviation in frequency of the input signal from a datum.

9. Electronic apparatus as claimed in any preceding claim, in which the converting means comprises a differential amplifier arranged to receive said differential voltage.

10. Electronic apparatus substantially as hereinbefore described with reference to and as shown in Fig. 1, or Fig. 1 as modified by Fig. 6, of the accompanying drawings.

11. A method comprising obtaining a differential output voltage, dependent on an input signal characteristic, from an RF input signal and converting said differential voltage to a single ended voltage with respect to a datum voltage.

1 2. A method as claimed in Claim 11 and substantially as hereinbefore described.