TM 11-6625-486-14&P
will also conduct but will not affect the useful output of vfo
(2) Low frequency mixing. Transistor Q301 is a
signal.
low frequency mixer and high-gain audio preamplifier.
The base of Q301 is forward-biased through voltage
divider R307, R303, and R304.
With the emitter
(3) Crystal Oscillator signal. During the crystal
grounded, Q301 operates like a forward-biased diode
calibrate or CHK function, the signal is injected at the
followed by a direct-coupled amplifier. A negative going
junction of CR302 and L302. These two components
input signal will cause a positive going output signal to
function as a ringing circuit for generating either a 1-MHz
appear at collector load resistor R305. Low frequency
or 2.5-MHz harmonic spectrum. Diode CR302 conducts
heterodyning in Q301 and diode rectification action
and introduces positive going pulses into the mixer
develops rf, af, and dc signals at the collector. The audio
network. When calibrating the high frequency vfo, the
signal fed to the main audio amplifier is recovered by
positive half-cycle of oscillator signal will add to a similar
coupling capacitor C304 and audio choke L303 which, in
polarity of vfo signal and cancel an opposing polarity.
addition to removing the dc component, presents a high
When calibrating the low frequency vfo, the oscillator
impedance to rf while still allowing audio signals to pass
signal will act in the same manner as for the high
through. Low frequency rf signals, appearing at the
frequency vfo signal. The mixing of vfo and crystal
collector of Q301, are fed back to the ANT. jack through
oscillator signals produces a sufficient number of crystal
C308, L307, L306, and L305 for the purpose of reinforcing
checkpoints for ranges B and C without using an external
harmonic frequencies developed by the low frequency vfo
source of diode biasing voltage for CR302; however, on
on range A. The impedance of this network is such that
range A, it was necessary in the AN/USM-159 to use
the desired frequencies will pass from Q301 to the ANT.
some bias. This is accomplished by taking negative
jack but neither range B or C vfo frequencies will pass in
voltage (0.08 volt) from the crystal oscillator circuit and
the reverse direction. Although L305 and L307 appear to
feeding it through S703, R308, and L301, to the cathode
be iron-core chokes, they are not chokes in the ordinary
of CR302. This voltage causes CR302 to generate more
sense. Each inductor is not a coil but a short length of
harmonics. Although diode CR301 has a tendency to
wire (lead of L306) inside a piece of ferrite material which
oppose the crystal oscillator signal, higher order
contributes inductance at certain frequencies.
For
harmonics will pass through the stray capacitance of
passage of desired frequencies, the inductors present no
CR301 and be radiated from the meter antenna the same
appreciable rf resistance; whereas, at higher frequencies,
as a vfo signal.
the inductors behave as iron-core rf chokes and present
maximum rf reactance.
b. Mixer Functions.
(1) High frequency mixing. The high frequency
c. Overload Protection. The network consisting of
mixing of signals in the diode network produces complex
C309, CR305, and CR306 in the AN/USM-159A and
waveforms containing the fundamental frequencies, plus
AN/USM-159B (fig. 5-9) acts to prevent damage to mixer
their sum and difference frequencies, plus harmonics.
diode CR301 and other components in the mixer circuit if
During frequency measurement, the unknown input signal
high amplitude transients are accidentally applied to ANT.
is applied through either C301 or C306 and CR301 where
jack J703. C309 blocks dc, but passes rf components.
the signal heterodynes against the internal vfo signal and
CR305 and CR306, connected parallel with opposing
produces a difference frequency within the audio range.
polarity, act as shunting resistances to transients of either
polarity when the applied amplitude exceeds the diode
signal as well as an outgoing vfo signal, a number of
forward breakdown voltage of approximately 0.7 volt.
other frequencies are also developed. In the diode mixing
Signals in the normal amplitude range of the mixer are
network, all high-order harmonics are removed by low-
not affected since the resistance of both diodes is high
pass filter L304 and C302.
The remaining low
when the applied voltage remains below the forward
frequencies (including audio) are applied to the base of
breakdown voltage.
Q301, a low frequency mixer.
5-11
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