TM 11-6625-486-14&P
cause some negative dc rectification of vfo signal with a
5-5. Mixer Circuit
tendency to produce additional harmonics, these affects
are small in comparison with the stray capacitance of
CR301 which is sufficient to pass high frequency rf.
The mixer circuit performs the heterodyning function and
Resistors R301 and R302 provide constant input and
generates harmonics for the crystal oscillator and the
output impedances for the meter overall frequency
variable frequency oscillators. Crystal diodes generate
ranges. Capacitor C301 couples high frequencies and
harmonics and a transistor functions as a mixer and audio
C306 serves a similar function for lower frequencies. The
preamplifier. During operation of the meter, the mixer
circuit including L305, L306, L307, and C308 does not
circuit receives dc power continuously.
influence high frequency output signals appearing at the
ANT jack (b (2) below).
a. Harmonic Generator Function.
(2) Low frequency vfo signal.
The low
(1) High frequency vfo signal. The vfo output
frequency vfo develops strong fundamental frequencies
signal is composed of strong fundamental frequencies
and some high order harmonics of lesser amplitudes.
and some higher order harmonics of lesser amplitudes.
The desired harmonics up to 65 megahertz are produced
To generate sufficient harmonics up to 1,000 megahertz,
by harmonic generating diode CR301. The vfo signal,
the vfo output signal is squared. This is accomplished by
applied through isolating coil L301, causes the diode to
diodes CR303 and CR304 which clip the negative and
conduct on the negative going portion of signal and
positive portions of vfo signal. Diode CR303 clips the
produce harmonics (the nonlinear characteristics of a
negative going portion and diode CR304 clips the positive
diode produce harmonics).
At the fundamental
portion. The square wave, rich in harmonics up through
frequencies, very little signal will pass through the stray
the tenth harmonic, passes through C307 to the mixing
capacitance of CR301 or R302. As a result, the output of
network. Capacitor C307 removes the dc component.
CR301 will constitute a pulsating dc signal rich in
The rf portion of vfo signal passes through the stray
harmonics. The ac component of the signal passes
capacitance associated with CR301 and is radiated out of
through C301 or C306 to the ANT. jack. Diode CR301
the ANT. jack through C301. Although diode CR301 may
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