However, its output, which is being added directly to the output of unit #1, is experiencing its
maximum gain condition. Consequently, any temperature-induced shifts of this unit must be
negligible even below its logging range, as they contribute to the active logging region of the
overall transfer function. To minimize this potential problem and thus facilitate the process of
temperature tuning, DLVA#2 should be set to start logging at as high a power as possible. The
point here is that this procedure reduces the low-power gain of DLVA#2, and thus minimizes the
size of, e.g., any temperature-induced baseline shifts of this unit.
Similar considerations apply to tuning for minimum recovery times. It is again significant where
the onset of logging (overall gain) is set for DLVA #2. To see this, consider the case where a very
large pulse is closely followed by a much smaller one. The tail of the first pulse in the second
DLVA will contribute to the summed output during the second pulse, throughout the power range
where DLVA #1 is active. This unwanted contribution will be minimized by reducing the low-power
gain of unit #2, i.e. by setting the start of logging as high as is practical.
If minimum recovery time is needed, the low-power DLVA (#1) should also be used (but not
necessarily set) so that its logging range of interest starts at fairly high power (to the detector).
The reason for this is that perfect limiting is difficult to achieve, and the first detector may see the
effects of high power in spite of the limiter, thus making good recovery more difficult to achieve.
This problem is also minimized by having the logging range of interest start at higher power. Of
course the price for this approach is the need for additional RF gain, but the additional ease of
tuning may make this well worthwhile.
Since most dual DLVAs only require 65 to 70 dB of dynamic range, each DLVA needs to cover
only about 35 dB, which makes the implementation of the above suggestions quite practical in
many instances. For easy tuning of dual DLVAs covering about 70 dB, we strongly suggest killing
L1 by tying pin 9 to ground. To further simplify tuning of dual units requiring only 65 dB of dynamic
range, ANADYNE will consider marketing modified versions of the L-17C in which A3 and/or the
output amplifier have been cut out, if there is sufficient demand for such versions. Cutting out both
A3 and the output stage will also drop the quiescent power consumption of the IC by about 40%.
In small quantities, such modifications can be accomplished by laser- cutting traces on the existing
die, albeit at some additional cost. If there were sufficient demand, we could make wafers of
specially- modified LVAs by implementing the necessary design changes. Please let us know of
your interests in this regard.
The procedures for the correction of any effects of temperature on, e.g., dual DLVAs, are basically
the same as those for single units. However, since any zero-power offsets of the high-power unit
(#2) will affect the first unit throughout its logging range, it would be advisable to monitor the
outputs of A1, A2, and A3 on both units during the correction process.
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