Letter to the Editor, Electronic Engineering, July? 1955, p405





A Simple Frequency Comparison Circuit

Dear Sir - I was very interested to read the letter by M. G. Beauchamp (May issue) describing his frequency comparison circuit.  I have been using a " Magic Eye " as a beat indicator and can confirm that for such a simple and inexpensive circuit the results are very good, e.g., a frequency difference of 0.05c/s is easily distinguished, a figure which compares favourably with the 0.025c/s accuracy given for a more elaborate C.R.T. method (W. S. Wood. "Engineering", Vol. 171, p.216).

The circuit shown below was designed for adjusting to equality frequencies from 25c/s upwards and differs from that of Mr. Beauchamp in that the input signals are mixed externally in the common cathode load of V1 and V2, and the resultant beat signal is fed to the indicator via a low pass filter.  One advantage of this method is that for any input frequency, only the low beat frequency is fed to the indicator so that the indicator may be located some distance from the instrument with little alteration of performance.

The circuit shown below is suitable for input signals greater than 2V, but where greater sensitivity is required V1 and V2 may be reconnected as amplifiers with common anode load. With this arrangement, however, a stabilized H.T. supply is usually necessary since slow variations in H.T. voltage are passed with little attenuation to the indicator and give rise to spurious deflexions.  An alternative method, for which a stabilized supply is not required, is to amplify the signals and couple to VI and V2 via high-pass filters, e.g.. a 0.005F coupling capacitor and a 1 megohm grid leak would be suitable for a single stage pre-amplifier.

If, in addition to adjusting frequencies to equality, it is required to compare different frequencies (i.e., n is not equal to unity) it would be an advantage to add a distorting circuit such as that described by Mr. Beauchamp.

Yours faithfully,



A Simple Frequency Comparison Circuit, as described in Mr. Beck's letter