Re: Topband: FT-2000 for serious TB DXing?

["Tom Rauch" <w8ji@contesting.com>]

Yaesu radios traditionally have run very high gain at the 
end of the IF section, after all the filters. Since there is 
no selectivity or filtering in that area, the detector stage 
gets hit with a broadband noise. Both the upper and lower 
sideband of that noise add to the output. This is the source 
of wideband hissing noise when we disconnect the radio from 
the antenna with a narrow filter selected.

It is different in every radio and the effect is rather 
subjective. The effect also depends on the filter you have 
selected. A wide filter passes more noise and broader noise 
from the front end, and masks the very wide bandwidth noise 
from the last IF gain stages. The effect is subjective 
because many people aren't bothered by the noise, some are, 
and every receiver can be different.

A second factor is anything we do to increase gain in front 
of narrow filters reduces IM and blocking performance of the 
radio. In most FT1000 series radios I've tested the effect 
almost tracks the added gain. I add 10 dB 70 MHz IF gain, 
close-spaced IM gets worse just like I added 10 dB of signal 
from both generators. This is because the second mixer and 
later stages are worse than the very good first mixer, they 
are the trouble spots. If you don't reduce the noise that 
actually bothers you by at least the amount you increase 
signal levels, the dynamic range decreases.

It's my suggestion for those bothered by noise from the very 
last IF stages (after the narrow filter) you just add an 
audio filter. Don't mess with the gain distribution, filter 
the noise before it reaches your ears. This way you get rid 
of the noise without reducing close-spaced dynamics. Some 
people will never be bothered by that noise anyway because 
the filter in their head can disgard the low level but very 
broad bandwidth noise  and focus on the narrow bandwidth 
stronger noise in the filter window.

My FT1000MP MKV's are not an issue at all for that broadband 
noise. My FT1000D is, but the noise doesn't really bother 
me. I also added a small audio filter that I use on CW so I 
can't hear the 2kHz to 20kHz faint hissing that might drive 
some people nuts.

I think narrower roofing filters are a great thing. Adding 
gain in front of the narrow filters is something I would 
never do in my receivers.

73 Tom


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