Feb 17, 2008

FM 455KHz IF filter choce

It is not only on CW/SSB mode that I want to explore dx at band noise level and beyond.
Even FM DX on VHF/UHF can be exciting. Especially during early mornings, when due to temperature inversions several distant stations turn up. Today the PI3ZLB Beacon at 120km showed at S9+30 instead of the usual S3. Next to that also LX0HI(Luxemburg), and the repeater from Koblenz (Germany) could be contacted on the same frequency (145.725).

There is another way of digging slightly deeper; using a smaller IF filter to get a better s/n.

In todays FM transceivers you will find either the CFU (4 element) or
the CFW(6 element) type 455Khz ceramic filter for FM. The CFU is mechanically smaller. The larger CFW has better characteristics in terms of steeper skirts and stopband attenuation. See the Murata website for exact specs.
The choice of filter type depends on what bandwidth you prefer.

Some commonly used sub versions:

CFx455B 30 Khz

CFx455C 25 Khz

CFx455D 20 Khz

CFx455E 15 Khz

CFx455F 12 Khz

CFx455G 9 Khz

The most commonly used FM IF filter is the E-type(15KHz). That is a bit wide, but just suitable if you need to do packet.
I used to have an Icom 271 with the E-type filter and, like all it suffered from near channel interference (12.5KHz channel offset). A custom installed F-type filter helped.
My Icom 821H has a special E version filter which has an extremely flat response for even better packet results, while retaining 15kHz selectivity. However it suffers from the same interference on stronger local off channel signals. Has the most lovely audio though..

The Kenwood TS790 uses a CFW455F (12Khz) in each IF strip for FM and performs much better.
Todays competition, like Icom (746 and 910) have two selectable FM filters (narrow and wide); F (12Khz) and G (9KHz) type, but using the 4-pole version (CFU).
I have had the opportunity to test both Icoms extensively on the FM narrow/wide filter settings and
found the following:

12Khz vs 9KHz differences:

1. Lowering interference; despite the low stopband specs using only 4-poles, the better selectivity of the narrower 9Khz filter does help avoiding "near channel interference"
2. Better sensitivity; smaller BW creates better S/N ratio at weak
signal levels
3. The smaller IF 9KHz filter results in slightly less audio bandwidth

How much 'usable evidence' did I find on the air during the above tests ?

1. QSO was possible yes/no. In most occasions the next-channel
(12.5Khz) interference completely dissappeared
2. QSO was possible yes/no, reception can be dramatically improved

3. I'm not into Hifi. But the narrow BW does have a negative side
effect: stations which are overmodulating (= more swing) will appear outside the filter, causing audible drop-outs

What filter upgrade did I choose?

A local amateur-parts shop (Barend Hendriksen) has all types available
at only 6.13 euro a piece. Since I like dx'ing on FM as well, I have swapped the original F-type (12Khz) filters in my TS790 for the G type (9Khz). You might consider a home made optional outboard switchable filter section and make your own FM narrow/wide, but I could not find a button on the frontpanel to spare for that.
On my IC821 I went a different route. At the main receiver I removed the 15KHz and exchanged it for 2x 12Khz 455KHz Murata filters in series. See red cornered section in the picture where I glued both filters together.Comparing signals when doing VFO A/B swap shows evident results.
The IC821 was carefully realigned using my Marconi 2955, and showed so much IF gain, that I could easily spare the extra attenuation of a second ceramic FM IF filter in series, without any signal loss.
Checking known FM repeater frequencies brings more stations from Germany, Belgium and even France.

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