RADIO FREQUENCY DETECTOR

Abstract: A method of detection of radio frequency signals wherein the radio frequency signal is fed to a null offset terminal (1) of an integrated circuit (I.C.) operational amplifier, and the inverting input terminal (2) of the I.C. is maintained at a potential a predetermined proportion of the potential difference between the output terminal (6) of the I.C. and a reference potential.

Description: RADIO FREQUENCY DETECTOR Technical Field This invention relates to a method for detecting radio frequency signals and is of particular use when it is desired to detect radio carrier wave signals over a broad band or spectrum of radio frequencies. By use of the method radio frequency signals which are also modulated at an audio frequency may be both demodulated and amplified by simple and relatively inexpensive means, As herein used the term "radio frequency" (r.f.) means a frequency in the portion of the electromagnetic spectrum between audio frequency and the infrared portion of the spectrum typically between 10 KHz and 100,000 Mhz. Background Art The invention is based on the discovery that an r.f. carrier signal fed to the offset null terminal of an integrated circuit (I .e . ) operational amplifier powered by a power supply produces a signal at the operational amplifier output indicative of the r.f. signal strength. Integrated circuit operational amplifiers are amplifiers having an inverting input terminal, a non-inverting input terminal, and an output terminal and are intended for use to compare a signal input at one of the input terminals with another signal (or a reference signal) fed to the other input terminal to produce an output signal responsive to a difference in the input signals-. In a typical use the operational amplifier functions as a comparator. For example, a reference voltage is applied at the inverting input and an analogue signal is supplied to the non-inverting input terminal, the output indicating whether or not the analogue signal is, for example, positive with respect •to the reference. While an ideal operational amplifier would have zero output voltage for zero input at the input terminals there is in practice alway some output voltage at no input which is called an offset. Offset changes with temperature, time and supply voltage producing drifts. Offset null terminals are provided to enable drift compensatory adjustment of the operational amplifier, or to enable precise adjustment of a reference voltage applied at one of the inverting or non-inverting input terminals. Disclosure of the Invention According to one aspect the invention consists in a method for detecting the amplitude of a signal alternating at a radio frequency, comprising the steps of: a) selecting an integrated circuit operational amplifier of the type comprising an inverting input terminal, a non-inverting input terminal, an output terminal and having an offset null terminal which is connected to the base emitter junction of a transistor, b) injecting the signal into said base emitter, and c) maintaining the non-inverting input terminal at a potential which is a predetermined proportion of a potential difference between the output terminal potential and a reference potential whereby to obtain at said output terminal a signal indicative of the amplitude of said injected signal. According to a second aspect the invention consists in a method according to the first aspect wherein the signal injected is modulated at an audio freguency whereby to obtain at said output a signal having a frequency corresponding to said audio frequency. Brief description of the drawings Fig. 1 is a schematic diagram useful for explaining the invention. By way of example only, the invention will now be described with reference to its use in radar detecting apparatus. Such apparatus typically comprises a microwave receiving transducer including a local heterodyne oscillator for producing an r.f. signal of an intermediate frequency in response to reception by the transducer of microwaves in a band to which the transducer is tuned. The amplitude of the intermediate frequency signal varies in response to received microwave signal strength. Hitherto, the intermediate frequency r.f. signal has typically then been modulated at an audio frequency and passed to an r.f. frequency preamplifier, prior to feeding the preamplified signal into a diode detector which demodulates the signal to produce audio frequency pulses. The audio frequency pulse signal is then amplified by an audio frequency amplifier and fed to circuit means for providing an audible or visable signal indicative of r.adar reception by the transducer. Diode rectifiers used to demodulate the signal have resulted in poorly shaped noisy signals which are detrimental to sensitivity in subsequent stages. In apparatus using the method of the present invention the r.f. preamplifier is for example a type commonly used as a video " amplifier, having an intermediate frequency band width of approx. 2-60 MHz with an amplification of about 400 V/V. The preamplified signal comprising noise or an r.f. signal is chopped by means of an F.E.T. switch at a frequency of for example 3 KHz. The r.f. signal from the r.f. preamplifier so modulated at 3 KHz is fed to the compensation null terminal 1 (Fig. 1) of a LM741 series integrated circuit operational amplifier. LM741 series operational amplifiers are general purpose operational - amplifiers available from National Semiconductors and BUREA O PI are direct plug in replacements for LM709 (or equivalent) i.C.'s and are well known to those skilled in the art. The LM741 operational amplifier comprises a non-inverting input terminal 3 which is connected internal of the I.C. package to the base of one differential transistor and has an inverting input, terminal 2 which is connected internally of the package to the base of another differential input transistor. Each differential input transistor in turn has a collector load constant current source which comprises a transistor the base emitter of which is connected internally of the I.C. to an externally extending offset null terminal 1 or 5. The internal schematic and connection diagrams of operational amplifiers of the LM741 type operational amplifier have been published by the manufacturers and are herein incorporated by reference. Other types of operational amplifier I.C. having an offset null terminal connected to a base emitter transistor junction may be used. In the present example the I.C. is supplied with power from an external source via terminals 7 and 4 in the usual way but the non-inverting input 3 is connected to earth potential and the inverting input terminal 2 is connected to the junction of two resistors l and R2 forming a potential divider. One of the resistors, Rl, is connected between the OMPI operational amplifier output terminal 6 and the inverting input terminal 2 and is for example 9,000 ohms, the other resistor, R2, is for example 1000 ohms and is connected between the inverting input terminal 2 and earth potential. .A signal is produced at the output terminal 6 of the operational amplifier which is both demodulated and amplified, that is to say in the present example there is produced a signal which is a square wave signal of frequency corresponding to the chopping frequency and which is substantially free of r.f. components. The signal has an amplitude which varies with the. amplitude of the r.f. signal received. If two offset null terminals are provided on the operational amplifier I.C. as in the type shown in fig. 1 it is possible to select the phase on which the detector operates, by selection of offset null terminal 1 or 5 into which to inject the r.f. signal. o There is a 180 phase difference such that an r.f. signal on one offset null terminal drives the I.C. output terminal high while the other drives the ouput low. It is thus also possible to use differential inputs to mix and process normal input audio signals simultaneously with the r.f. Industrial Applicability As will be recognized by those skilled in the art, the use of a single I.C. component to function both as a detector and amplifier is very convenient for many uses. "BURE OMPI In other uses, an aerial may be used to receive signals and/or tuning means may if desired be employed to narrow the frequency band of a r.f. signal prior to injection to an offset null terminal. Although earth potential has been used as a reference potential .in the example described that is not essential. The amplification may be varied by varying the values of the resistance dividing network so as to alter the potential at the inverting input terminal relative to reference potential and to the output terminal.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. A method for detecting the amplitude of a signal having a voltage alternating at a radio frequency, comprising the steps of: a) selecting an integrated circuit operational amplifier of the type comprising an inverting input terminal, a non-inverting input terminal, an output terminal, and comprising an offset null terminal which is connected to the base emitter junction of a transistor. b) injecting the signal into the base emitter of said transistor. c) maintaining the non-inverting input terminal at a potential which is a predetermined proportion of a . potential difference between the output terminal and a reference potential whereby to obtain at said output terminal an output signal indicative of the amplitude of said injected signal. 2. A method according to claim 1 wherein each of said input terminals is connected with the base junction of a respective differential input transistor, at least one of said differential input transistors has a collector load constant current source comprising a transistor and wherein said offset null terminal is connected to the base emitter of said constant current source transistor. 3. A method according to claim 2 wherein said operational amplifier is a type LM741 or substantial equivalent. 4. A method according to any one of the preceding claims wherein said proportion is selected so that variations in amplitude of the input signal are amplified in the output signal. 5. A method according to any one of the preceding claims wherein the -input signal is chopped at an audio frequency. 6. A method according to any one of the preceding claims wherein the non-inverting input is maintained at said reference potential.

Inventors: FREW W, GREEN D
Applicants: FREW W, GREEN D, MARK II ENTERPRISES PTY LTD
Application year: 1980
Main classification: H03D1
IPC codes: H03D1