[CQ-Contest] Re: NAQP Power and AGC

[T A RUSSELL]

On Fri, 14 Aug 1998 01:20:25 +0100 "Barry Kutner" <w2up at mindspring.com>
writes:
>
>On 13 Aug 98,, AD6E at aol.com <AD6E at aol.com> wrote:
>
>> 
>> Hi again Tom,
>> 
>> I found a real expert in this field: AE0M works in the hearing aid 
>industry
>> and is well versed in the human hearing field.  Tony told me that 
>for
>> unrelated voice applications (such as telephones) there probably 
>needs to be 2
>> or 3 dB difference to be able to say that one is louder than the 
>other.
>> However, for a "trained ear", and that probably includes all of us, 
>it is very
>> possible to distinguish strength differences betweens two tones in a 
>noisy
>> environment with only a few tenths of a dB difference.  I that means 
>it really
>> does makes a difference between running 100W and 150W after all. 
>Guess I'm
>> lucky no one took me up on that challange... hi
>> 
>> 
>Al - You are forgetting about the effect of AGC. I presume most, if 
>not all, of us use the AGC almost all the time. Since this pretty 
>much equalizes the audio level of what we hear, the small power 
>differences shouldn't make a difference.
>73 Barry
>--

The AGC will equilaze audio output from different signals ONLY 
when there is only one signal in the receiver passband.

When there are several signals in the passband, the STRONGEST
signal will set the AGC voltage which controls the receiver gain and
weaker signals will produce less output.  

This assumes that the  AGC time constants are longer than time 
differences between signals.  If the AGC time constants are TOO 
short, it will follow varying signal levels which makes it very difficult
to sort out different signals on the same frequency.  If AGC time
contants
are LONG, weak signals are not heard in pileups.

My preferred receiver operating condition is to reduce the RF gain
control, which raises the AGC threshold, leaving the receiver in a
LINEAR mode below AGC threshold.  This way, there is no AGC
pumping and I can hear weaker signals during gaps or pauses
from stronger stations, such as when trying to hear a DX station
returning under a big on-frequency pileup.  Guys who use SLOW
AGC are often clueless as to when the DX station replies, until
everyone else finally stops transmitting.

de  Tom  N4KG




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