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Re: Icom IC 910 ( again)!!

Keith Bainbridge wrote:

I've now had the chance over the past week to evaluate the Icom IC910 as compared to my FT 736R. In previous discussions I said my initial feeling was the ft736 won hands down. After operating AO-40 this past week or so I would like to hear from others using the 910H. My biggest concern is the background noise level, when switching the antenna directly between the FT736 and the 910, the signal is AUDIBLY stronger on the 736 , forget S meter readings for now,with a much lower white noise level. However the signal on the 910H is much clearer but has a background noise level that makes it almost unreadable without my external DSP

Wayne's comments:

First, I'm surprised that the noise level is an issue with any receiver when you use a 2 meter dish for AO40 reception.  You should be able to hear the transponder noise floor most of the time with your 2-meter dish.  The transponder noise floor should be higher than the noise floor of your preamp/downconverter, which should be MUCH higher than the noise floor of your receiver.

Second, you need to consider what kind of IF filtering you have in each of the radios.  From your description, it sounds like the IC-910 has a stock ceramic filter (2.7 kHz bandwidth, low Q) in the last IF, and the FT-736R has an optional crystal filter (1.8 or 2.1 kHz bandwidth, high Q) in the last IF.  The optional filter would reduce the noise floor compared to the stock ceramic filter, but the clarity and fidelity of the audio would be lower because of the narrower bandwidth.  Sharp, narrow filters reduce noise and adjacent interference, but they also reduce the fidelity of the desired signal.

If you do have significantly different IF filtering in the two radios, you should be able to hear the difference in the noise spectrum.  The wider filter will allow much more bass and treble in the noise (assuming you aren't limiting the audio spectrum later with an audio DSP).

Another possibility is that you may accidentally have a menu set incorrectly on the IC-910H so that the radio is using an unusually wide IF filter such as a 6 kHz wide "AM" filter.  I'm not familiar with the IC-910H, so I don't know if such an option even exists.

Wayne Estes W9AE
Mundelein, IL, USA
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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