[amsat-bb] Re: MMIC for LEO preamp?

i8cvs domenico.i8cvs at tin.it
Tue Dec 2 09:11:30 PST 2008

----- Original Message -----
From: "Curt Nixon" <cptcurt at flash.net>
To: "i8cvs" <domenico.i8cvs at tin.it>
Cc: "AMSAT-BB" <amsat-bb at amsat.org>
Sent: Tuesday, December 02, 2008 3:44 PM
Subject: Re: [amsat-bb] Re: MMIC for LEO preamp?

> Good Morning Domenico:
> In step 7 you wrote:
> 7)Without moving the audio and RF setting of the receiver start to play
>     tuning and adjusting everything is possible in your preamplifier i.e.
>     capacitors and inductances of the input network as well potentiometers
>     to set the active device current until switching the noise source ON
>    OFF the maximum variation on the AC voltmeter is reached  and this
>    condition correspond to the lovest possible Noise Figure obtainable
> Looking for MAXIMUM variation in noise...is this correct or should it be
looking for MINIMUM variation??
> Than ks..I'll give it a try.
> Curt
> KU8L
Hi Curt, KU8L

Switching the noise source from OFF to ON you must look for the
MAXIMUM variation in noise levels on the AC voltmeter.

During the above test the current flowing in the diode must be keep constant
so that as soon you get a comfortable reading possibly near full scale on
the AC volmeter with the source switched ON do not change the setting of the
potentiometer of the noise source described in the following Fig-8


When the source is OFF the white  noise generated by the 50 ohm resistor
is at room temperature of 290° kelvin and when the source is ON the white
noise generate by the diode and sent to the 50 ohm resistor is greater i.e.
it is like to keep the resistor at a higher temperature

Tuning the preamplifier for the lovest NF and switching the noise source
from OFF to ON if you reach a difference from say 2 volt to 5 volt it is
much better than if you get only a difference from say 2 to 4 volt because
switching the noise source for the same ON/OFF noise levels you get a
greater S/N ratio in the AC voltmeter  corresponding to a lover NF of
your preamplifier.

Please let me know the result of your testing

73" de

i8CVS Domenico

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