[hpsdr] Alex Filter Switching Frequencies

[Scott Cowling]

Hi Phil,

I suggest that we implement #2 (using Lyle's suggested frequencies), 
which would fix the "early switching" problem and keep MARS operators happy.

We can then add an additional bit to the C0 command 0b0001001x in bit 
6 of byte C2 called

"Alex - Enable manual filter selection independent of frequency 
(0=disable, 1=enable)"

Then bytes C3 and C4 can contain the filter selections.  If 16 bits 
is not enough, then we can add another C0 command to select the filters.

In the Mercury code, when the "manual filter enable bit" (which is 
normally always zero in current implementations) is set high, you 
prevent the frequency from selecting the filter.

As long as the "manual filter enable bit" is zero, the manual 
selection bits in C3 and C4 (or the selection bits in the additional 
C0 command) are ignored and filter selection is done as it is now.

This would allow all existing software to work as-is, but allow new 
software (or updates to existing software) to set the filters 
manually if desired.

You could even put a button on the software GUI for "auto/manual" 
select of the filters.

I can write up an addendum to the USB protocol document to cover this 
if that is the direction we decide to go.

73,
Scotty WA2DFI


At 20:09 2011-09-09 +0800, Phil Harman wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>All,
>
>Good to see that Alex is making its way into users hands and appears 
>to be working OK.
>
>We need to review just how the various filters on the Alex boards 
>are selected .
>
>When I first wrote the Mercury FPGA code that drives Alex,  PowerSDR 
>was using a 9kHz IF.  Hence the frequency sent to the HPSDR boards 
>was the indicated frequency less 9kHz.  In the Mercury FPGA code I 
>added 9kHz to the frequency being sent by PowerSDR and used this to 
>select the approproate HPF and LPF pair.
>
>The idea was that the 'intelligence' as to what filters to select 
>was in the FPGA code and there was no need for the PC code to send 
>anything more than the required frequency - which it did by default.
>
>I gather that the latest versions of PowerSDR use a 0Hz IF in which 
>case the filter selection will be incorrect at the high end of some 
>bands. A while ago I did try setting the IF frequency to 0Hz in 
>PowerSDR but from memory I found the AM demodulator was distorting 
>at zero beat. I can't reproduce this problem so assume it has been fixed.
>
>We have a number of options in relation to how the filters are 
>selected - these include:
>
>1. Simply remove the 9kHz offset code in the Mercury FPGA code and 
>switch filters at the current frequencies. The current logic is:-
>
>For the HPFs
>if (frequency < 1.5MHz)  bypass
>
>else if (frequency < 6.5MHz)  1.5MHz HPF
>
>else if (frequency < 9.5MHz) 6.5MHz HPF
>
>else if (frequency < 13MHz)  9.5MHz HPF
>
>else if (frequency < 20MHz)  13MHz HPF
>
>else 20MHz HPF
>
>and for the LPFs
>
>if (frequency > 29.7MHz) 6m LPF
>
>else if (frequency > 21.45MHz) 12/10m LPF
>
>else if (frequency > 14.35MHz) 17/15m LPF
>
>else if (frequency > 7.3MHz) 30/20m LPF
>
>else if (frequency > 4MHz) 60/40m LPF
>
>else if (frequency > 2MHz)  80m LPF
>
>else 160m LPF
>
>2. Remove the 9kHz offset and alter the frequencies that the filters 
>switch at. This would help MARS operators.
>
>3. Remove the filter slection from the FPGA code and instead allow 
>the PC code to specifically select the filters. This could be simply 
>linked to the Band selected or a full GUI similar to that used to 
>select the Penny(Lane) open collector outputs or the Alex antenna selections.
>
>When working through the above we also need to consider split/full 
>duplex operation and the use of transverters.
>
>Perhaps we could have a preliminary discussion following the 
>Teamspeak session on Friday (USA) evening followed by some 
>brainstorming via the reflector?
>
>73 Phil....VK6APH
>
>
>
>
>
>
>
>
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