If the undershoot has the form shown in Fig. 7.3.2(a), increase P
T
, as T1 is too short.
When the appropriate T1 is chosen, set P
S
just small enough to give no undershoot. Actually, it
should be backed off just slightly (leave about +1/4 dB), or undershoots may appear at low
temperature. Fig. 7.3.2(b) shows the effect of a properly-tuned net.
Fig. 7.3.2(a)
Fig. 7.3.2(b)
Next, reduce the power by 10 dB. If the tail is not sufficiently reduced at this power, add a Type 2
net, and adjust the RC values of this net to reduce the tail.
Continue in this fashion, using smaller and smaller time constants for the nets, until satisfactory
results are achieved. ANADYNE has achieved recovery times of 200 nanoseconds over a range
of -44 dBm to + 1 dBm with 5 nets (3 on A1 and 2 on A2). For a range -35 dBm to +5 dBm, we
have achieved 200 nanoseconds recovery with a single net, and can get down to about 100
nanoseconds with extra nets.
As mentioned above, while the procedure is initially tedious, we have found that once recovery-net
values have been established for a given detector and amplifier configuration, these same values
can be "pre-selected" for subsequent units, provided that the detectors are reasonably matched.
7.3.3
Dealing With Temperature-Dependent Detector Video Resistance
Some users have reported substantial temperature-dependence in the effectiveness of the Type 1
recovery nets described above. In such instances (assuming they are using temperature-stable
capacitors in the nets), the problem has been principally caused by variation with temperature of
the video resistance of the tunnel-diode detectors being used. Putting a 15-ohm resistor in series
with the detector as described in footnote 5 (page 20) helps somewhat, but the remaining
problems may still be unacceptable.
An alternative approach is to connect the A1 recovery nets to the non-inverting A1 input, where
they are essentially unaffected by any changes in the video resistance of the detector (which is
connected to the inverting input). We show below two kinds of recovery nets which are useful for
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