----- Original Message ----- From: "Curt Nixon" [email protected] To: "i8cvs" [email protected] Cc: "AMSAT-BB" [email protected] 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
and
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
http://users.tpg.com.au/users/ldbutler/AmpNoise.htm
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