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Re: 2.4 GHZ Down Converter Noise VS IF Gain??



Hello Dave,

Noise figure is dominated by the first active device and the passive
losses ahead of it to the antenna, like feed line losses, relays,
filters, connectors, and any other passive loss.

If the RX downconverter has multiple stages, RF amplifier, mixer,
IF amp #1 and IF amp #2, then noise contribution from stages after
the first RF amplifier is very low, as long as each stage has gain.

Excess gain reduces the spurious free dynamic range, causing
spurious distortion products and possibly reduced performance from the
IF radio's AGC.

One way to reduce or even to determine if excess gain is present is 
to use a noise figure meter and to reduce stage gain until the NF is
reduced by 0.1 dB. Another method is to use a SINAD meter and reduce
stage gain until the SINAD reading just starts to be reduced.

SINAD is signal + noise + distortion. These meters are common in
communication service shops and cost 100's of $$ where the
noise figure meters cost 1000's of $$$.

Consider bypassing the downconverter IF amplifier stages not needed.

Stan, WA1ECF   Cape Cod   FN41





Dave wrote:
> 
> The combination of events has caused me to ponder the following any easy
> answer I hope for the RF noise figure Guru's
> 
> Many people including my self put attenuation between their receivers and
> their downconverters if for no other reason than to get the better apparent
> ( to the ear) signal to noise and to keep the downconverter from driving
> their receivers into clipping or at least into the non linear portion of
> the AGC's range.
> 
> Recently as some of you may recall during in a bout of experimentation I
> had a "stupid moment" and transmitted into MY Cal Amp 31732 frying the two
> IF transistors. While trouble shooting and reverse engineering the Cal Amp
> circuit ( I'm still looking for a schematic diagram) and ultimately
> replacing the IF transistors it became obvious to me that the IF circuit
> lends itself to having the gain adjusted as there are resistors from both
> emitters to ground. Increasing the value of these resistors  from their
> current vales of about 2 ohms in the first IF and 5 ohms in the second IF
> to a higher value would certainly lower the gain  (a relative easy thing to
> do). Both of the IF transistors are pretty "beefy" little jobs rated at
> around 1W dissipation each and in their current bias configuration they get
> warm to the touch!
> 
> So my question is this. What happens to the actual noise figure of the
> Downconverter if I lower the gain of the IF amps by increasing the  values
> of the emitter resistors? I know what happens at the interface to my 2M
> receiver/ IF it is much happier with out all that hiss in its face same as
> when I put in the attneuator.
> 
> Dave
> NR1DX
> nr1dx@arrl.net
> 
> "A man who picks a cat up by the tail learns a lesson he can learn no other
> way" .... Mark Twain
> 
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