likely cause is one of the following: a) a poor connection to the detector; b) a poor connection to a
decoupling cap; or c) an oscillation on a power line.
NOTE: If the A1 output is not within about 15 mV of zero, check to see that there is no DC level
on the RF input. To test this, the RF should be disconnected, and if this fixes the problem, there
are problems with the RF input source. If there is any DC on the RF center connector, an inside
DC block should be used. If there is no DC on the RF center connector, but ground loops exist, do
not use an inside-outside DC block; rather, clean all RF connectors and try again. If there is still a
ground loop, it has to be removed. An inside-outside block will give unacceptable pulse-shapes.
Adjust P1 to zero the output of A1 to
1/2 mV. Observe the output of A2; if it is negative, connect
P2 to V
CC
; and if positive, connect P2 to ground. Adjust P2 to zero the output of A2 to
1/2 mV.
Repeat this A2 procedure for A3, using P3 for the adjustment.
5.3.4
Apply the RF power at which you wish to start logging. Adjust the gain of A1, using P4,
until the output of A3 is 18 mV. Now measure the gain of A1, as follows: Switch the unit off, and
apply an RF signal of about -10 dBm to the detector. Measure the voltage amplitude at the input
of A1. (Note that if you leave the unit power on, the detector output will read zero, since the A1
input is a virtual ground.) Leave the RF power at this same level, switch the unit power on, and
measure the output of A1. The ratio of the second measurement to the first is the gain of A1. If
the gain of A1 is less than 4, and minimum TSS is important, follow the procedure outlined in sub-
section 5.3.5 below. Otherwise, replace P4 with the equivalent fixed resistor, R2. Provided the
detectors are well- matched, subsequent units can pre-load this value of R2.
The above procedure for measuring the gain of A1 is only approximate, as it depends somewhat
on the specific detector characteristics. A better measurement can be made at a lower input
power, say -30 dBm. However, this requires a very sensitive voltmeter and correction for any
small DC offsets. Using Herotek 2018 (2 - 18 GHz) tunnel diode detectors, we observe an A1 gain
of about 8.2 with R2 (or P4) at 600 ohms. With P4 at 300 ohms (as above), your gain should be
about 4.1.
5.3.5
If TSS is an important consideration, and the gain of A1 at the start of logging is less than
4, short pin 9 to ground (this kills L1), and repeat the procedure of sub-section 5.3.4 above. If the
gain of A1 is then still less than 4, and you really need to start logging at this high a power, we
suggest you call ANADYNE, as it may be advantageous to use specially-modified ICs.
5.3.6
The desensitizer can now be set. Put in a 10k pot, P6, as shown in Fig. 2(a). Re-zero the
output of A1 by adjusting P1. Set P6 to ~3k. Now drop V
CC
by ~20mV by touching a resistor
across the voltage-setting resistor of the regulator. Observe the change in V
out
(pin 18) when you
do this. Adjust P6 until there is no change in the output voltage with a change of 20 mV in V
CC
.
Replace the pot with the equivalent resistor, RS3. This value can be used for all future units with
this configuration.
-16-
 |
 |
 |
 |
 |