Filter Selection

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NN4F
Posts: 128
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Location: Ridgeville, SC
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Filter Selection

Postby NN4F » Tue Aug 25, 2015 5:14 pm

Hey Tech Support...

Question about the built in filters, are the selected by the LO frequency, i.e If I'm listening to the USAF on 11.175 and my LO is at 8Mhz, I'm sure it's the 0-12 Mhz filter, but if I'm on 11.175Mhz, and the LO is at 13Mhz, am I using the 12-30Mhz filter??? Hmmmm, what causes the filter choice, I can't see any difference in operation at the moment...

Paul - NN4F
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Paul Jones - NN4F, Charleston SC
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Tech_Support
Posts: 199
Joined: Mon Jun 01, 2015 7:00 pm

Re: Filter Selection

Postby Tech_Support » Tue Aug 25, 2015 7:05 pm

Hello Paul,
That is a very interesting question and to answer it with certainty I would have to check with the guy that defined the spec for the API. Unfortunately, he is away on vacation until next week, so please bear with me. What I think will occur is that if the LO is set to below 12 MHz, then the 0-12 MHz filter would be selected and if above 12 MHz, the 12-30 MHz filter would be selected. Obviously the filter isn't a brick wall filter, so even at the widest bandwidth the effect of the filter might be hard to see at the transition point. Let me make a note of this issue and get back to you with a more definitive answer next week.

Sincerely

SDRplay Tech_Support
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Tech_Support
Posts: 199
Joined: Mon Jun 01, 2015 7:00 pm

Re: Filter Selection

Postby Tech_Support » Tue Sep 01, 2015 1:49 pm

Hello again Paul,
I can confirm that the LO setting is used to determine which of the RF filters is selected. A 1 Hz resolution is used, so 11.999999 MHz setting will select the 0-12 MHz filter and 12 MHz will select the 12-30 MHz filter.

Incidentally, when developing the new API, we also found a bug which meant that the correct filter was not necessarily being selected. This bug has now been corrected, but it might explain why some people have found that the sensitivity appears to be a bit better with the new API.

Sincerely

Tech_Support
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F1BJB
Posts: 104
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: Filter Selection

Postby F1BJB » Wed Sep 09, 2015 7:27 am

Tech_Support wrote:I can confirm that the LO setting is used to determine which of the RF filters is selected. A 1 Hz resolution is used, so 11.999999 MHz setting will select the 0-12 MHz filter and 12 MHz will select the 12-30 MHz filter.

Hi
I think the usual way of paralel switching overlapping octave filters is wrong.
It would be more usefull to use cascade switching so that in the overlapping region both adjacent filters could be selected.
It would introduce a few problems like gain correction but using two banks of odd/even paralel filters
each having a bypass possibility in cascade could help.
In fact the desing of SDRplay is not the most suited to this modification because ,depending on frequency,
RF path are sometimes different.
A nice addition would be some sort of output ,may be of the open collector type, to allow users to switch
external filters , antennas or preamps.
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BrianG61UK
Posts: 1
Joined: Sun Jan 31, 2016 8:21 pm

Re: Filter Selection

Postby BrianG61UK » Sun Jan 31, 2016 8:40 pm

F1BJB wrote:
Tech_Support wrote:I can confirm that the LO setting is used to determine which of the RF filters is selected. A 1 Hz resolution is used, so 11.999999 MHz setting will select the 0-12 MHz filter and 12 MHz will select the 12-30 MHz filter.

Hi
I think the usual way of paralel switching overlapping octave filters is wrong.
It would be more usefull to use cascade switching so that in the overlapping region both adjacent filters could be selected.
It would introduce a few problems like gain correction but using two banks of odd/even paralel filters
each having a bypass possibility in cascade could help.
In fact the desing of SDRplay is not the most suited to this modification because ,depending on frequency,
RF path are sometimes different.
A nice addition would be some sort of output ,may be of the open collector type, to allow users to switch
external filters , antennas or preamps.


No need for outputs on the RSP. Just modify the software that's controlling the RSP to send info out via another port of some kind.

If you want to do something cleverer with the filters then I guess the best way would probably to create the required band pass by selecting both one of several low pass filters and one of several high pass filters. The relatively wide band pass filters currently used are almost definately made up of largely separate low pass and high pass parts anyway.
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