Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Post by Robert1 » Thu, 29 May 2003 21:09:40

Hello:

Have a JRC NRD 545, but am a newbie re using it.
But, the folowing question is probably general in scope, irrespective of
receiver, I would think.

Could someone explain, please, the difference in attenuating a "too strong"
input signal
by using the radio's Attenuator (button; -20 db) vs just turning down the rf
gain pot ?

I understand that the attn works immediately at the antenna input, and the
rf gain pot on the IF,
but when, and under what conditions,  would one use each ?

Thanks,
Bob

 
 
 

Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Post by J999 » Fri, 30 May 2003 12:48:41


Quote:>Hello:

>Have a JRC NRD 545, but am a newbie re using it.
>But, the folowing question is probably general in scope, irrespective of
>receiver, I would think.

>Could someone explain, please, the difference in attenuating a "too strong"
>input signal
>by using the radio's Attenuator (button; -20 db) vs just turning down the rf
>gain pot ?

Good question !!

I have an old R71a and this is how I use mine:

The attenuator I will use when there are strong signals _somewhere_ in the band
I'm listening and they are overloading the front end. They may be100khz or more
away and causing images, like on the MW band. I leave the AGC on, RF gain full
on, and turn on the attenuator to eliminate the images hoping that there is
still enough signal to copy.This works on Long Wave too, but I will loose a lot
of gain for the weak signals.

However, if there is either a strong station on an adjacent frequency to the
station I'm trying to hear (within a few khz or less), or there are static
crashes covering the signal, I turn the AGC off and throttle back the RF gain a
bit. Now, the offending station isn't desensing the receiver and I can still
copy the weak targer. THis technique may give you a headache, but at least you
can still copy the weak station even though the QRM my be absolutely terrible !
[ doing this for 24 hours during Field Day is not for the faint of heart ! ].

The other way to use the RF gain is when listening to a loud station but you
want to eliminate some of the background noise. Leave the AGC on, and back off
the RF gain to leave just the loud station booming through the speaker with
limited snap, crackle, pop in the background.

Hope that helps.

73 !

John WB9UAI
Milwaukee

 
 
 

Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Post by Paul Vanass » Fri, 30 May 2003 18:42:17

Hello Bob:

The RF Gain control is the most valuable device on a receiver. While most
people keep the pot cranked up as far as it will go, many veteran radio
enthusuasts recognize that most incoming RF noise is not linear in relation
to the incoming signal. By using a combination of Volume and RF Gain, one
can make an inaudible signal fully readable. The goal of the RF Gain control
is to get rid of the noise (or most of it) BEFORE the amplifier amplifies
the remaining signal.

I usually run my RF Gain at 50% and adjust from there.

 
 
 

Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Post by Paul Vanass » Fri, 30 May 2003 20:49:40

Addendum to Post:

Of course, all of the aforementioned juxtaposed and in contrast of an
Attenuation circuit on a radio which is designed to prevent the front end of
a radio from overloading. In the old AM days, incoming signal overloading
was very common, this distorting the signal (and sometimes damaging the
radio). The only time I use my attenuator is when I'm desperately trying to
get rid of serious noise by using a combination of Attenuation, RF Gain,
Notch, and Volume controls. If all this doesn't work, it's usually time to
switch over to DSP via the radio itself or a PC.

 
 
 

Using Input Attenuation (button) vs Using RF Gain Pot For Attenuating Input Sig.: Differences ?

Post by Pete KE9O » Sat, 31 May 2003 03:34:50

Normally, I don't comment on these types of things, but in this case, I
will.  If an input attenuator is used, the system NF will be degraded by the
amount of attenuation that is switched in.  For instance, if you started out
with a 7dB NF, switching in a 10dB attenuator will degrade the NF to 17dB.
Now, for the I.F. gaiin control................while reducing the I.F. gain
can make the receiver more pleasant sounding for some folks, it does nothing
to improve the system NF.  The S/N ratio of the incoming signal that reaches
the antenna cannot be changed; we are talking about the ratio fo the
incoming signal to the escess noise of the environment, which would be
around 15 to 30dB, depending on the listener's location.
The only way that an I.F. gain control can improve the S/N ratio of a
receiver, is if the receiver is so poorly designed that the devices in the
I.F. system are biased to a point where they physically get hot, thus
creating excess thermal noise.  I haven't seen this happen yet, but that
doesn't mean that it can't.
On a final note, a couple of ways to improve the S/N ratio of a system are
by..................using a low noise RF amplifier ahead of the system, at
the antenna point of the feedline, or by narrow-banding the system, either
by using a selective, tuned front end, or by using narrower I.F. filters.
In the final two ways, you are narrowing the aperture of the system at two
different points, but only by using an RF amplifier, or by limiting the
front end bandwidth, can the system NF be improved.  A narrower I.F. filter
does nothing to improve the system NF, but it does give the impression of a
"quieter" system.
I hope this helps!

Pete


Quote:> Hello Bob:

> The RF Gain control is the most valuable device on a receiver. While most
> people keep the pot cranked up as far as it will go, many veteran radio
> enthusuasts recognize that most incoming RF noise is not linear in
relation
> to the incoming signal. By using a combination of Volume and RF Gain, one
> can make an inaudible signal fully readable. The goal of the RF Gain
control
> is to get rid of the noise (or most of it) BEFORE the amplifier amplifies
> the remaining signal.

> I usually run my RF Gain at 50% and adjust from there.