9.
EXTENDED-RANGE (DUAL OR TRIPLE) DLVAs
An extended-range DLVA can be made by using more than one detector- LVA pair, amplifying the
RF power to one or more of the detectors, and summing the outputs. The L-17C is especially
well-suited to this type of application, since provision has been made to sum the log outputs of the
LVA ICs prior to the output amplifier stage by using the S+ and S- pins (nos. 8 and 15). Thus, it is
possible to take the total output from two L-17Cs without the use of an external summing amplifier
(see Fig. 9). In utilizing this feature, we would advise taking the summed output from DLVA #1,
and dropping the gain of the output of DLVA #2 by using ~100-ohm resistors for the feedback loop
of this output stage (R17 and the R15 net).
Fig. 9
Before deciding to utilize the S+ S- feature for summing, as opposed to an external summing
amplifier, several associated factors need to be considered. First, S+ and S- are high-impedance
nodes, and are very sensitive to capacitive loading, including that associated with the internally
bonded wires in the IC package itself. This will result in some slowing down of the L-17C output
stage, as discussed below. Also, when using the S+ S- summing feature, the nominal transfer
function slope is reduced by a factor of two, e.g. from 50 mV/dB to 25 mV/dB. If you wish to
increase this gain with minimum effect on the output rise time, you will probably want to increase
the feedback resistance on the output stage of DLVA unit #1, and decrease it on unit #2. If you
need 50 mV/dB or more over a 70-dB dynamic range, you will in any case have to use a post-
amplifier, since the L-17C output is limited to about 3.2 volts.
Our experience in using single L-17Cs on the PC36-1 test board is that the rise time of the output
amplifier stage is slowed to about 14 nanoseconds when the output gain is set for 50 mV/dB, if the
S+ S- pads are bonded out. The rise time is faster at lower output gains, and several methods for
output speedup are given in section 10.1. If you do not plan to utilize the S+ S- feature, you might
prefer that these pins not be internally connected, for the sake of a faster rise time. You may order
packaged L-17Cs either with S+ and S- bonded out (pinout #40), or without S+ and S- connected
(pinout #41).
-31-
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