Hi Bill,
If you worked RTTY you would find out that the "250Hz" filter is ideal
for recovering weak ones and for 20M during RTTY contests. The dual
passband filter fits very nicely within it. I don't care what
bandwidth you call it. Don't eliminate it as a choice.
I was astounded the difference it made during yesterdays DL RTTY
contest. Fine tuning + the selectivitiy made the 20 KHz RTTY band seem
like 100KHz wide while doing search and pounce. A/B comparisons between
it and the 400Hz filter did indeed show significant benefit.
Spectrogram spectra, with a band loaded with signals, strengthen this
A/B comparions conclusion.
Some offending signals simply fell off the edge of the 250 Hz filter
slope. Part of this benefit is undoubtedly due to the capture effect of
FM like modes. Strongest wins.
73 de Brian/K3KO
Bill W4ZV wrote:
Brett Howard wrote:
So am I to assume that the 8-pole filters are not going to need to worry
about this as they are all at 0 anyway? What is the advantage of
getting two matched pairs of 5-pole filters when you can get two 8-poles
for only 10 bucks a filter more?
I'm assuming that its 100 + 100 + 30 for matched 5-pole filters or 125 +
125 for the 8-pole filters. I can see there being a great advantage if
you already have a 5-pole and want to match it in your sub RX. But if
you're just getting 2 at the same time it seems like the 8-pole makes
for a good option too.. At least I hope so cause diversity receive was
the main reason I figured I'd go for the 8-poles... :)
You're correct that 8-poles have no offsets and avoid this problem. When I
ordered my 5-poles there was a $40 per filter discount to 8-poles and nobody
(including Elecraft) understood that the Sub RX filters needed to matched if
using 5-poles. EI6IZ on the Field Test team discovered this in December
when he saw slight frequency offsets as the WIDTH control toggled different
filters.
But the real reason I ordered my 500 and 200 combination is purely
bandwidth. Here are Elecraft's measurements:
Filter BW(-6dB) Shape Factor
200 224 4.0
250 370 2.1
400 435 2.1
500 565 3.1
For weak CW signals or in contests I prefer a wider bandwidth. For weak
signals the additional bandwidth gives my ears a better context in which to
apply my ear/brain's internal 50 Hz filter. In contests, I also want to be
able to hear stations calling me off-frequency as well as hear what's going
on around my run frequency (so I can chase off those folks who send "?" once
and then start CQ-ing 250 Hz above me).
I would actually prefer 8-pole filters for rejection reasons, but the
bandwidth choices above are simply wrong for my use. The 400 is too narrow
and the 250 is too wide (and only 65 Hz between the two!). When I need a
narrow filter, I want a *truly* narrow filter. The main use I have for the
200 is in nearly simplex pileups with lots of S9+++ signals (think 160m DX
pileups).
Another reason one might want 8-poles is when using N8LP's LP-PAN and SDR
software for a panadaptor/waterfall display. The 5-pole offsets can cause a
similar problem here as you rotate WIDTH through different filters.
However, with the recent addition of Elecraft's programming commands to read
the internal filter offsets, N8LP says WU2X can correct for the offsets in
his PowerSDR-IF software.
The bottom line to me is that I choose filters primarily for their bandwidth
and I prefer the 500/200 combination even with their offset warts. If
Elecraft offered different BW choices in the 8-poles, I might prefer them.
Quite frankly, given the current $25 price differential, I don't see why
Elecraft doesn't go to all 8-pole filters and make some better bandwidth
choices (e.g. at least octave differences at the low end...say 600 and a
*true* 250...not the one above which is actually 370). I wouldn't be
surprised to see Inrad eventually do this even if Elecraft doesn't.
73, Bill
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