Ramsey Electronics COM 3 - Page 25

reason for the external transistor stages is that the
mixer within the 3359 is contained within the AGC
loop. This means that the mixer must be operated
over a linear region in order that the AGC loop and
AM detector operate properly. After the AM detector
is driven, the IF signal can be limited and applied back
into the 3359 chip for quadrature detection and
processing. Audio output from the MC3359 is
amplified by op-amp U20 and applied to analog
switch U21. Squelch action is also provided in the
MC3359, the amount of squelch is set by resistor
R134. Note that the squelch output, on pin 16,
controls the analog switch U 21.
Even when the receiver is in the AM mode,
the squelch in the FM receiver MC3359 is used. In a
FM detector, even though it is receiving AM, the
signal still quiets and therefore the squelch still works
properly whether an AM or FM signal is being
received.
The AM detector consists of diode D 31 and
op-amp U 20. The 455 KHz filtered signal is
envelope detected by D 31 and amplified by U 20
where the demodulated signal is available at pin 8.
Another section of U 20 amplifies the filtered DC
component of the demodulated signal and drives the
AGC amplifier U 19, which provides an AGC range of . .
over 60 dB. Appropriate switching of the audio signal
between the AM and FM sections and the squelch
action, is provided by analog switch U20.
When the Com 3 is in the receive mode, the
microprocessor is instructed to offset the oscillators ©
by 9.5 MHz, which is the IF. It does this by offsetting
the coarse loop and the fine loop at the same time.
5.11 16 KHz REFERENCE GENERATOR
The 16 KHz reference which is used:
throughout the Com 3 is derived from counter U9.
ሀ9 takes the 4 MHz signal from the crystal oscillator
and divides by 250. U 9 is run on 12 volts because
its specifications do not allow it to divide properly on
a5 volt supply at a frequency as high as 4.0 MHz.
5.12 FREQUENCY ERROR DETECTOR
The frequency error detector is novel in that
it is completely temperature stable, not relying upon
a quadrature detector, pulse counter, or other
analog system, it is a true digital counting system
which indicates the frequency error. J
The converted 455 KHz signal from the
receiver section of the Com 3 is -amplified and
applied to an eight bit binary counter whose output
goes to an R-2R resistor ladder network. The output
of the resistor ladder network goes into a sample
and hold gate/buffer, then to an operational amplifier
Stage. The op-amp is used to properly center the
output voltage and provide a gain factor to give a
precise voltage/KHz output for frequency error
input. The eight bit counter is clocked by the 455
KHz signal from the IF and is gated with another
counter. This counter takes the 16 KHz input used
within the service monitor, and divides it by 64. The
output is a 250 Hz square wave which gates the
counter for a period of 4 milliseconds. At the end of
that 4 millisecond period, the sample and hold gate
is closed, sending the output of the resistor ladder
network to the op-amp, and then on to the bargraph
display. After the sample and hold gate is closed,
both the counters are reset again and the same
process is allowed to continue. In this way, if the
455 KHz signal is a contstant frequency, there will
be a constant voltage at thé output of the resistor
ladder network at the end of each count period, and
therefore, a constant output voltage as frequency
error. However, if the 455 KHz signal is somewhat
higher in frequency, the eight bit counter will count
up to | a higher output number and the resistor
ladder network will produce a higher output voltage.
This will reflect in the frequency error meter output.
If the 455 KHz signal is lower in frequency, the
eight bit counter will produce a lower number and
the resistor ladder network will produce a lower
voltage. Thus, a lower output voltage' for the
frequency meter error output. Note that in this
circuit there is nothing dependent upon
temperature or component tolerances, it is a
" completely digital system.
5.13 AUDIO AMPLIFIER
The audio amplifier is a standard LM 380/2
watt audio amplifier, it performs basic audio
amplifiication functions such as speaker driver for
receiver mode, and speaker driver for modulation
mode to enable you to hear what modulation is
taking place.