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




----- Original Message -----
From: Dave <nr1dx@arrl.net>
To: <amsat-bb@amsat.org>
Sent: Wednesday, August 07, 2002 6:14 PM
Subject: [amsat-bb] 2.4 GHZ Down Converter Noise VS IF Gain??


> 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
>

Dave,

>From the theoretical point of  view the equation describing this matter has
been discussed already many time in BB but as a roule of tumble remember
this:

If powering up your converter the noise level  measured at the audio output
of your 2 meter receiver increases minimum by 10 dB than the overall noise
figure of your receiving system is not degraded and is very close to the
noise figure of your converter alone.

If powering up your converter the increase of noise level is less than 10 dB
than the overall noise figure of your receiving  system is degraded and it
is greater than that of  your converter alone.

If powering up your converter the increase of noise level  is more than 10
dB than the overall noise figure of your  receiving system  cannot be better
than that of your converter alone and your 2 meter receiver is not happy
because it can be overloaded and gain compressed.

To measure the noise increase at the audio output of your 2 meter receiver
the S-meter is a bad and meaningless instrument.

Use the following procedure:

1) Switch  OFF the AGC  of  2 meter receiver

2) Put the audio gain control knob full clockwise

3) Put the RF gain control  knob full counterclokwise

4) Connect an AC voltmeter 5 volt fs or less at the
    audio output

5) With converter power OFF adjust the RF gain control
     to a convenient noise level voltage for example 1 volt
     AC and call this voltage V1

6) Power converter ON and read the new voltage
     and call this voltage V2


The increase of noise level in dB is computed by

20 log      ( V2/V1)
           10


I hope this help a bit

Tank you for QSO CW/SSB on AO40 orbit 812

73" de i8CVS Domenico


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