Hi Mike,
Well, there are many possible explanations (at least as many
explanations as there are circuit stages between the antenna port and
the audio output) for one rig to be noisier than the other.
Is it really noisier though? Suppose that both rigs have identical gains
and noise figures in all stages and that they have identical number of
stages and identical mixing schemes. (Yes, I know this is probably not
the case for the rigs you are comparing. I am using this "thought
experiment" to explain how one rig might "seem to be" noisier when they
are the same.) Now suppose that the AGC action in the two rigs is
markedly different. While the maximum total gain from RF input to audio
output of the two rigs is identical, when there is very low signal
input and the AGC is not yet controlling the gain, AND at a specific
signal level input (such as 50 uV) they may also have identical S meter
readings, and even identical gains, at some intermediate input signal
level one rig's AGC may be reducing it's gain more than the other. The
one with the higher gain at that intermediate input signal level will
seem noisier.
With the AGC operating it is really hard to know how noisy the rig
really is.
With the AGC defeated, you could put your 50 uV signal into each rig,
turn the RF gains up to maximum and adjust the audio gains for equal
measured audio output levels. Now you have the rigs adjusted for equal
total system gains (roughly, assuming that at this signal level the
receiver noise is insignificant) Now remove, or reduce to a real low
level, the input signal. The receiver with the higher measured audio
output level is the noisier receiver.
With the AGC operating you don't really know exactly what is going on.
DE N6KB
Now, using the same antenna.
The noise level on the Argonaut V was running between S-5 and S-9
On the trition, very little S-meter movement was seen and it was just
not that noisy
so I decided to test the receiver's sensitivity with the HP signal
generator. I injected the signal in place of the antennas
At 50uV I got S-9 on both rigs. At 1uV I could detect the signal. At .
5uV I could still hear the signal from the generator. Although the
triton had a harder time hearing at that level.
Applying the attn on the Argonaut V, reduces the noise level, but as
suspected reduces the signal strength of in coming signals, too
So bottom line? Why is the Argonaut V so much noisier than the old
Triton?
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