[TenTec] Re: Al Gavenas' position on Front-End atenuation

Wed, 09 Apr 2003 19:38:11 -0700

George & Group,

I would just like to add.... especially  for all those that were finding
fault with the the new Argonaut V because it does NOT have an "RF"gain
control,  I have tried numerous times  to find ANY improvement in S/N
ratio by adjusting the RF gain on my new ORION and have not yet found
any circumstance when reducing the RF gain was of any advantage.

Reduction of the RF gain on some  older BA tube type gear does seem to
have an advantage but on the ORION it is not of much use.  Maybe T-T
could upgrade the flash memory to re assign this control to changing the
T-T Logo Dot from Black to Red!   :-)

73,
Bill - K6WLM

"George, W5YR" wrote:
> 
> Thank you, thank you, thank you, Allan!
> 
> I have been corresponding with Al on this plus doing my homework plus
> conducting some fairly interesting experiments using an advanced audio
> analysis program. The bottom line for me is that with four radios on the
> desk  the recommended procedure does nothing that I can perceive or measure
> toward improving signal-to-noise ratio. And I would be greatly alarmed if it
> did.
> 
> Following is the response I made to Al's original posting:
> 
> "I have an Icom IC-765, a 756 PRO, a 756 PRO2 and a K2 on the desk. I tried
> your recommended approach and it does nothing beneficial on any of the
> radios.
> 
> "Back in my youth (1946) when my National HRO-5TA1 was the hot stuff, your
> approach
> worked because there were no AGC systems that could work with CW signals.
> Besides, with *two* high-gain r-f amps ahead of the mixer and marginal  AGC
> (it was called AVC back then!)
> at best, if you didn't back off on the RF gain - which really did control
> the gain of the r-f amp stages - you could
> overload them and get all manner of cross-mod, overloading, etc.
> 
> "I really think that this business of running the audio wide open and riding
> the "RF" control is a holdover from the old days when it was applicable to
> the radios of an era past. But, with today's radios, I really have to wonder
> how it could do much good since the operator is primarily acting to defeat
> the very carefully designed and implemented multi-loop, distributed AGC
> control system.
> 
> "Our experiences seem to have been different, since I find with all four
> rigs
> mentioned above that letting the designer's AGC system do its job cannot
> really be improved upon by my fumble-fingering a couple of controls.
> Especially since receivers made in the past decade or so rarely have
> anything in the r-f stages to control with the "RF Gain" control! "
> 
> Al has indicated that he will track down the QST reference for me.
> 
> The experiments I conducted just this afternoon are as follows:
> 
> With a noise generator driving the PRO2 to an S-meter reading of S8 on
> 40-meter CW, I injected a continuous carrier (RTTY) signal from the
> companion PRO at the same S-meter level of S8.  With both signal and noise
> present, the S-meter reading was S9+15 dB. The noise level within the 800 Hz
> passband was -55 dB on the spectrum in the absence of the signal, while the
> signal alone peaked at -33 dB.With signal present, AGC action reduced the
> noise floor to -72 dB. The S/N measured by the program with an 800 Hz
> passband in the PRO2 was 28 dB. The peak signal to noise level spectral
> difference was 72-33 = 39 dB.
> 
> I then arbitrarily reduced the "RF Gain" (which controls the IF gain via AGC
> system bias and the bias on a PIN diode *after* the first mixer - nothing in
> the "r-f" section of the radio) such that I measured a noise level of -75 dB
> from the noise generator, compared to -55 dB with max RF Gain setting. That
> occurred with the gain control set to about 9o'clock. With the same signal
> injected again, the peak signal was -37 dB for a difference of 38 dB between
> peak signal and noise level. The S/N as measured by the software remained at
> 28 dB. All these values fall well within experimental tolerance.
> 
> So, Allan, I greatly appreciate your confirming the position that I had
> taken with Al and in other exchanges on other reflectors. I fear that this
> "reduce the r-f gain and full-blast on the audio" is about to become another
> fetish similar to the 1:1 SWR craze. There are doubtlessly instances with
> certain receiver designs under certain operating conditions where the
> operator can do a "better" job with manual "r-f gain" control than can the
> receiver circuitry, assuming that the operator even has control over "r-f
> gain" with the knob so labeled. But, as you point out, this is 2003 and our
> current radios just cannot be compared to or operated in the same manner as
> our 1946 models, no matter how hard we try!  <:}
> 
> 73/72, George
> Amateur Radio W5YR -  the Yellow Rose of Texas
> Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
> "In the 57th year and it just keeps getting better!"
> <mailto:[email protected]>
> 
> ----- Original Message -----
> From: "Allan Henry Kaplan" <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, April 09, 2003 5:17 PM
> Subject: [TenTec] Re: Al Gavenas' position on Front-End atenuation
> 
> > The article to which Mr. Gavenas refers was, alas, one of the most
> > misinformed and misleading pieces to ever appear in QST!  Sometimes you
> > can decrease the effect of atmospheric noise as a distraction to a
> > wanted signal by turning down the "RF Gain" or inserting an RF
> > atenuator.  The catch is:  That strategy works only if the desired
> > signal is substantially louder than the noise.  If the signal is weak --
> > not a whole lot greater than the external noise -- RF attenuation (or
> > gain control) will degrade the net signal-to-noise ratio.  If one runs
> > the AF gain wide open and controls speaker-blasting with the RF gain, he
> > emphasizes the receiver's internal hum and noise with respect to the
> > weak signal.  In many cases, the weaker signals simply will not be
> > heard!
> >
> > Running AF gain wide open and riding the RF gain control is just BAD
> > ADVICE unless you are in a QSO with pretty loud signals -- KWs across
> > town on 75 meters, perhaps.   If you are after DX or QRP that is the
> > WRONG way to go.
> > 73, Allan, W1AEL.
> > _______________________________________________
> > Tentec mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/tentec
> 
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