5-16
The first mixer is a conventional +7 dBm double balanced type, which provides
excellent dynamic range in this application.
5.5.3
THE 455 kHz IF AMPLIFIER
The first 455 kHz IF amplifier is Q10, a grounded-gate JFET which provides some
gain and serves as a wideband 50 ohm termination for the mixer. The output from
the amplifier is passed through FL3, a 6-pole ceramic bandpass filter with a nominal
bandwidth of 4 kHz. Output from the filter is then passed through emitter follower
Q11 to the second mixer.
5.5.4
THE SECOND MIXER/POST AMPLIFIER
The second mixer is a double balanced commutation mixer, which uses a quad
HCMOS analog gate (U16) as the switching element. Push-pull signal drive is
provided to the two switch arms through a wideband transformer. (Note that two
gate elements are used in each switch arm.) Push-pull local oscillator drive is
applied to the switch actuator pins from the output of the local oscillator divider
chain. Both output arms of the switch are summed together, resulting in a double-
balanced switching mixer with excellent dynamic range and good local oscillator
balance.
Output from the mixer is passed through a low pass filter consisting of L26 and
C107 to a low noise operational amplifier stage with gain of 10 and a phase splitter,
which, as in the Main Receiver, drives the A/D input.
5.5.5
THE LOCAL OSCILLATOR CIRCUITRY
Both conversion oscillator frequencies are derived from the same crystal controlled
source in the following fashion. Crystal oscillator U14 operates at 5285.0 kHz. This
oscillator is temperature compensated and maintains a frequency stability of less
than ±4 ppm over the voltage and temperature range of the equipment. Trimmer
capacitor C116 is used to set the oscillator frequency to exactly 5285.0 kHz at TP10.
Output from the oscillator is buffered by gate U13 and then divided to 2642.5 kHz
by U12A. This signal is sent to both the first mixer and the second local oscillator
divider chain. The difference frequency between the first local oscillator and the
2187.5 kHz signal frequency is 455 kHz.
U11, U12B and U10A form a symmetrical divide-by-three counter. The resulting
880.3333 kHz signal is then further divided by two in U10B. The Q and notQ
outputs of U10B are at 440.4167 kHz and, when mixed in MIX-4 with the 455 kHz
IF signal, results in the 14.5833 kHz last IF frequency which is amplified by U3 and
U5A/B and then sent through the A/D converter to the DSP for further filtering and
demodulation.
To Next Page