Re: [amsat-bb] Downconverter noise

[Scott Townley <nx7u@xxxxxxxx>]

Available noise power output from any active device can be stated as:
FGkTB
where F is the noise figure referred to the input (which is how it's 
usually specified), and G is the available gain.
For the AIDC-series, assuming available gain=37dB and the noise figure is 
~1dB (but you can see from the equation that a with a gain of 37dB, that 
even several tenths variation in the NF makes no difference) the available 
noise power is -102dBm (2.5kHz RX bandwidth).
What your S-meter does with this obviously varies, but -102dBm is 
significantly higher than your RX noise floor!
With the dish fitted, the available noise power should increase slightly, 
as antenna/sky noise have been added.  But for a "good" system it will not 
be detectable on your S-meter.
All that noise is why on some receivers, it's not a bad idea to pad down 
the IF.  Remember that the bandlimiting devices in your receiver are 
*after* the RF front end--so if you're the RF amp/1st mixer then B is much 
much higher, and the noise power seen is proportionally much higher 
too.  Too much noise power leads to saturation of the mixer and lots of 
other bad things :-)


At 04:27 2003-05-27, Jon Ogden wrote:

>On Tuesday, May 27, 2003, at 01:53 America/Chicago, Howard Long wrote:
>
>>I might have misunderstood the question, but of the 50 or so AIDC 3731's
>>I've distributed here in the UK, all of them have produced a very
>>significant increase in noise level and S meter reading when hooked up to
>>the half a dozen different radios I've used them on (and no antenna, dish or
>>feed connected). S5 is about ball park but due to the vagueries of S meters
>>it might be S3 or it might be S7.
>
>Well, then I guess I stand corrected.  But it just doesn't make sense.
>If the d/c is at S5 with no antenna and no signal, then you are already at 
>least 10 to 15 dB above the noise floor of your radio.  Does the noise 
>actually go DOWN once the antenna is mounted on the dish and put in place?
>
>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'm confused..
>
>73,
>
>Jon
>NA9D
>
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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

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