Patent GB-955555-A (GB)
Title: Frequency modulation measurement
Inventor: NELSON BRUCE ELMER
IPC: G01R29/06
Filed: 1961-02-10
Abstract: 955, 555. Frequency analysis; voltage measurement VARIAN ASSOCIATES. Feb. 10, 1961, No. 5092/61. Heading G1U. The amplitude of noise or other modulation voltages producing frequency modulation of an R.F. generator 11 is measured by a frequency discriminator 20 whose output is then analysed as a spectrum of modulating frequencies by a wave analyser thereby giving the relative average power of the frequency modulating frequencies throughout the sideband spectrum of the R.F. generator. As shown, the R.F. generator 11 feeds a hybrid magic- T with one arm coupled to a cavity resonator 16 tunable to the frequency fo of the generator and another arm including an attenuator 17 and adjustable short 18 so that at frequency fo the output is zero but varies in magnitude in proportion to departures from fo and with phase reversal on opposite sides of fo. This is then reduced in frequency by a local oscillator 21, e.g. klystron, in abalanced modulator 19 and, after amplifying, passed to a phase sensitive amplitude detector 25 receiving, as other input, a carrier signal reduced to the same I.F. directly from the generator 11 and of equal or opposite phase as adjusted by a variable phase shifter 27. The output of detector 25 is therefore a signal varying in magnitude and polarity with the magnitude and sense of the instantaneous frequency deviation of generator 11. This is passed through a calibrated attenuator 37 and video amplifier 38 to a mixer 39 receiving a frequency adjustable in steps from a local oscillator 41 and with output passing through a narrow, e.g. 1 kc./s, pass-band filter 44 and amplifier to a voltmeter V so that the average amplitude or power of each 1 kc./s. section of the sideband spectrum of generator 11 due to frequency modulation by noise &c. may be detected in turn. Voltmeter V is calibrated by using an oscillator 51 of known frequency e.g. 20.79 kc./s. whose amplitude is adjusted by an attenuator 52 before frequency modulating the R.F. generator until the amplitude of the unmodulated carrier shown on an oscilloscope is zero, this arising when Jo(M F ) = 0 where M F  is the modulation index #f/fm, fm the known modulating frequency and Jo the zero order Bessel function i.e. when #f/fm = 2.405 or #f = 50 kc./s. for oscillator 51. By comparing the outputs of the wave analyser meter 48 corresponding to this calibration deviation of 50 kc./s. and the output due to noise, amplified as necessary by adjusting an attenuation calibrator 37 in series with the frequency discriminator output, a measure of the proportionate average frequency deviation due to frequency modulation by noise throughout each section of the sideband spectrum of the R.F. generator may be obtained and hence the corresponding R.M.S. noise power. This may be displayed on an oscilloscope 46 using a swept frequency local oscillator 65, in place of adjustable oscillator 41, with frequency swept by a sawtooth generator 56 supplying the timebase sweep of the oscilloscope. The spectral distribution of noise power due to frequency and amplitude modulation combined may also be displayed by taking the direct output of generator 11, reduced to an I.F. value, from I.F. amplifier 34 and mixing it at 54 with a local oscillator frequency swept by timebase sawtooth generator 56, the mixer output passing through narrow, e.g. 800 c./s,pass-band filter 58 to the oscilloscope Y-terminals. The noise spectrum due to amplitude modulation alone may also be detected at V or displayed by amplitude detecting the direct output of generator 11 in a detector 31 and passing it to calibrated attenuator 37 by switch 36. An oscillator 73 and meter 75 are used for calibrating the noise power due to amplitude modulation alone.