[CQ-Contest] Fwd: [FCG] riding the RF gain...

Mike W4EF at dellroy.com
Wed Dec 5 19:44:50 EST 2001

Hi Clive,

The 90dB figures that are typical of many competition grade
HF receivers, are a measure of the receiver's overall dynamic
range, not of the difference between a weak desired signal and a
3rd order distortion product. For instance, if the two interfering
signals are each ~90 dB above the receiver's thermal noise floor,
then the resulting 3rd order products in our competition grade
receiver will be right at receiver's thermal noise floor. Thus a
desired signal that is 20dB above the thermal noise floor will have
carrier to interference ratio of only +20dB (not +90dB as
might be suggested by the dynamic range specification).

Continuing with a similar example, if I increase the levels of the
interfering carriers by 10dB (both carriers are now 100 dB
above the thermal noise floor), then the resultant third order
products will jump up from the thermal noise floor by 30 dB.
Now my C/I ratio will be -10dB instead of +20dB. In this case,
reducing the RF gain of the receiver may offer an improved C/I
ratio because the third order products may fall at a faster rate
than the signal drops and the thermal noise floor rises (this
will be true if the 3rd order distortion is taking place after the
point in the receiver's RF/IF chain where the AGC is applied).

I think this is what Tree was getting at (e.g. he was talking about
Signal to Distortion ratio instead of overall receiver Dynamic

73 de Mike, W4EF.............

----- Original Message -----
From: "Clive Whelan" <clive at gw3njw.fsworld.co.uk>
To: <cq-contest at contesting.com>
Sent: Wednesday, December 05, 2001 4:56 PM
Subject: Re: [CQ-Contest] Fwd: [FCG] riding the RF gain...

>  Tree N6TR wrote:
> > Typically, they show 3rd order products down about 30 or
> > 35 db from the two signals.  With reduced RF gain, it is
> > more like 50 or 55 db.  This might improve the ability to
> > hear a weak signal in noise, or lots of QRM.
> >
> Hmm don't really understand those absolute figures. Have I
> understood correctly. I take this to mean the following:
> Signal A on ( say) 21025
> Signal B on ( say) 21028
> say at equal strength
> IMD products audible on 21022 and 21031 30 or 35 dB down?
> If so this would be horrendous and the band would be full of
> fragments of broken CW when Station A and B were key down
> together but not just one or the other. This was the original
> problem with the TS870S ( see Radcom and QST reviews ca. 1996),
> and the IMD products were *only* 56dB down, as opposed to 90dB
> or more for signals spaced >15kHz. This was said to be due to
> the inherent problems of DSP filtering and poor grade roofing
> filters, but I suspect was rather inadequate gain distribution,
> i.e. too much RF and not enough IF. I believe this was addressed
> by Kenwood sometime in 1996. However the 870 is still not
> perfect and I experienced the above problem on 21Mhz in the
> recent CQWW CW, finding it necessary to introduce 6dB of
> attenuation, in addition to operating in the AIP mode ( minus
> one of the RF amps I think).
> I have *never* experienced this with the FT1000MP nor e.g the
> TS930, and suspect the same would apply to the TS850. N.B.  that
> this does not mean that the FT1000MP is a better ( contest) rig
> than the TS870S, in fact I believe the opposite to be the case,
> and I have recently retired my FT1000MP/TS930 combo in favour of
> a pair of TS870Ss, the reasons for which are beyond the scope of
> this note. Of course everyone has their personal preferences
> -caveat ymmv!-, and my only real point is that surely IMD
> products only 30-35dB down  can't be right?
> Of course I may have misunderstood in which case I am happy to
> be disabused.
> 73
> Clive
> gw3njw at gw7x.org
> Contest Cambria-http://www.gw7x.org
> --
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