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Output Buffer Amplifier @
The Output Buffer Amplifier is a complementary pair of transistors in which Q8 acts as a
source follower boosted by Q9. The current through FET QS is set by R58:
The total current through QS and Q9 is set by R59. The input is selected by the BW5 line
from either CR15 in the LC mode or CR14 in the crystal/mode.
Crystal Filtering Circuits @ @
The bandwidths 3 kHz, 10 Hz, and 30 kHz are obtained by crystal filtering in the First
Crystal Pole and Second Crystal Pole circuits. The crystals are used in series resonant mode
and can be modeled as a series resonant circuit with a parallel capacitance. (See Figure 3.)
The parallel capacitance (C,) and series resistance (&) are not desired and are compensated
for in the circuit, as shown in the equivalent circuit of a single pole of crystal filtering. (See
Figure 4.)
Since the two crystal filtering circuits are almost identical, the following description is for the
First Crystal Pole circuit only.
The PIN diode CR7 functions as a variable resistor at 21.4 MHz. As the resistance is lowered
by increasing the current in the BW63 line, the bandshape becomes narrower. The bandwidth
of one pole widens to approximately 80 kHz when CR7 is turned off completely at the 30
bandwidth of all five poles taken together.
A simplified schematic of a crystal pole, including compensation for R, and C, in the crystal
and input capacitance of the buffer amplifier, is shown in Figure 5.
I FET =
Figure 3. Quartz Crystal, Simplified Schematic
Figure 4. Crystal Filtering Pole, Equivalent Circuit
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