-4
and pinout #40 versions. As ANADYNE obtains more accurate data on the cost and yield of the laser
cutting procedure, there may be a modest price increase for cut ICs.
E: Good Recovery and Crossover-Matching in Multi-Arm DLVAs
We have recently been building 2-arm DLVAs of various dynamic ranges so that we may better anticipate
any problems which our customers may encounter. One such problem is saturation of the input transistors,
which may occur if the tunnel-diode detector in the low-power arm is heavily overdriven. This can happen if
the signal from the preceding RF amplifier is imperfectly limited prior to entering the detector, but it is
normally only a problem if the gain of A1 is set high, and there is no series resistance between the detector
and the A1 input.
The problem is saturation of the A1 input transistors, which distorts the waveform and leads to poor baseline
recovery. The reason this occurs is that the video resistance of the tunnel diode detector goes down to a
very low value at such high RF powers, which has the effect of greatly increasing the voltage gain of A1,
thus causing the input transistors to saturate. Putting 15 to 20 ohms of series resistance between the
detector and the A1 input will decrease the severity of this problem, which in any case should be examined
by checking the A1 output on the low-power arm at the maximum RF input power.
If A1 saturation is still found to be a problem, you may counter this by using 4 diodes and 200 ohms, all in
series, across the A1 feedback resistor. This approach prevents overdriving A1 by limiting at about 3 volts.
It is a simple and effective procedure, but the limiting is not temperature-independent. It is, of course,
possible to design a temperature-independent limiting circuit, but this is a little more complex than another
alternative, which is to use little or no linear extension on the low-power arm, also killing or greatly reducing
the contribution of L7 by adjustment of R110. The effect of temperature on limiting should then not be a
significant problem in the crossover region, as A1 does not contribute significantly to L6 above 2 volts.
In spite of killing or greatly reducing L7, and using little or no linear extension, good crossover matching can
still be achieved. The size of the RF pad for the high-power arm may provide some matching flexibility.
Another trick is to feed a little of the low-power arm A1 output to the high-power arm non-inverting A1 input.
In order to minimize temperature effects, do not use up more than about 1 dB of the first log stage in the
high-power arm. Remember: saturation of the A1 input should not be a problem in the first place (with
tunnel detectors), if the A1 gain is not set too high, and/or the input has ~15 - 20 ohms in series. If more
dynamic range is needed at the high end of the high-power arm, and an external summing amplifier is used,
the A1 output may also be directly fed to the summing amplifier input in order to obtain a substantial true
linear extension.
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