Patent GB-697457-A (GB)
Title: Improvements relating to apparatus for indicating the amplitude or phase versus frequency characteristic of an electrical network
Inventor: LEYTON ERIC MCPHAIL
IPC: G01R27/32
Filed: 1950-04-01
Abstract: 697,457. Determining phase/frequency or amplitude/frequency of an amplitude :modulated transmitter. ELECTRIC & MUSICAL INDUSTRIES, Ltd. March 27, 1951 [April 1, 1950], No. 8306/50. Class.37. To determine the amplitude/frequency characteristic of an amplitude modulated transmitter 1, Fig. 1, its carrier wave is amplitude modulated with oscillations of a first cyclically variable frequency, the modulated carrier being mixed with second oscillations of a variable frequency the frequency of which differs from that of the first variable oscillation by the carrier and a fixed frequency so as to obtain oscillations of a fixed frequency whoseamplitude deflects a cathode-ray beam in one direction, the deflection in the other direction being in accordance with the frequency of the first oscillations. An oscillator 2, Fig. 1, stabilized in amplitude by a feed-back rectifying circuit 13, produces oscillations varying cyclically from Fc + Fd - Fm to Fc + Fd + Fm, where Fc is the carrier frequency of the transmitter 1, Fm is the variable modulation frequency and Fd is a small fraction of the maximum value of Fm. The output from 2 is mixed at 3 with oscillations of frequency Fc +Fd from 4 maintained at constant amplitude by a feed-back circuit 5, 6. The resulting oscillation Fm modulates Fc in 1, the output from which is mixed at 7 with the oscillations from 2 to produce the constant difference frequency Fd .which passes through a band-pass filter 8, is amplified.at 9, rectified at 10, amplified and fed to the plates of a cathoderay tube 12. The other plates of the tube are fed with a wave-form to give a deflection in accordance with the variation of oscillator 2. A marker corresponding to a known frequency is imposed on the cathode-ray tube trace by mixing the output of 3 and a variable oscillation from 17 in 16 to produce a difference frequency which when below a minimum value, i.e. when the frequency of oscillator 17 equals Fm will pass through filter 18 to produce a pulse for brightening or darkening the cathode-ray tube trace, so that a mark appears in every cycle-of variation of oscillator 2 at a point determined by the setting of oscillator 17. The transmitter may have its modulation equipment only tested by providing an artificial load and feeding the output with a carrier wave of frequency Fc from an independent oscillator to the mixer 7, Fig. 2 (not shown). 'Or, the radio-frequency part of the transmitter may be tested by feeding it with the varying frequency Fm and the carrier from an independent oscillator, Fig. 3 (not shown). To determine the phase/frequency characteristic the circuit of Fig. 4 is used. The output of filter 8 is applied to an amplifierlimiter 26 to give oscillations of frequency Fd of constant amplitude for application to mixer 27. The carrier frequency Fc from the oscillator in the transmitter is applied to mixer 28 together with the frequencies Fc and Fd from oscillator 4; so that a frequency Fd having a phase independent of change produced by the transmitter is produced for comparison in 27 with the frequency Fd delivered by amplifier 26. The resulting voltage from 27 is proportional to the cosine of the phase difference and is applied to the cathode-ray tube. The modulating and high-frequency amplifying stages of the transmitter may be tested separately as for the amplitude/frequency characteristics.