[Icom] Filtering at 9 MHz vs. 455 kHz

[Adam Farson]

Hi Guys,

Here is an argument for installing 9 MHz and 455 kHz filters with equal or
comparable bandwidths and shape factors:

Assuming that the filter passbands overlap (PBT or Twin PBT in centre
position), the composite adjacent-channel selectivity at the stopband (-60
dB typ.). will be superior to that of one filter alone.

In addition, for PBT or Twin PBT to operate correctly, the passbands of the
two filters should at least be similar. Thus, for example, the FL-80/FL-44A
pair ensures a much tighter and more aggressive PBT action than FL-80/FL-96.
Pairing 250 and 500 Hz CW filters (in any order) will make the PBT very
sloppy.

A pair of 500 kHz CW filters in a radio equipped with PBT or Twin PBT is a
very acceptable compromise. Whilst the adjacent-channel selectivity with the
effective bandwidth reduced to 250 Hz via the PBT is not quite up to that of
a pair of 250 Hz filters, it is still very acceptable in many cases.

In a receiver, the filter closest to the first mixer does most of the work
in terms of determining the IF bandwidth and filtering out the unwanted
sideband, out-of-band signals etc. If only one filter is installed, it
should be the 9 MHz filter. In the original, unmodified IC-765, the 9 MHz
filter is bypassed when enabling IF Shift. This allows strong out-of-band
signals to ride down the IF strip and overload the third mixer (9 MHz/455
kHz). This degrades close-in dynamic range.

Conversely, in an analogue transmitter which shares the filters with the
receiver, the first filter should be as close to the balanced modulator as
possible, to ensure adequate suppression of spurious products. This means
that the transmitter should always have a 455 kHz filter.

Thus, we end up with a transceiver fitted with crystal filters at both IF's.

You can view the passband curves of popular Icom IF filters here:
http://www.qsl.net/ab4oj/icom/filters/filters.html

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:farson@shaw.ca


-----Original Message-----
From: icom-admin@mailman.qth.net [mailto:icom-admin@mailman.qth.net]On
Behalf Of Guillermo
Sent: Thursday, September 12, 2002 19:59
To: icom@mailman.qth.net
Subject: Re: [Icom] Filtering at 9 MHz vs. 455 kHz


	Hello Rick, I've read all other reply messages posted until now (six
answers, mine the seventh). I agree mostly with Jordan's reply, and want
to add this: in my humble opinion, it would be better to have 500 Hz in
9 MHz, and then another 500 Hz or if you want a 250 Hz filter at 455 KHz
IF. You "pre-shape" incoming RF signal first with the 500 Hz filter at 9
MHz, and then make more "vertical" skirts with the 455 KHz 500 Hz filter
and even a narrower BW with the 250 one.
The skirts of the second filter (500 Hz at 455 KHz), won't add too much,
if something, if you install the already narrower 250 Hz BW filter. My
choice would be to add another 500 Hz filter at 9 MHz.

Best regards
Guillermo - LU8EYW.


"Clifford, Rick" escribi�:
>
> ICOM'ers,
>
> I have an IC-746 (original, not PRO) and do primarily CW on 40m. After
> suffering through several instances of adjacent frequency interference, I
> decided to buy a filter. The IC-746 has two places where filters can be
> placed, the 9 MHz and the 455 kHz IF stages. So now I needed to find out
> which 500 Hz filter to buy. The suggestion from the HRO store was to
filter
> at the 455 kHz stage (naturally, the more expensive of the two options),
> while a call to ICOM suggested that both stages should be filtered (an
even
> more expensive option).
>
> I ended up installing the more expensive ($180) 455 kHz filter (500 Hz
> bandwidth). I am quite pleased with the performance, but wonder if I will
> get significantly improved filtering performance if I also place a 500 Hz
> filter in the 9 MHz IF stage.
>
> Since I have been wondering what the 250 Hz filter is like, an alternative
> approach is to buy the 250 Hz filter and put it in the 9 MHz stage. Then,
I
> should be able to switch in either 250 or 500 Hz.
>
> Does any have an opinion about how these filters should be used across the
> two IF stages? And, is there enough difference between the 250 Hz and 500
Hz
> filters to justify having both?
>
> Thanks and 73,
>
> Rick
> KF6UEB