[1000mp] CW Cap Mod and I.F. Mod not the Same - Over and Out

[Tom Rauch]

> I.F. hiss due to insufficient previous stage gain to over come the
> internal noise of the next stage, and audio high frequency response

True enough.

The noise in the FT1000's comes from the fact gain distribution is 
poor at the tail end of the IF. The last narrow filter in the 455 IF 
is somewhat starved for signal level, and most of the IF gain is in a 
dual-gate FET 455kHz amplifier immediately after the 455 filters.

When you switch to narrow CW, the IF system **after** the 455 filters 
responds to unfiltered wide-band noise from that single FET. The 
audio system not only gets noise from the unfiltered full bandwidth 
of desired sideband, it has reciprocal noise from the opposite 
unneeded sideband.

Meanwhile, the narrow filter reduces noise from earlier stages in 
direct proportion to the selectivity increase. If you switch from 
2.4kHz to 250Hz, noise from early stages drops around 10dB.

The real problem is the lack of selectivity AFTER most of the IF gain 
is obtained.

> pass band at audio frequencies (cw or other wise) are two different
> topics..... and two different and unrelated mods.

Actually they are closely related.

There are two ways to reduce broadband noise. One method is to add a 
filter after the high gain area of the receiver, either in the very 
low frequency last IF stages or audio stages. The other is to 
increase level into the last narrow filter, and reduce gain after 
that filter.

The first method, adding late filters, improves intermodulation 
dynamic range and blocking dynamic range. The second method can hurt 
dynamic range, especially if the gain is added early in the receiver. 
 
> The cap mod for the cw audio stage frequency response high end roll
> off is just that and nothing more. It doesn't reduce the I.F. stage
> noise one iota .... only affects the audio frequency response.
> 
> Too many folks here are using the term 'hiss' as a single source item,
> and it is not. Band noise 'hiss', rf amp noise 'hiss', i.f. noise
> 'hiss', diode noise 'hiss', mixer noise 'hiss', audio frequency

Diode noise hiss is a myth, encouraged by the pathological changes 
people imagine they hear when signal diodes are replaced with PIN 
diodes.

Of course if spent several hours and a few hundred bucks replacing 
diodes that never needed to be changed in the first place I guess I'd 
imagine it improved the receiver also! Like waxing my car and 
noticing how much better it runs.

Virtually **all** of the annoying hiss you hear in the FT1000 series 
receivers comes from the dual-gate FET following the 455kHz filters. 
Circle that component, and put a noise-dart in it.
> response 'hiss', etc etc are not the same and have to be treated
> individually. Noise that is passed from one stage to another can
> accumulate, but only if it is in frequency range of the stages that
> pass them and handle them, or the harmonics and distortion products
> inter relate. The audio stages do not handle i.f. frequencies.
> 
> Put a noise analyzer in the I.F. stages, and put the CW cap mod in and
> out of the cw audio stage, and you will see ZERO effect in the I.F.
> stages. Period!
> 
> You cant fix the effects of the I.F. at audio frequencies. You can
> mask the effects of noise at the audio side, but that doesn't take
> care of the i.f. side. Signal to noise ratios, noise blanker
> performance, and dsp functions are all affected by what comes to it
> from the earlier i.f. stages, and are improved by the Inrad
> modifications. Not by the cap mod.
> 
> This is also not a situation of agreeing with me or not on the CW cap
> mod. As I have said before. It is a user choice and I have made my
> choice and you should make yours. The technical side of it has nothing
> to do with the agreeing with X Y or Z persons, so your best to leave
> the emotional side out of it. Confusing sources of hiss, and mixing
> circuit effects is not helping the situation either .....
> 
> End of topic from me on this side. It is just a shame to see so many
> misleading statements put out, that are technically dis connected.
> 
> 73 from Bill - WD8ARZ
> wd8arz@ix.netcom.com
> 
> 
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73, Tom W8JI
W8JI@contesting.com