On Fri, 2008-02-01 at 21:56 -0600, Bob McGraw - K4TAX wrote:
> I've been reading this thread with interest. There's a couple of things
> that I'd like to point out.
>
> First, there is clearly the need to run to audio gain at a level for
> comfortable listening. However, running the gain "wide open" or at a very
> high setting does leave one exposed to likely ear damage, headphone damage
> or speaker damage in the event of some accidental feedback condition that
> might develop. Be careful, headphones are about the most dangerous device
> one can place over or in our ears.
I believe there needs to be at least two gain control loops in the
modern radio. For user comfort the output needs control, the classic
AVC. Not necessarily so strong control it removes variations in signal,
but good enough it brings up the weak signals, but prevents ear
blasting from strong signals. This needs to work only on the actual
signal bandwidth at the speaker or headphones. It should not pump on
rejected (notched or outside the audio bandpass) signals. Then the
digital and analog front end needs AGC to keep the level of the spectrum
applied to the A/D(s) as high as possible without exceeding the top end
of the A/D dynamic range.
>
> As to RF gain, keep in mind that the A-D and D-A processing clearly has a
> limit as to the dynamic range it can accurately handle. Exceed this range
> and the system will produce all sorts of rude sounds and results. Now to
> keep the A-D and D-A systems within their correct signal handling range
> there is an internal AGC function that is not user accessible. At the same
> time in order to maintain the highest S/N ratio for reception, some type of
> gain control system is required, thus enter the user AGC system. That
> system too has its limits of input dynamic range vs. output dynamic range.
> Now to operate within that range comes the "human interface" thus the
> dreaded manual RF Gain control.
It appears to me that one AGC system is trying and failing to accomplish
both tasks.
>
> The noise floor of these radios is extremely low. Clearly many dB below the
> atmospheric noise and antenna thermal noise. This is necessary to "hear the
> weak ones". At the same time the gain of the radio, input to output is
> quite high. Again needed to hear the weak ones. Typically I find that
> after setting the AF gain at a modest level, it is then quite easy to find a
> RF gain setting for the band conditions and signals present. I really find
> that I don't have to "ride the RF Gain" when tuning the bands. The internal
> AGC systems handle the signals quite well. Changing bands or antennas
> systems or even sometimes beam heading changes the noise floor and thus some
> minor RF gain adjustment is needed.
>
> Perhaps we are not being clear in out terminology between AVC and AGC. In
> case you thought so, they aren't the same. Today's digital radios don't
> have AVC systems so don't expect the AGC system to do what a AVC system
> does.
>
> I'm still not convinced that the PRE AMP on condition is really needed for
> most HF operation as it provides some 6 to 24 dB of gain. In most cases on
> HF, again atmospheric noise will be the weak signal limiting factor.
Unless the antenna efficiency is very poor, such as a Beverage or very
small active antenna.
> There
> are certain exceptions that apply. However, adding gain via the PRE AMP
> will raise the signal and will also raise the noise thus reducing the
> dynamic range that can be handled by the receiver AGC systems. In my case
> the use of a PRE AMP is one that is mounted at the antenna and its purpose
> it to make up for the signal loss in the feedline between the antenna and
> the receiver thus maintaining or improving the NF of the receiver. I
> believe we must get away from the thinking that more gain will allow us to
> hear weaker signals. Actually it is more gain and less noise that
> accomplishes this task. Thus, reduction in RF Gain is the objective.
>
> 73
> Bob, K4TAX
>
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|