output of the detector (K = 700) is about 44
V, and the main output is 6.4 x 4 x 16 x 6.5 x 44
V =
117mV. Since the log transfer slope is 50 mV/dB, the output at -36 dBm = 117 mV + [(42-36) x
50] mV = 417 mV. The question now is: "What input voltage will give an output of 417 + 50 = 467
mV?". Since we are 6 dB above the start of logging, the gain is reduced by 4, so once again we
can tolerate a tail-induced input of 75
V. However, without a pulse, i.e. looking only at the tail,
this would now give an output of:
0.89 x 75
V x 6.4 x 4 x 16 x 6.5 = 178 mV
The factor of 0.89, above, results from the fact that some non- linearity will be in evidence at this
input voltage. This result is to be compared with the 50 mV tail output derived from case 1.
We can draw the following conclusions from this analysis:
1.
For a given maximum input power and a given true recovery time, the amplitude of the
tail as seen on a scope depends on where the unit is set to start logging.
2.
For recovery to a given accuracy at a fixed minimum input power (-36 dBm in the
previous example), it does not matter where the unit is set to start logging; the actual
recovery time will be the same, even though the scope display looks worse when the
unit is set to start logging at a lower power.
3.
Good recovery times are much easier to achieve if the desired power (or signal) at the
onset of logging is increased (as opposed to how the unit is set). This is obvious,
because the important parameter is:
V
signal
V
thermal tail
and if V
thermal tail
is fixed, it pays to increase V
signal
at minimum power in order to improve
recovery time. This might be achieved by, e.g., using a detector with a higher K.
RECOVERY TUNING OF THE L-17C
7.3.1 General Guidance
For optimum recovery results using the L-17C, we have found it most effective to use two different
types of recovery nets. DLVAs using the L-17C behave differently in this respect than most of the
other DLVAs we have examined. In the case of most DLVAs, when a recovery net is placed on
the input amplifier and tuned to flatten the tail at maximum input power, the output will undershoot
at lower powers. With DLVAs utilizing the L-17C, the behavior is exactly the opposite.
In general, if a recovery net is placed on the L-17C's A1, and tuned so that it is just weak enough
to avoid undershooting at maximum input power, it will not provide enough recovery at (P
max
- 10
dB). However, a recovery net placed on A2 has just the opposite effect, namely it gives a strong
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