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).
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.
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 0.005µF coupling capacitor
and a 1 megohm grid leak would be suitable for a single stage
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.
H. V. BECK.