Re: Best Gain Settings for 137 mhz reception


W0LEV
 

"Free" gain setting allows the user to set the RF, IF, and mixer gains independently. 

I am now on my third AirSpy.  The first two, AirSpy Ones, are laid to ruin by close-by lightning strikes (front end damage and USB loss).  So far......  ALL have (and do) need an FM Stopband filter before feeding RF to the input SMA connector.  The strongest FM signal here at my location is -35 dBm on the antenna I use for 20.1 MHz solar and Jupiter emissions.  That isn't 'too' strong as signals go, but its enough with the other stations in the FM band, all less than -35 dBm, to cause several intermod products to show up in a frequency slice from 18 through 26 MHz from the AirSpy.  The filter I use is -40 dB throughout the FM band.  With that, I have no problems with FM broadcast.

Here is another solution from AirSpy.US:


These preamps are filtered for the band of interest and will reduce the problem with the FM BC band while still giving  you better S/N (nower NF).  From my measurements, the AirSpy typically runs with a NF around 4 to 5 dB.  They claim better with the versioin 2, but I have not measured mine.  The NF is also pretty constant over the advertised frequency range.  This is more than adequate for HF, but at and above 50 MHz, you can gain better S/N by using a filtered, 'reasonable' gain preamp.  Spend the $53 from AirSpy America and cure all your problems.  Personally, I'd design and build what I need, but not everyone has the test equipment to assure what they build is what was intended in the final product.   

BTW:  The FM SBF filters from Amazon have a rather marginal track record.  Some work, others don't, loose components, bad solder - typical Chinese start-ups.

Dave - WØLEV

 

On Sat, Sep 9, 2017 at 1:40 PM, doug <dmcgarrett@...> wrote:

On 09/09/2017 09:01 AM, Ruben Navarro Huedo (EA5BZ) wrote:
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) --> 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.
A filter will not improve SNR unless your SNR is being limited by interference--i.e., overload by local FM stations. If the FM is not overloading your receiver, your SNR is
being determined by the noise figure of the front end. You haven't mentioned whether you're using a preamp now. If so, the SNR is determined by the NF of the
preamp AND its gain. If you add a preamp and the FM interference goes up to the point of receiver overload, then put a filter AFTER the preamp. Of course, the
preamp itself must not overload on out-of-band signals like FM, aircraft radio, etc. You need a preamp with a high compression point.
I don't know what free gain mode is. What I have told you is general knowledge of receiver performance that applies to any receiving system.
--Doug, WA2SAY, retired RF engineer.





--
Dave - WØLEV
Just Let Darwin Work

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