Frequency Meter FR-4/U

One of the latest and most accurate analog frequency meter, designed to measure frequencies in the range from 100 kHz to 20 MHz over seven bands. 0.001 percent accuracy, calibrated against an internal thermostated crystal reference.

Depending upon the frequency the set operates as heterodyne direct or indirect frequency meter. in the first case the unknown frequency beats with one harmonic of the 10 kHz internal reference. The mixer output in the 15 to 20 kHz band is the compared on the CRT display against the frequency of an internal 15 to 20 kHz interpolation oscillator. In the indirect mode, the unknown frequency beats with the frequency, or its harmonics, generated by a proxy oscillator, continuously compared against the harmonics of the internal reference on the CRT display.

Quite remarkable the analog division chain used to generate the 10 kHz reference starting from the thermostated 1250 kHz crystal quartz. It uses three divide-by five stages, each one based upon a closed loop containing a mixer feed by the input signal from one side, a tank circuit tuned at one fifth of the input frequency at its output and a multiplier by four feeding the other side of the mixer.

In questo link sono elencati 398 modelli, di cui 350 con immagini e 210 con schemi.
Elenco delle radio e altri apparecchi della MILITARY U.S. (different makers for same model)