I have built a number of DEM pre-amp kits I am fortunate enough to have the loan of a Noise measuring set - & have found that the tuning for lowest noise & highest gain are often WAY apart 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)
Re MMICs - I have built a large number of these for various freqs & purpose, using DEM PCBs Besides the poor NF, these amps are VERY broadband, totally unsuitable for head-end preamp (IMHO <g>)
Rod Adkins WI6M nr San Diego 17917 Lyons Valley Road Jamul, CA 91935
[Original Message] From: Curt Nixon cptcurt@flash.net Cc: amsat-bb@amsat.org Date: 11/26/2008 7:19:46 AM Subject: [amsat-bb] MMIC for LEO preamp?
SO...now I have made a few contacts on AO-51 and SO-50. Now I need to get better ears.
I have a very simple mode J setup using a couple of old mobil rigs--the rx unit is an IC 2350H. 10 el cheap yagi through 60ft of LMR400.
I'm considering options for a mast preamp for rx only. I'll be using a diplexer for desense ahead of it, and eventually add coaxial switching so I can use TX.
I'll feed with power through extra wire in rotor cable.
Looking at the Hamtonics, DEMI kit and a hombrew from scratch.
Any experiences with the DEMI units as kits?
with there relatively high NF, are the MMIC devices like the ERA
series, at all appropriate for simple mast preamps for entry level application?
Thanks to all.
Curt KU8L _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
----- Original Message ----- From: "Rod Adkins" adkins92rod@earthlink.net To: "Curt Nixon" cptcurt@flash.net Cc: amsat-bb@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
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
----- Original Message ----- From: "Curt Nixon" cptcurt@flash.net To: "i8cvs" domenico.i8cvs@tin.it Cc: "AMSAT-BB" amsat-bb@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
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
----- Original Message ----- From: "Curt Nixon" cptcurt@flash.net To: "i8cvs" domenico.i8cvs@tin.it Cc: "AMSAT-BB" amsat-bb@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
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
In my previous message I wrote:
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.
I would like to add:
Supposing that in OFF condition you read 2 volt and 5 volt in ON condition of the noise source it happen that during tuning procedure the gain of the preamplifier changes so that the voltage in OFF condition changes as well but you cannot move anything in the setup so that it is best to look for the MAXIMUM variation of the ON/OFF voltages as a RATIO instead of the difference because the ratio Von/Voff is indepentent from the gain and greater the ratio lover the NF
73" de
i8CVS Domenico
participants (3)
-
Curt Nixon -
i8cvs -
Rod Adkins