tail suppression at (P
max
- 10 dB), but has less effect at P
max
. Consequently, by using a judicious
combination of Type 1 (A1) and Type 2 (A2) recovery nets, excellent recovery at all powers can be
achieved.
Type 1 Recovery Nets. An example of a Type 1 recovery net (for a case in which A1 is used as a
transimpedance or inverting amp) is shown in Fig 7.3.1(a), below. The specific net described uses
more components than are strictly necessary, but it is easy to tune, since P
T
determines the time
constant, and P
S
the strength. P
T
and P
S
will be replaced after tuning with the fixed resistors
RR1B and RR1C, respectively [see Fig. 2(a)]. The value of RR1A is usually
5k, and is not very
critical. If the desired value of P
S
is too large, one can increase RR1A.
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Type 2 Recovery Nets. These are simply RC nets of the type shown utilizing RR4A and CR4 in
Fig. 7.3.1(b). See also Fig. 2(b).
NOTE: We have sometimes observed a small undershoot at high power with long pulses (
100
sec) when the L-17C is used with certain detectors. If you have such an undershoot, it should be
corrected prior to recovery tuning. To make this correction, put in an anti-recovery net on A1,
using R3 and C5A [see Fig 2(a)]. With the detectors which we found to cause this behavior, the
undershoot was removed by using such an anti-recovery net with R3 = 3.3 Megohms and C5A =
27 pF.
7.3.2
Specific Procedure
Set the RF power to the maximum level to be utilized, and the pulse width to about 10
sec.
Observe the tail. It will not be an exponential, but can be approximated by a sum of exponentials.
Estimate the longest exponential time, T1 (T1
5
sec). Use a Type 1 net, with P
S
at its maximum,
and set P
T
so that R1x CR1 = T1, where R1 is the value of P
T
. Reduce P
S
until the tail starts to
disappear, and continue to reduce P
S
until the pulse undershoots.
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We have found it efficient to build several of these nets with different capacitance values
(e.g. 10 to 500 pF) on perf. boards with legs placed to fit into the appropriate holes on the PC
board.
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