first log stage if one wishes raise the start of logging yet further.
(2)
The log transfer slope is set by
the output amplifier gain only. The L-17C ancillary circuitry for which the PC36-1 board is set up is
shown in Figures 2(a), 2(b) and 2(c).
INPUT AMPLIFIER
This amplifier (A1) is unipolar, producing only positive outputs up to a maximum of 4 V. It can,
however, be run in either the inverting or non-inverting modes. (A1 can be made to swing a
negative voltage under certain circumstances, which will be discussed subsequently.) If the
application calls for the use of tunnel-diode detectors, the inverting (transimpedance) mode is far
better. When the detector is to be used at very high power (
5 dBm) it may be useful to put 10 to
15 ohms in series with the detector (this raises the RF power level at which A1 limits), but
otherwise this is not desirable.
The L-17C also works well when used with Schottky diode detectors. However, since the
collectors of the input differential pair (pins 1 and 36) normally sit at about +0.5 V, when Schottkys
used at high RF power care should be taken not to saturate the input transistors. The procedures
for use of Schottkys are detailed in Appendix D.
Normal A1 compensation, C4 [see Fig. 2(a)], is 39 pF, but this value is not very critical. The
waveform is sensitive to the capacitor C2. This capacitor compensates for the input capacitance of
the detector, and its value depends on the detector and on the gain setting of the amplifier.
Without any capacitance the amplifier will overshoot, and with too much it will oscillate. A value of
1 to 3 pF is normally sufficient, although a larger value may be required for low-frequency
detectors, or if the gain of A1 is set very low. We set R1 at ~20 ohms, as the input bias current is
so low that having R1 < R
V
(the detector video resistance) does not upset behavior over
temperature. C1 (for noise reduction) is not normally used, and should not be used with low
values of R1. The other components are discussed later.
The second and third amplifiers (A2 and A3) are both non-inverting, and, in general, do not need
any gain adjustment. Both limit at about 0.5 V. Note that there are 200 ohm resistors in the IC
which are in series with the output pins of both of these amplifiers. They can be slowed down by
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(2)
Reducing the gain of A1, i.e. starting logging at a higher power, does not normally affect TSS. It
does, however, affect the baseline noise, and if the gain of A1 is set so low that the noise from A2
becomes significant, TSS will then be affected. It is obvious that TSS is not otherwise affected,
since TSS is simply a measure of the detector and amplifier noise, which are essentially
unaffected by the overall gain. The effect of gain on behavior over temperature and on recovery
time is discussed in sections 8.2 and 7.2, respectively.
-8-
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