[Icom] RE: Filtering at 9 MHz vs. 455 kHz

[Hsu, Aaron]

Sorry for the bandwidth (no pun intended <g>), but I tried to reply direct to Rick and it bounced.  Here it is for the world to see...

---------------
Rick,

I don't have an Icom HF rig, but analog filter chains basically work the same on any radio.  Here's the basics...

A "perfect" filter should cut off *everything* below and above the filter's bandpass.  So, a filter with 300Hz to 2.7KHz bandpass should not allow *any* frequency below 300Hz or above 2.7KHz through.  If plotted on paper, it should look like rectangle with 90 degree vertical lines at 300Hz and 2.7KHz and a perfectly flat horizontal line connecting the top of the two vertical lines (this horizontal line represent the frequencies between 300Hz and 2.7KHz).  Now in the real world, it's impossible to have a perfect analog filter (even digital filters aren't perfect).  The vertical lines are more at an angle - anywhere 1 degree to 89 degrees depending on the quality of the filter.  The shape factor is determined by taking the bandwidth of the -6dB points divided by the bandwidth of the -60dB points.  The closer you get to 1.0, the steeper the skirts the the closer you get to "perfect".

Lower IF (e.g. 455KHz)filers have "shape factors" closer to 1.0 because it's easier to build high "Q" filters in low frequency ranges.  However, this often comes at a size penalty as component sizes get larger at lower frequencies.

Higher IF (e.g. 9MHz) filters have higher "shape factors" - the skirts are further from being "90 degree" vertical.  Component sizes generally are smaller, but it's harder to build high "Q" filters as frequency rises.

So, which one is better???  It's a toss-up.  I asked this same question many years ago and got the following two answers...

By using a higher IF filter (9MHz), you prevent the overload further down the IF chain.  In other words, filter out the crap at the door before the crap overloads the rest of the system.  The downside is that the filter skirts are wider, so even though you're getting rid of the crap early, you're still letting in crap (though to a smaller extent).
 
By using a lower IF filter (455KHz), you have much better filter skirts so you can get rid of the crap that's near what you want to hear.  The down side is that there's a lot of crap hitting the filter to begin with and it's been amplified early in the IF chain.

So, again, which is better?  It's personal preference.  The true engineer would have you put the filter early in the chain.  Our ears would have you put it later in the chain.  The optimal condition is to have filters in both.  I originally got a 455KHz filter for my Kenwood HF rig.  I eventually got the same filter for the higher IF (8.83MHz).  You *can* hear the bandwidth difference if you listen for it, and if you do lots of work with nearby signals, you'll start to notice that the higher IF filter does prevent overload.

One more thing...the "type" of filter also matters a GREAT deal.  Ceramic filters are cheap (inexpensive) and have poorer shape factors.  Crystal filters are more expensive, but have *much* better shape factors.  If determining what filter to buy based on cost, defintely keep this in mind...the ceramic filter *will* be cheaper, but at the cost of performance.  This may be one reason why the higher IF filter costs less and performs worse than the lower IF filter.

I would *HIGHLY* suggest that you get filters from InRad.  They are *VERY* well known in the DX and Contesting world as the best filters you can get for amateur radios.  Cost is about the same as the OEM filters, but much better shape factors.  You can find InRad at

  http://www.qth.com/inrad

If you click on the links to the performance curves, you can get a general idea of what filter shape factors look like.

73,

  - Aaron Hsu, NN6O (ex-KD6DAE)
    {nn6o}@arrl.net
    {athsu}@unistudios.com
    No-QRO Int'l #1,000,006
    . -..- - .-. .-   ".... . .- ...- -.--"


p.s.  I'm not a filter expert...just passing on what I've learned.  Comments and corrections are always welcome!


-----Original Message-----
From: Clifford, Rick [mailto:rick.clifford@eds.com]
Sent: Friday, September 13, 2002 11:33 AM
To: icom@mailman.qth.net
Subject: [Icom] Filtering at 9 MHz vs. 455 kHz


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