Re: Best Gain Settings for 137 mhz reception


doug
 

On 09/13/2017 03:47 PM, jdow wrote:


On 2017-09-13 10:35, doug wrote:

On 09/13/2017 06:40 AM, Ruben Navarro Huedo (EA5BZ) wrote:

Today i have received the BCFM Notch filter and it is doing a GREAT Job.
Finally i bought RTL-SDR BCFM Filter.

One more question:
Where is the best place to install this filter?
My setup now is:
Antenna --> BCFM Filter --> LNA4ALL --> Coax --> Airspy R2
From the display, it seems to be doing its job. The FM station is 32 dB below your desired signal. Since the filter is a notch, it should have almost no attenuation in the desired band, and thus you have it
where it will do the most good. --doug, WA2SAY
selective cuts above
If the LNA4ALL is not being overloaded and the problem is simply AIRSPY saturation moving the filter after the LNA will purchase a modest noise figure improvement. If the LNA4ALL is generating IMD then the filter must go up front.

As a side point if the filter works placed after the LNA4ALL he can probably place it at the AIRSPY end of the coax. This will make it easier to take the filter out of the chain if he starts working on frequencies the BCFM stations are not a problem for the R820T.

{^_^} Joanne
Joanne is correct, of course, and in my earlier posts, I suggested putting the filter after the preamp, for obvious technical reasons, but in this case, it seems that the purpose of receiving the signals
the OP wants to hear is fulfilled as he/she desires. I don't think we need to go into a discussion of cascaded noise figure and third-order intercept here. It really is sufficient to almost all users to
connect things in whatever order works, and to explain, as Joanne and I have tried to do, what will work, and what might not. To possibly simplify things, I remember one contributor to this discussion
saying, use as little preamp gain as necessary to reduce overload problems. Amen to that!

--doug, WA2SAY

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