SEA 222 Operator's Manual page 43

5.3.1
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5.3.2
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5.3.3
5.3.4
carrier diode double balanced mixer assures minimal cross modulation
and intermodulation distortion in the receiver front end. The 2 MHz
high pass filter provides some protection from the very large signals
generated by nearby standard broadcast transmitters.
THE BILATERAL 45 MHz 1.F. AMPLIFIER
The output from Al is the desired signal, upconverted to 45 MHz and
filtered by Fl, a two pole monolythic crystal filter with a bandwidth
of about 15 KHz. This 45 MHz signal is then amplified by the first
I .F. Ampifier. The amplifier stage is somewhat unique, in that it
consists of a tuned MC1350P integrated circuit amplifier and a tran-
sistor buffer surrounded by a P.I.N. Diode array which a11ows the
direction of signal flow through the amplifier to be reversed by
properly biasing the switching diodes. (See Figure 5.3.1.) The
advantage of this type of amplifier stage is that the direction of
signal flow can be reversed at will, allowing the entire receiver
system to "double" as a transmitter. This process minimizes the
transceiver circuitry and allows considerable simplification in the
design.
SECOND MIXER AND 6.4 MHz SSB FILTER
Once the 45 MHz I.F. Signal is amplified in
IJl,
it is applied t.o
the second double balanced mixer, A2, along with the VCXO signal at
about 38.6 MHz. The DIFFERENCE frequency output, at 6.4 MHz is then
passed through F2, a narrow bandwidth lower sideband filter. The LOWER
sideband at this point is equivalent to a signal frequency UPPER
sideband signal, since the first conversion INVERTS the information
band, while the SECOND conversion to 6.4 MHz does not.
6.4 MHz I.F. AMPLIFIER AND PRODUCT DETECTOR
The twice converted, amplified and filtered signal from the 6.4 MHz
filter (F2) is then passed through the second I.F. Amplifier. This is
another tuned bilateral MC1350P Amplifier, similiar to the first I.F.
Amplifier except for operating frequency. The output from the second
I.F. Amplifier is then applied to A3, the product detector, along with
a BFO signal at 6.4 MHz. The product detecter is still another double
balanced hot carrier diode mixer, which assures a low distrotion, low
noise detector with good dynamic range.
LOW NOISE AUDIO PREAMPLIFIER
Integrated circuit ll3 is a low nois-e audio preamplifier which uses
the type LM387 dual preamp chip. The first stage (ll3A) is configured
as a non-inverting amplifier with a stage gain of approximately 35.
The non-inverting configuration is userl to assure minimum noise figure.