Re: Best Gain Settings for 137 mhz reception
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.toggle quoted messageShow quoted text
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....
On 2017-09-09 07:28, David J Taylor via Groups.Io wrote:
From: Ruben Navarro Huedo (EA5BZ)