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

Thu, 2 May 2002 18:42:00 +0100

Hi Tom,

I know that you were experimentating the change of the post 455khz filter
FET with a GASFET (in FT1000MP or FT1000D)..
Please give more detail about this modification and how it has performed.
Tks es 73
Ian IK4EWX

----- Original Message -----
From: "Tom Rauch" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 02, 2002 11:07 AM
Subject: Re: [1000mp] CW Cap Mod and I.F. Mod not the Same - Over and Out

> > 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
> > [email protected]
> >
> >
> > > http://mailman.qth.net/mailman/listinfo/1000mp