An approach which further decreases the capacitive loading of pin 20 is to sever this pin right at
the IC package. The output DC balance may then be adjusted by appropriately selecting RT3 and
RT4, or, if this compensating net is not used, by connecting an appropriate value of resistor from
pin 21 to V
CC
or V
EE
, as needed. In practice, we find it is usually easiest to adjust the output DC
offset by a very slight offset adjustment of either A2 or A3.
We suspect that some of the capacitive loading originates in the IC package itself, and that in
order to achieve the fastest possible rise time, it might be best not to bond this pad when
packaging. In any event, the procedures described above allow rise times in the 10 to 12
nanosecond region, and package loading effects will be absent in hybrid configurations.
ANADYNE plans to explore the situation further, and will provide additional information when
available.
In the meantime, as mentioned previously (in section 6.10), the fastest possible output-stage rise
time, in either pinout version, will be obtained when the output gain is kept low (25 mV/dB or less),
and the output load is also kept low (or an external drive transistor is used). If both the fastest
possible rise time and a high transfer slope are needed, an external buffer amplifier should be
utilized.
ANADYNE is presently implementing several major improvements for the L-17C. Among these
improvements are reduced sensitivity of the S+ and S- junctions to capacitive loading, and a faster
rise time for the L-17C output stage.
5. If you would like to try building a prototype dual-DLVA using PC36-1 test boards (we have not
tried this yet), you might experiment with an arrangement where two of the boards are placed
back-to-back (with the component sides out to permit tuning), and also turned end-to-end with
respect to each other, in order to minimize the length of the S+ and S- connections between the
two boards. The boards can be fixed in their relative position by small bolts in the corner holes,
with the spacing between them set by nuts and/or washers. Let us know how it works!
-38-
 |
 |
 |
 |
 |