HP 85662A Troubleshooting And Repair Manual page 384

A / W
ADJUSTABLE I
IN BUFFER „ „ . , „ , . , „
A M P L I F I - K
s
GENtKATED
BY R+0 BY +02
P O S I T I V E
FEEDBACK
Figure 5. Crystal Pole, Simplified Schematic
The SYM adjustment Cl3 compensates for C
0
by producing a current ( - I
x
) that is equal
to the current (Ii) through C
0
of the crystal but opposite in phase. These currents cancel,
nullifying the effect of C-o-. The positive feedback from the collector of Q2 generates a
negative output resistance that cancels R* of the crystal. This is approximated by resistor R16
in the Input Buffer Amplifier and XTL adjustment R40 in the Buffer Amplifier.
The input capacitance of the Buffer Amplifier, printed circuit board capacitance, PIN diode
capacitance, and the centering (CTR) capacitor C29 are in parallel resonance with L8. These
components have negligible effect on the band shape. As long as C29 has sufficient range to
"dip" the band shape, they can be ignored in analyzing the remainder of the circuit.
The PIN diode CR7 controls bandwidths from 3 kHz to 10 kHz. For the 30 kHz bandwidth,
CR7 is back biased and R26 sets that bandwidth. If the 30 kHz bandwidth is much too
narrow, even with CR7 back biased, the circuit might be loaded by a bad buffer amplifier
(Q4, Q5). If the bandwidth is only slightly narrow, it may be widened by selecting a new
value for R26*. If the narrowest bandwidth (3 kHz) has too little gain, and it cannot be
increased enough by XTL adjustment R40, then the crystals may have too high a series
resistance (defective crystal), the output resistance may not be negative enough (defective
Buffer Amplifier or Q3), or the crystals may have drifted too far apart in frequency.
LC Filters (6) (G)
The two LC filtering circuits are used for the wider bandwidths (100 kHz through 3 MHz).
They are similar in function; the Second LC Pole circuit is described. A schematic of the
simplified equivalent circuit is shown in Figure 6.
SYM
4 A4A8