SEA 222 Operator's Manual page 49

5.4.8
5.4.9
5.4.10
5 .4 .11
THE UP CONVERTER, 45 MHz I.F. AMP AND FILTER
· The up converter mixer is . a double ba 1 anced di ode ring type mixer
with inputs from the 38.6 MHz VCXO and the 6.4 MHz I.F. Amplifier. The
mixer output is amplified by the bilateral 45 MHz I.F. Amplifier. The
operation of this amplifier is the same as the 6.4 MHz amplifier except
for frequency. The SUM of the two frequencies is selected by the 45
MHz I.F. Filter. Selecting the sum frequency results in a LOWER
sideband 45 MHz SSS signal.
The high impedance 45 MHz monolythic filter is matched to the low
impedance amplifier and mixer stages by low
Q
PI networks to provide
for a ripple free, low loss filter termination. The filter output is
applied to the input of the down converter or signal mixer.
SIGNAL MIXER AND LOW PASS FILTER
Al is another double balanced ring diode mixer. The output from
this stage is the down converted 45 MHz SSS signal. This signal has
been changed to the desired OPERATING frequency by mixing with a signal
from the VCO and the use of the DIFFERENCE frequency here results in a
frequency inversion. Thus the output signal is an UPPER sideband SSB
signal as required. The mixer output is passed through a seven section
elliptical function low pass filter which provides some 50
d~
of
rejection for the image and I.F. Frequencies above 23 MHz.
TRANSMITTER SIGNAL PREAMPLIFIER
The signal output from the low pass filter is switched through CR7 to
the input of the transmitter preamplifier. This is a two stage wide
band amplifier, with the first stage configured as a two transistor
direct coupled pair. The first transistor, Q3, is connected as a
corrunon emitter stage which is directly coupled to Q4. Q4 is an emitter
follower which provides power gain and impedance matching for the out-
put signal. DC feedback from the emitter of Q4 to the base of Q3 sta-
bilizes the circuit operating point over a wide temperature range, while
the use of VHF transistors provides a very wide frequency response.
Q5 is driven from the output of emitter follower Q4 and is configured
as a transformer coupled grounded emitter amplifier. DC feedback is
provided by the emitter resistor, R107. The output from this stage is
nominally +10 dBm. From JS, the SSB signal goes to the PA/Filter board
for further amplification. '
TRANSMITTER PREDRIVER
The transmitter low level signal is applied through a 3 dB
p~d
anrl a
wide-band transformer to the base of
011.
011 is a
2N3866
connecterl in
I