HP 85662A, Troubleshooting And Repair Manual

Page: 383 / 594

Output Buffer Amplifier
The Output Buffer Amplifier is a complementary pair of transistors in which Q8 acts as a
source follower boosted by
The current through FET is set by R58:
I
The total current through and is set by R59. The input is selected by the BW5 line
from either CR15 in the LC mode or CR14 in the
Crystal Filtering Circuits
The bandwidths 3 10 Hz, and 30 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.)
Figure 3. Quartz Crystal, Simplified Schematic
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.)
Figure 4. Crystal Filtering Pole, Equivalent Circuit
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
when CR7 is turned off completely at the 30
BW setting. For a five-pole filter, the bandwidth of each pole is about 2.6 times the
bandwidth of all five poles taken together.
A simplified schematic of a crystal pole, including compensation for and in the crystal
and input capacitance of the buffer amplifier, is shown in Figure 5.
3