Ramsey Electronics COM 3 - Page 23

supply bus of the Com 3, and the other is to prevent
any references that are used within the 1024 MHz
loop from leaking back out onto the power supply
line bus.
The 1024 MHz Offset oscillator is switched in
and out of the circuit using PIN diodes CR24 through
CR27. When these diodes are turned on, the output
frequency from the coarse loop (506 - 1017 MHz ) is
applied to mixer X4 along with the 1024 MHz
oscillator. The difference frequency goes into a 600
MHz low pass filter and a hybrid broadband amplitier
which provides approximately 25 dB of gain. There
its output is applied to another 600 MHz low pass
filter and then switched back to the output through
the PIN diodes. When the pin diodes are turned off,
the signal simply bypasses around the mixer X4, and
the 25 dB amplifier.
To prevent spurrious signals from being
generated, the 1024 MHz oscillator is simply
switched off when it is not needed. This is
acomplished by diode D29, and resistor R208 which
simply supply a negative 10 volt bias to the base of
the 1024 MHz VCO transistor Q5. Op-amp U 17
drives the PIN diode switches and the on/off control
for the VCO transistor Q5. This device is driven by
one of. the extra available data outputs on the
synthesizer chip U14.
5.5 Output Step Attenuator
The output step attenuator goes from 0 - 80
dB in 20 dB steps. It consists of two 20 dB sections,
and one 40 dB section. Each section can be
switched in or out of the circuit through the use of
PIN diodes. For 0 dB attenuation all attenuators are
bypassed, for 20 dB attenuation only one 20 dB
attenuator is in the circuit, for.40 dB attenuation the
40 dB attenuator is.switched in, for 60 dB a 20 and
the 40 dB attenuator is switched in, for 80 dB all.
attenuators are switched in.
The PIN diodes are driven off of op-amp U2
which is a 4 section device. The attenuator is turned
on by a high level, and turned off by a low level. In -
this circuit a high level is +12 volts, and a low level is -
10 volts. The attenuator operates over the
frequency range of 10 MHz to 1 GHz, and flat to
within approximately +1.5 dB. The attenuator is
composed of surface mounted capacitors and
surface mounted resistors. NE
U2, the driver for the attenuators, is
controlled by the additional outputs of the
synthesizer ICs . The synthiszer ICs have available
extra outputs that are not in use within the
synthesizer, but which are decoded . In this case
the attenuators are controlled by the output of the
microprocessor, the data is fed along the same line
to the synthesizer chip, the synthesizer simply
ignores that data and controls the output pins
which drive the comparators:of U2.
5.6 4 MHz Reference Standard
The 4 Mhz crystal that is used in the crystal
oscillator is a small HC-18U type. Each individual
crystal has been tested and data has been taken to
plot out its charateristic, curve with respect to
temperature. Then appropriate temperature
compensating capacitors are matched with the
crystal. After that has been acomplished the crystal
- is encased in a very small micropowered oven. The
crystal is wound with several turns of nichrome wire,
. and a darlington power transistor and thermistor are
put in contact with the crystal. The entire assembly is
then potted with thermally conductive epoxy and
enclosed in a styrofoam shell to insulate it from
external temperature changes. The crystal, with the
nichrome wire wrapped around it, and the transistor
all in close proximity makes for a very small
micropower oven which has very low temperature
inertia. This allows it to warm up quickly, and not be
affected by external temperature variations. The
crystal oscillator is composed of transitor Q15, and
related parts with buffer stage Q14 following it . The
output of the crystal oscillator is 4 MHz with a level of
approximately 0-4 vpp. The 4 Mhz is used
throughout the service monitor and all frequencies
generated and received. within the service monitor
are referenced back to the 4 MHz standard. The
high stability timebase option utilizes similiar circuitry.
The crystal used in the high stability timebase is
much more closely tested, and tolerances on the
temperature compensating components and the
oven components are much more closely held
which allows an increase in stabilty (0.5 ppm).