SDRplay Community Forum

I have found the Gain Reduction control but this doesn't work like a typical RF Gain on a conventional receiver. RF Gain controls flatten the signal and reduce the signal-to-noise relationship. Is there any way to do this using HDSDR?

Dear kb9rz,
The RSP gain control is indeed different from a conventional receiver. The first thing to note is that what is controlled is gain reduction from Maximum gain and not gain as such. If you are using HDSDR, with the slider at the top of its adjustment rang and the Gain reduction setting at 0 dB, you are at maximum gain. This will be far to high for any real world use case and so you will need to reduce the gain by anything up to 60 or 70 dB or maybe more depending upon the band you are operating in and the type of antenna you are using.
The second point to note is that the LNA gain is not controlled directly. The LNA has a single gain step and the point at which this step is applied within the overall gain reduction curve is controlled by the LNA trip threshold. The default setting is 59 dB, which means that the overall receiver gain will have to be reduced by more than 59 dB before the LNA gain reduction step is applied.

More details behind the way gain control and AGC works on the RSP can be found at:

http://www.sdrplay.com/docs/SDRplay_AGC ... e_r2p2.pdf

Sincerely

SDRplay Tech_Support

Dear SDRPlay team,

I've read the technical note about the AGC and made some tests. From that I noticed that the LNA On causes ghost signals (from non-linearity or overload) in the shortwave ranges, even at the moment I do only have a Discone on roof as antenna (which is not a powerful shortwave antenna at all). Switching the LNA On populates 10m with BC stations, switching it off all is fine.
So, one could switch the LNA off and all is fine ... unfortunately, on the VHF/UHF bands the LNA helps to increase the SNR, so in most cases it should be switched on in these bands.
It would be fine if this frequency dependent LNA on/off switching (or the LNA threshold, as it is implemented now) could be done automatically in the extio.dll. Just to be able to define different LNA settings for different frequencies. If you would go this way, the next small step is to make the mixer gain control also frequency dependent. Then, one could program the right LNA/gain settings that fits exactly to the antenna used.
What do you think, does this make sense?
I've seen that in the SDRConsole is something similar done, isn't it? However, I could not reproduce the same effects as using your extio.dll and HDSDR. Maybe the implementation in SDRConsole is still buggy.

Eckhard

Hi Eckhard,

I don't think this would be a good idea because the situation is very dependent on antenna used any many other factors. For example with my (very badly situated) antenna the LNA even helps with false reception AM broadcasts, because in my case these are caused by some spurs originating inside the RSP between the baseband amp and the LNA. Redistributing gain from the baseband amp to the LNA helps in my case.

edit: sorry, I've read your post too fast at first, I see you meant it configurable. That way it's a good idea.

It would be good if you could modify your DLL so that it emulates how conventional AGC works, and put a "RF Gain" control under the ExtIO button. This is certainly doable technically. (I'll help you do it if you want.) Otherwise the experience is not a good one for me on SSB, personally, I'm afraid. I like everything so far except this.

Dear SDRPlay team,

thank you very much for introducing the preset profiles into the extio.dll GUI! That's really nice to have!
One question about that: would it also be possible to make it automatically frequency depended switched? Just define lower and upper frequency boarders a certain profile should become valid? That would make it even more easy for the user to handle the characteristics of the used antenna. However, if you would do this, the bandwidth (resp. sample rate) should not be stored within the profile!
It seems that the extio.dll interface (at least for HDSDR) supports reading back the LO. Found this code on HDSDR webpage in FAQ section:

long EXTIO_API GetHWLO(void)
{
return (long)( glLOfreq & 0xFFFFFFFF );
}

What do you think, would this be possible?

Thank you very much for your hard work!

73 Eckhard

Thank you for your input. I guess that this would mean that you would have to set the frequency before loading the profile rather than switching to the profile you want and then setting the frequency. I'm not sure at this point that we want to apply that restriction but I see what your saying.

We will be reviewing how the profiles are working and make some improvements in coming releases. I will make a note of your request to be looked at for that review.

Thanks and best regards,

SDRplay Support