[TenTec] Radio for soon to be newbie General....

Tue, 8 Apr 2003 16:56:24 -0500

Al, how does a linear front end have a "threshold" above which noise cannot
pass and signal can?

You lost me on that one!   <:} I am one of those guys that need "educating."

Especially considering that most rigs today use the control labeled "RF
Gain" to actually vary the IF gain in the receiver . . . many designs have
no manually gain-controllable amplifying devices between the antenna
terminals and the first mixer. Only fixed gain preamps and fixed loss
attenuators.

What is the difference in the output S/N ratio as perceived by the listener
from the audio signal between inserting attenuation up front before the
first mixer and reducing the gain of one or more stages in the IF?

I really would like to understand what is going on here since so many people
are recommending that "RF Gain" controls should always be reduced and the AF
gain set up high.

On many radios all this does is gain up the internal noise and hum from the
audio system and reduce the signal level available to the first mixer, etc.
But, if the front end is linear, as it must be for good IMD performance,
etc., the ratio between signal power and noise power is set at the first
filter following the antenna terminals and does not change with changes in
gain in that linear portion of the receiver.

Puzzled . . .   <:}

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: "Al Gavenas" <[email protected]>
To: <[email protected]>
Sent: Tuesday, April 08, 2003 11:23 AM
Subject: Re: [TenTec] Radio for soon to be newbie General....

> It seems like most hams want to run their rigs with the RF gain
> wide open and ride the volume control to adjust the audio output
> to a comfortable level.  What you should do, is run the audio volume
> control wide open and ride the RF gain control to adjust
> the audio output to the comfort level.
>
> There was something in QST a while back about this and it makes a lot of
> sense.  What comes in the front end of your rig is the sig you
> want  along with unwanted  noise.  Hopefully, the sig rides above the
> noise.   The trick is to amplify the sig and not the noise at the
> front end of the rig.  By adjusting the RF gain what you want to do
> is push the noise down to below the threshold that the front end
> of the rig can grab and amplify.  But you do want to amplify
> the sig.  The noise blanker helps on some types of noise but I think you
> can knock out more noise with proper use of the RF
> gain control than you can with a noise blanker.  The purpose
> of the filters in the rig has to do with selectivity and not noise.
> Selectivity has to do with selecting the one sig among the many
> out there that you want to hear.  Have I made myself clear????
>
> I think a lot of hams need to be educated about this.
>
> Al Gavenas WA6IQD
> >>>> [email protected] <<<<
>
> On Tue, 8 Apr 2003 07:34:37 -0700 (PDT), James C. Owen, III
> <[email protected]> wrote:
>
> > Hi Gene,
> > On the Jupiter you get a popping and static much like you do
> > with the noise blanker on the Corsair when it's set too high. Not
> > a problem though, turning on the 20db att. takes care of it.
> > About the  ONLY place that I have had a problem is on 75M at
> > night. If a signal is within the 20Khz passband and is at least
> > 25+ Db over S9 then I might get the problem on the station to
> > which I'm listening. However you can't tell the difference except
> > on the S meter when you turn the Att on, the garbage just goes
> > away. Sometime but less frequently I'll have the problem on 40m
> > with the SW BC. I have NEVER had it on any of the other bands
> > even during a major contest.
> > Believe me this is NOT a problem.
> >
> > 73 Jim K4CGY --- Gene Buckle <[email protected]> wrote:
> >> > I like my Pegasus and use it as my prime radio on HF.  For
> >> the money you
> >> > pay for this rig you get a lot.  It''s easy to
> >> > use and has wonderful filtering.  Major problem with the rig
> >> is front end
> >> > overload with real strong signals, but you cant
> >> > have everything with an $800 radio.  Get the tuning pod when
> >> you buy the radio.
> >> >
> >> What happens with "front end overload"?
> >>
> >> g.
> >> kc7afe
> >>
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