Re: Help with distorted transmit audio

[Steven.Zabarnick]

Jack's measurements of the K3 AGC on the link listed below are quite
interesting. As my noise level on the low bands is usually at least S3 or
4, his plots indicate that the noise (S3 is approx -90 dBm) is either in
the linear range of the AGC, or just barely in the software AGC range.
Don't we really want the band noise in the  flat part of the AGC curve,
down near -130 dBm? Would operating with the ATT on and/or the RF gain
turned down help move the band noise to this part of the curve, resulting
in a higher S/N ratio for signals just above the background noise?

Steve N9SZ



Barry N1EU wrote:

>Steven.Zabarnick wrote:
>>
>> I'm somewhat puzzled by the behavior of the AGC THR adjustment with
>> respect
>> to band noise. My understanding is that the AGC THR setting adjusts the
>> level at which the AGC is activated. Thus, I would expect when listening
>> to
>> band noise (QRN) that if the noise is activating the AGC, using a higher
>> number would prevent AGC activation, resulting in a lower background
band
>> noise.
>>
>A higher number means that AGC compression begins at a higher signal
level,
>so a higher number means you'll hear more noise.
>
>take a look at the graphs at
>http://www.cliftonlaboratories.com/elecraft_k3_agc_and_s-meter.htm#AGC_SLP_and_AGC_THR

>
>73,
>Barry N1EU


_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Don Wilhelm-4]

Steve,

If you attempt to use AGC to change the noise level, you will also be
potentially pushing all the signals down to that level as well (how much
so depends on the amount of AGC slope you have dialed in.

A receiver's sensitivity is measured at the antenna input end, and if I
understand correctly. what you are proposing can only change the level
at the output (speaker or headphones).
The receiver sensitivity can be related to its internal noise level (for
any given gain, more internal noise reduces the sensitivity at the
input) - and that is a very different thing than band noise.  The amount
of noise picked up by the antenna and fed to the receiver is effectively
'just another signal'.

If the band noise is a -90 dBm, one would ideally want the AGC threshold
to be at something above that - say -85 dBm for best reception (no
matter what the meters are indicating).  Set that way, signals will
activate the AGC and not the noise - the human ear/brain can perceive
signals down to (and below) the noise floor with a little practice.  
With band noise at -90 dBm, the K3 receiver sensitivity is still -140
dBm, but the band noise covers up actual signals until their level is
greater than -90 dBm, no receiver can change that situation.

If band noise activates the AGC, the receiver will not hear as well
because the AGC has reduced its gain.

Band noise is just another 'signal', but unlike real signals it is
broadband and unwanted.

73,
Don W3FPR

[hidden email] wrote:

> Jack's measurements of the K3 AGC on the link listed below are quite
> interesting. As my noise level on the low bands is usually at least S3 or
> 4, his plots indicate that the noise (S3 is approx -90 dBm) is either in
> the linear range of the AGC, or just barely in the software AGC range.
> Don't we really want the band noise in the  flat part of the AGC curve,
> down near -130 dBm? Would operating with the ATT on and/or the RF gain
> turned down help move the band noise to this part of the curve, resulting
> in a higher S/N ratio for signals just above the background noise?
>
> Steve N9SZ
>
>  
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com