a. An unknown signal (F) is fed into a harmonic mixer.
A swept oscillator
input signal.
A 1-MHz signal from the counter time base is used as a phase
detector and locking frequency for the output signal of the mixer.
This circuit
phase-locks the proper harmonic (N) of the swept oscillator signal (f) with the
input signal (F) at a precise 1-MHz offset.
b. The input signal (F) is also applied to a second mixer which has the same
swept oscillator signal (f) but with the addition of a 1-kHz signal from the
counter.
When the phase detector locks the swept oscillator, the signal in this
second mixer will be N (f=1kHz) which is mixed with the input frequency (f).
c. When the offset is exactly 1 MHz, then F=fN +/-1 MHz. The output of the
second mixer is 1 MHz plus N kHz. The 1-MHz signal from the counter is mixed with
the output of the second mixer and the resultant signal is N kHz. The mixed signal
(N kHz) is then applied to a digital N computer circuit along with the 1-kHz signal
from the counter.
d. This circuit then divides the N kHz by 1 kHz and the result is N pulses.
The N pulses are gated in conjunction with the counter time base signal.
This
extension multiplies fxN +/-1 MHz so the counter can read out the unknown input
frequency directly with the 1-MHz offset.
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