Manufactured in the 1940s for military purposes, the BC-221 frequency meter used technology rarely seen today to measure radio frequencies. The operator could either plug it in or use it with internal batteries, but the batteries produced enough voltage to cause a severe shock to the user, and it could not come in physical contact with the equipment it measured. The BC-211 came in either an aluminium alloy case or a finished wooden box.
Called "SCR-211 sets" when including an antenna and headset, the BC-211 came in models with letter suffixes, usually indicating the manufacturer. Bendix corporation manufactured the "A" model, and Philco built the "E" model, with other manufacturers also contributing. Although the circuits were basically the same in all models, the dimensions of the case, the arrangement of the controls, the part numbers and locations of the components, and their identification numbers in the circuit diagrams differed between models.
Rather than connect a signal to the meter to "count" the frequency as in today's equipment, the BC-221 used an antenna to pick the signal out of the air. The operator then tuned internal signal generators to create a similar signal, while monitoring an audible tone on a headset. When the created signal approached the same frequency as the measured, the audible tone dropped in frequency, and it became inaudible when the two were the same. The operator then read the frequency from the markings on the dials.
Frequency and Power Range
An internal 1MHz crystal oscillator, along with two variable oscillators with ranges of 125 kHz to 250 kHz and 2MHz to 4MHz, allowed the BC-211 to measure frequencies up to 20MHz, per its design specifications. Because of the many additional internal harmonics it generated, it could actually measure frequencies well above that, but using an antenna for the input signal limited the power range of what it could measure to only what the antenna could detect.
The internal oscillators could only be as stable as their components, and the value of other components had tolerances. With extensive procedures for calibrating and tuning, the accuracy was the best available for the time, but would be questionable by today's more rigid standards. The manual does not show any accuracy specifications.
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