problem, particularly for hybrid construction. To overcome such problems when using the L-17C,
we have developed a very simple "voltage desensitizing circuit", as shown in Fig. 2a.
A voltage error on V
CC
or V
EE
produces a signal at the junction of the 2k resistors (RS1 and RS2),
which is fed into the input in such a way as to cancel the offsets produced at the output by V
EE
and
V
CC
variations, even if these are at relatively high frequencies. The circuit will work for either V
CC
or V
EE
fluctuations, since the dual- tracking power-supply rejection of the L-17C is excellent. Set-
up of this circuit is described below in sub-section 5.3.6. The circuit also ensures linear
temperature shifts; while great care was taken in the design of the L-17C to ensure temperature
effects on the IC would be linear, regulator shifts over temperature are by no means linear.
Consequently, unless the voltage desensitizer is used, non-linear shifts may be introduced by the
regulators. The desensitizer has the added advantage of minimizing the effects of inevitable long-
term regulator drifts. For optimal performance, we stress the need to include this ancillary circuit
and to follow the suggested procedure for selecting RS3. Also, see the stabilization system in
Appendix C.
INITIAL SETUP
5.3.1
The following steps should be observed in the initial setup: In place of the detector,
temporarily connect a 50-ohm resistor from the A1 input to ground. Connect the regulators, and
set the voltages with a dummy load of about 200 ohms on the + and - lines to
6 V (
+
10 mV).
Install RS1 and RS2, and trim them so the junction of the two is within 3 mV of ground. Remove
the 200 ohm loads, and connect the IC and the following list of components:
1. Power supply decpl. Caps
9. C13 (100 pF)
2. R1 (20 ohms)
10. R14 (400 ohms)
3. C3 (150 nF)
11. P8, (a pot to replace R20,
4. C2 (start at 2 pF)
S1, and R15 for first-time
5. P4 (set at 300 ohms)
use; set at 2k to start)
6. C4 (39 pF)
12. R17 (= feedback res. value)
7. C5 (100 nF)
13. Q1 (if used)
8. C9 (10nF)
5.3.2
Switch on the supplies. The order of switching on is irrelevant, as the L-17C is designed to
protect tunnel diodes from damage, no matter how the voltages are switched on. The current
draw should be about 40 to 50 mA on V
CC
and V
EE
. If there is a high current draw, you have a
faulty IC or (more likely) a faulty circuit. If the current draw is O.K., you may now connect the
detector.
5.3.3
Allow a couple of minutes for the unit to stabilize, and connect P1 to V
EE
if the output of A1
is negative, or to ground if the output is positive. The output should be within
10 mV of ground
before P1 is connected. Check A1, A2, and A3 for oscillations. If the unit oscillates, the most
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