Re: Best Gain Settings for 137 mhz reception


jdow
 

The U310 is my favorite goto RF JFET. Properly used it can perform WONDERFULLY. Common gate push-pull has very nice IMD properties, especially for even order IMD which is naturally fairly high in JFETs as they are square law devices.

{^_^}

On 2017-09-09 09:19, David Eckhardt wrote:
Here is the link to Chip's U310 (metal case) preamps.  The J-310 works as well.
http://www.ham-radio.com/n6ca/50MHz/50appnotes/U310.html
On Sat, Sep 9, 2017 at 9:54 AM, jdow <jdow@earthlink.net <mailto:jdow@earthlink.net>> wrote:
LNA4All has a lot of gain. In most situations 5 dB gain is enough to set
system noise figure well enough. 10 dB might be needed if you have a long
coax run. With 20 dB you almost certainly have too much gain involved. That
causes IMD from digital saturation effects without improving reception
particularly. For beset results you need to play with the individual gain
settings on the R820T2 chip. It will still be a little problematic. So a
specialized 162.500 MHz filter might be your way out of trouble. A gain
setting around 10 would be WAY WAY too high, regardless. I have a 20 dB
preamp here and have the expected problems with it. I need to use some
subtle filtering to just make up the cable loss and about 6 or 7 dB of extra
gain over the amplifier's useful range.
If GB is anything like the US pager transmitters are your likely source of
problems. "Good luck" filtering them They are witnin band for anything other
than SAW filters or very selective filters based on helical resonators. (The
latter are fun to play with. But, the mechanical design for such filters is
awkward.)
If you can find the major source of RF energy you can combine a selecting
162.5 MHz filter with a tuned coaxial stub to null the interference source.
And for God's sake, reduce the AirSpy gain - a lot. (I use around minimum
settings on R820T2 based RTLSDR dongles, which have even worse problems with
overload.)
In the final analysis, the number of digitized bits is going to set your
usable instantaneous dynamic range. If you improve noise figure 4 dB that
means interfering sources that just barely cause overload before will be on
the order of 7 dB into overload. (4 dB for NF improvement and 3 dB or more
for the extra gain needed to "set" the noise figure below the R820T2 noise
figure.)
Mumble mumble - where can I get a 10 dB attenuator that passes DC with no
attenuation? That's a start....
{^_-}   Joanne
On 2017-09-09 07:28, David J Taylor via Groups.Io wrote:
From: Ruben Navarro Huedo (EA5BZ)
Hello friends:
I am trying my Airspy R2 receiving NOAA and Meteor M2 sats.
My setup is:
Wimo TA-1 Turnstile Antenna (http://www.wimo.com/scanner-antennas_s.html
<http://www.wimo.com/scanner-antennas_s.html>) --> LNA4ALL --> Around 10
meters Aircell 7 coax --> Airspy
I have been trying with Linearity and using a Gain value around 10.
If i use a higher gain value i start to have some intermittent BCFM
interferences.
My question is:
Could i obtain better SNR values using Free Gain mode? What adjustment
is the best?
How do you think i could obtain better SNR values? Using another LNA?
Using a BCFM filter?
Thank's a lot for your help.
=======================================
Ruben,
Are you sure you need any extra gain?  I'm testing with Linearity, Gain:
14, Sample rate: 3 Msps, Decimation: 64, using an Airspy Mini.  Are you
sure what's causing the interference?  I get pager transmissions on both
138 and 153 MHz which show up as bands - some with a near 1 minute
repetition interval.
Ideally a filtered pre-amp, such as:
https://store.uputronics.com/index.php?route=product/product&path=59&product_id=94
<https://store.uputronics.com/index.php?route=product/product&path=59&product_id=94>
but unfortunately this is out of stock.  Therefore I would go for a
broadcast FM reject filter.  I see that Amazon offer one from a Greek
amateur - SV1AFN:
https://www.amazon.co.uk/FM-Notch-88-108-reject-receivers/dp/B074S8JBBZ/
<https://www.amazon.co.uk/FM-Notch-88-108-reject-receivers/dp/B074S8JBBZ/>
All being well the forthcoming Airspy HF+ will perform better.
73,
David GM8ARV
--
*Dave - WØLEV
*
*/Just Let Darwin Work/*

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