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

[i8cvs]

----- Original Message -----
From: "Rod Adkins" <adkins92rod at earthlink.net>
To: "Curt Nixon" <cptcurt at flash.net>
Cc: <amsat-bb at amsat.org>
Sent: Wednesday, November 26, 2008 4:45 PM
Subject: [amsat-bb] Re: MMIC for LEO preamp?

> I have built a number of DEM pre-amp kits

Hi Rod, WI6M

For many years beginning 1970 I have been buiding many low noise
preamplifiers from 2 meters to 23 cm tuning it for the lovest Noise Figure
using a very simple noise generator with a semiconductor diode IN21 of
the type depicted in the schematic diagram Fig-8 of the folloving paper.

http://users.tpg.com.au/users/ldbutler/AmpNoise.htm

> I am fortunate enough to have the loan of a Noise measuring set

Back in 1990 I have duplicated the Precision Automatic Noise Figure Meter
PANFI described by DJ9BV in DUBUS magazine 2/90 and 3/90 good up
to 10 GHz and I am very happy with it

> - & have found that the tuning for lowest noise & highest gain are often
>  WAY apart.

I agree with you but using  the above simple diode noise generator as
depicted in Fig-8 it is possible to manually adjusts a preamplifier for the
lovest Noise Figure even if at the end of the adjustement you don't know the
value of it in dB because the noise generator is not calibrated

The procedure to be used is a little bit tedious but at the end of the job
you will get the lovest possible Noise Figure obtainable from the
preamplifier under test.

1) Connect the noise source in front of your preamplifier or converter
2) Connect the output of your preamplifier to receiver or converter input
3) Switch OFF the AGC of you receiver and select mode SSB or CW

Note: In modern VHF/UHF receivers it is probably impossible to switch
          OFF the AGC but it is possible to connect the preamplifier in
          front of a downconverter with a 28 MHz IF output and then go to
          any old HF receiver normally using the AGC ON/OFF switching
          facility.

4) Adjust the receiver audio control to maximum
5) Adjust the receiver RF gain to minimum
6) Connect an analog AC voltmeter to the audio output of your receiver
     preferably the high impedance jack for the headphone in order to get
     a high AC voltage and if impossible interpose a small step up
     transformer between the regular loudspeaker jack and the AC voltmeter

7) Switch  ON the noise source and increase the RF control in order
     to get a confortable reading of the noise level on  the AC voltmeter
     for example in the order of  5 volt AC and take note of it

6) Without moving the setting of the RF control switch OFF the noise
     source and take note of the lover voltage level for example 3 volt AC

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

> My input would be - if you have no way to measure noise performance, buy
> the ready-built preamp - I have also gone this route, & been very
> satisfied (usual disclaimer)

> Rod Adkins WI6M nr San Diego
> 17917 Lyons Valley Road
> Jamul, CA 91935
>
My advise is to build the above noise sorce in Fig-8 and start to play with
it with the only precaution to replace the 50 ohm output resistor with
four 200 ohm resistors in parallel directly soldered like a star to the N
or BNC output connector using the shorter possible wires.
Using 1/4 watt carbon resistors from Allen Bradley helps to reduce
the associated inductance of resistor in order to get the output impedance
of the noise source as close as possible to 50+/-j0 ohm
Wenever is possible use a 6 dB or more fixed attenuator between the
noise source and the preamplifier input because the input impedance
of a preamplifier particularly GaAsFET when adjusted for the lovest NF is
WAY apart from 50 ohm so that the input VSWR is very high and a
50 ohm attenuator in between helps for many reasons.
Do not solder the cartridge of a diode type IN21 but use instead two
small copper spring like a fuse to old it in place.

I hope this helps the beginners to save money buying already made
commercial preamplifiers or of coarse to repair it and tuning it for
the lovest possible NF using the Ham ingenuity and a very inexpencive
noise source all  considering that the rest is needed  is  a receiver
and a AC voltmetyer and they are already available in any Ham shak

73" de

i8CVS Domenico