EMRFD Message Archive 1730

Message Date From Subject
1730 2008-06-19 15:20:35 bkopski SUPER TUNABLE TRAP
In working with wide band amplifiers in recent weeks I found the
EMRFD Fig 7.57 Tunable Trap idea to be very helpful. Specifically,
in watching the amplifier performance on my SA I was often not sure
if harmonics that appeared were due their presence in my test signal
source (an old Wavetek sweeper) or if the amplifier under test was
distorting. But I needed to cover from a few mHz to over 100 mHz
more conveniently so the Super Tunable Trap came into being.

The STT is pictured in an accompanying photo and is shown
schematically in an accompanying .pdf file.

This wide-ranging tunable trap is very easy to use with its three
overlapping ranges and in general can produce a 20 to 30 in a 50 ohm
system. As should be clear from the schematic, one 2P3T slide switch
selects one of 3 inductors and the other 2P3T slide switch selects
one or the other variable capacitor section, or both sections in
parallel. Generally, I can find the same tunable frequency with more
than one switch combination with one of the choices yielding the
deepest notch.

In operation, the STT is inserted in the test signal line from the
generator to the amplifier under test. It is then tuned to the
harmonic-of-interest in an attempt to eliminate it in the signal
source path. For an amplifier operating quite linearly the selected
harmonic is typically eliminated. For an amplifier that is
distorting, the selected harmonic is typically reduced. This is
because part of the total output is due source content and part is
often the amplifier contribution. In any case, since I'm pursuing a
myriad of amplifier designs, I found this versatile, not-too-
difficult assembly to be well worth the aside effort to build it –
and in this case I had all the parts in the junk box (a rare


Bob, K3NHI