[TenTec] Inrad 400 vs. TT 500 & 250 CW filters

[Darwin, Keith]

Hey guys,
 
Boy have I been having fun.  My new-to-me Omni V.9 came with an Inrad
400 Hz filter and TT model 282 250 Hz filter.  I already had a TT model
285 500 Hz filter.  So of course I plugged 'em all into the rig and
spent some time comparing and contrasting them head-to-head on the same
signals in the same rig.  Here's what I noted with them.
 
---- Bandwidth ----
The Inrad filter is narrower than the 285.  The difference is seen both
in the width of the passband and the steepness of the skirts.  Boy, that
Inrad is fabulous in this regard.  Tune in a signal, rotate the IF shift
knob and suddenly the signal goes away.  Very selective.  In contrast
the 285 filter allowed wider shifts on the IF knob before I lost the
signal and once a signal did hit the skirts it stayed stronger for
longer.  This is just the difference between and (apparent) 8 pole 400
Hz filter and a 6 pole 500 Hz filter.
 
Between the 285 and Inrad, the latter was the clear winner in the
selectivity department.  Very nice.
 
The 282 filter did provide narrower bandwidth than either the Inrad or
the 285.  Most of the difference was seen on the low side of the 282
filter.  It would stop those low signals that were on the edge of the
Inrad's passband.  On the high side, the 282 and Inrad were pretty
close.  Of course, a little IF shift action allows you to slide that
passband where you want it so in the end, selectivity is selectivity and
the 282 was the most selective of the bunch both close to the passband
and far away.
 
 
---- Stopband ----
The Inrad's stopband was not as good as the 285.  Strong signals could
be heard 5 KHz away as they leaked around or just snuck through the
Inrad filter.  The 285 would stop these signals dead in their tracks.
Often this low-level leakage is not an issue but if you're trying to
copy a weak signal in the passband and you have a strong signal in the
stopband it can cause enough QRM to confuse the brain.
 
The narrower 282 filter showed a bit of signal in its stopband as well.
Not nearly as much as the Inrad, but there was some.
 
For stopband performance, the 285 was the clear winner, 282 was a close
2nd place and the Inrad was noticeably behind in 3rd place.
 
 
---- Which one ----
After doing a lot of listening I decided the combination of the 285 and
282 was the way I would go.  The 285, with wider passband and shallower
skirts works a bit better for tuning across the band while still
providing pretty good CW selectivity.  When things get tough, I have the
282 in reserve to give maximum selectivity.
 
The Inrad filter is impressive in many ways.  If I did not have the 282
or needed to cut back to only one CW filter, I'd likely pick the Inrad
and use the Omni's great audio filtering to reduce or eliminate any
signals in the stopband.
 
- Keith -