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RE: Downconverter noise

At 01:25 PM 5/27/2003 +0100, Howard Long wrote:
>Hi Jon
>> I can tell you that form my experience you do hear an increase in noise
>> with the d/c on, but it certainly isn't enough to move the s-meter more
>> than a unit or less.  It's certainly not 5 units.
>I wonder how much gain your d/c has? The 37dB of the AIDC 3731/3733 is very
>high - many of us pop in 10-20dB of attenuation in the IF. Maybe your d/c
>has lower gain?
>The point of the d/c gain is to make the noise figure of the IF receiver and
>the feeder between it and the d/c become insignificant - and 37dB is
>certainly more than enough! The d/c's front end noise is being amplified too
>of course, which is what you're hearing in your radio when you plug in the
>Either way I think we're agreed that you should be able to hear a
>significant noise increase when the d/c is plugged into the receiver, with
>or without an antenna attached.
>Cheers & 73, Howard G6LVB

Howard has it right.  What you hear when you apply power to your convertor
(or preamp) without an antenna is the internally generated noise (the noise
figure is the measurement of this).  The higher your NF and gain the more
noise your receiver sees when you do this.  If you have a low noise preamp
ahead of the convertor then it amplies at a lower NF, but still there
should be a small detectable rise in noise when it is powered.  If you have
a fairly good system NF you should see a rise in noise when you connect a
dummy load (termination) at the input.  This is because the termination is
putting out roughly 290K thermal noise.  Your system Noise Temp should be
around 100K.  Connect an antenna and point it at the ground and you should
see the same small rise in noise (about 1/2 s-unit) if you have a good low
noise setup.  Point it straight up at the sky and it should drop off 1/2

The only thing that I can think that would affect this is a having a very
poor IF receiver (low sensistivity, gain).  If you are not able to detect
ground noise then your system NF is not good.

One of the standard ways of quickly checking a convertor or preamp is
hearing this noise rise.  My MKU-232/Drake combo rises only about 1 s-unit
at power up (with antenna).  The preamp has 40 dB gain and convertor only
about 15 dB gain.  I get about the same rise with my 2m EME preamp (two
cascaded ones with 31 dB total gain).  This is using a FT-847 with the
internal preamp off, agc and RFG control at max.

Hope this helps.

Ed - AL7EB

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