SEA 222 Operator's Manual page 48

5.4.5
5.4.6
5.4.7
is required to avoid intermodulation effects in later
circu~try.
The
output from the low pass filter is buffered by inverting amplifier U8D
and emitter follower Q2. The audio from Q2 is then passed through T/R
gate U4A and applied to the balanced modulator, A3. The gain of U8D is
preset to supply the correct modulating audio level to
A~
to insure
good carrier rejection and proper signal· level to A3 to insure good
carrier rejection and proper signal level through the I.F.· Amplifiers.
Note that adjustment of total transmitter gain is accomplished with
RlOl and NOT R80. R80 should be adjusted for 6 dB clipping as de-
scribed above. Increasing the audio level into the clipper will only
result in distortion and will NOT increase the peak output from the
transmitter.
TRANSMITTER BALANCED MODULATOR
Double balanced mixer, A3, is used as the transmitter balanced mod-
ulator. The processed microphone audio is applied to the OC coupled
port of A3, while the carrier signal at 6.4 MHz is applied to the L.O.
Port. The output from the R.F. Port is a double sideband suppressed
carrier signal which is then applied to the input of the 6.4 MHz
bilateral I.F. Amplifier.
CARRIER REINSERTION AMPLIFIER
Carrier is reinserted when desired through amplifier U32. This
integrated circuit is a voltage contolled attenuator, used here to
control the level of reinserted carrier. Normally in the A3J mode, the
control voltage port of U32 is held at +8 vol ts, which- cuts off any
output signal. When carrier is desired, the control computer switches
in a level control potentiometer which allows the output signal from
U32 to be adjusted to the desired level. Two control potentiometers
are provided: The A3A (-16dB) level pot, R29 and the A3H (-6dB) level
pot, R30. The reinserted carrier signal is routed around F2 and
applied to mixer A2.
6.4 MHz I.F. AMPLIFIER AND SSB FILTER
From A3, the 6.4 MHz double sideband suppressed carrier signal is
applied to the input of the bilateral 6.4 MHz I.F. Amplifier. This
amplifier and the ·45 MHz bilateral I.F. Amplifier, Ul, are both gain
controlled in the transmit mode by the ALC voltage derived from the
power amplifier.
The 6 .4 MHz crystal filter, F2, is a LOWER sideband filter with a
total passband of approximately 2.1 KHz. Passing the double sideband
suppressed carrier signal through this filter results in a LOWER
sideband 6.4 MHz, suppressed carrier signal which is then applied to
the up converter balanced modulator, A2.
32