The circuit generates low-level rf bursts having frequencies as high as 10 MHz, thus permitting field testing of high frequency receivers. A jumper-selectable binary fraction (liz to `1256 of the Y1 crystal frequency gates the output rf signal. Output amplitude (open circuit) is approximately 100 mV; output impedance is approximately 49 0. The rf source is a clock oscillator based on a 3.58-MHz, color-burst crystal and two inverting buffers. The oscillator drives two cascaded 4-bit binary counters, IC2, and the sync signal resets the counters with a logic-high pulse-logic low at the counters.
Select the desired output frequency by adding a jumper to one of the counter"s eight output lines, which provides an enable signaHor the two 3-state output buffers. The square-wave output at IC1, pin 11, is attenuated by R1, R2, and R3 to fix the output resistance at approximately 49 0. Resistor R3 is the only critical component; for clean gating, isolate it from the rest of the circuit.