SEA 222 Operator's Manual page 41

which protects the following circuitry from intermodulation problems.
The filtered 45 MHz I.F. Signal is then applied to the first bilateral
I.F. Amplifier, Ul.
After amplification in Ul, the 45 MHz I.F. Signal is .combined in A2
with a 38.6 MHz signal from the VCXO, down converted tp the 6.4 MHz
second I.F. Frequency and passed through F2, a narrow band signal
sideband filter. The filtered s i gna 1 is then amp 1 ifi ed in bi 1atera1
I.F. Amplifier, U2, and combined with the 6.4 MHz BFO frequency in
mixer A3. The output from A3 is a low level audio signal which is then
amplified by the low noise audio preamplifier, U3.
Note that the 6.4 MHz filter selects the LOWER sideband. This is
actually the UPPER sideband because of the frequency inversion which
occurred at the first mixer.
After being amplified to a suitable level by the low noise pre-
amplifier stage, the audio signal is then sent to three different
circuits: The squelch limiter/amplifier, U7, provides a hard limited
signal to the CPU. This signal is examined by the squelch software
routine which determines the presence of voice signals.
The 1 ow 1eve1 audio is a 1 so sent to the audio derived AGC circuitry
which consists of USR and USC. These two operational amplifiers
provide amplification and filtering to the audio signal which is then
rectified by CR15. The diode output is a fast attack, slow release AGC
voltage which is then applied to the AGC control buss of the I.F.
Amplifier stages through USO.
The third low level audio signal is sent to the signal audio pre-
amplifier/filter, USA, where it is approximately doubled in amplitude
and band limited to a cutoff frequency of about 3000 Hz.
After being amplified and filtered, the receiver audio is then passed
on to the front panel volume control where the operator can control the
audio level applied to the input of the power amplifier stage, U6. The
output from U6 provides the loudspeaker signal.
5.3 RECEIVE RF CIRCUITRY AND FIRST MIXER
As previously discussed, an incoming signal is first passed through
a system of cascaded low and high pass filters, a T/R relay and a
P.I.N. Diode device which is designed to protect the sensitive input
mixer from damage due to high RF overloads. On the RF board, switching
diodes CRS and CR6 are foward biased by the +lOV Rx buss, thus passing
the received signal to the double balanced mixer Al. The use of a hot
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