Re: [Discuss-gnuradio] Limited Rx BW on XCVR2450 daugtherboard

[Matt Ettus]

Hanwen,

Thanks for confirming the fix.  The slope you see with decimation 5 is 
indeed the fault of the CIC.  Because 5 is odd, we can't use a halfband 
filter, and instead need to use the CIC for all decimation.

Decimation or Interpolation which is a multiple of 4 will be the 
flattest, and multiples of 2 will be nearly as flat.  Odd numbers will 
always have that slope.

Matt

On 12/03/2009 06:10 AM, hanwen wrote:
> I've applied the fix to my receiver. It works. The attachment is the
> spectrum before and after the fixing at decimation rate 4 and 5. It
> shows at decimation rate of 4 the 3dB bandwidth is really widened from
> 7.5MHz to 9.5MHz.
> But when decimation rate is 5, the spectrum looks like an arch with over
> 10dB attenuation at age comparing with carrier center. I guess this
> imperfect frequency response is due to the CIC design. Is there a way of
> improve it?
>
> Thanks.
>
> 2009/11/26 Matt Ettus <address@hidden <mailto:address@hidden>>
>
>
>     Thanks for finding this bug.  I have applied your fix to my git
>     repo. I'm away from any equipment right now, but if someone could
>     confirm that this works, Johnathan can get it in the mainline code.
>
>     Matt
>
>
>
>     On 10/08/2009 01:26 PM, Stephen Lai wrote:
>
>         Hi All,
>
>         I've been having problems sending a 25 Msym/s BPSK stream
>         between two
>         USRP2's with XCVR2450 daughterboards. I've tracked it down to the Rx
>         circuitry limiting the passband to ~14 MHz when at least 20 MHz
>         should
>         be available.
>
>         Sifting through the db_xcvr2450.c/db_xcvr2450.cc files, I think
>         there
>         may be a bug which causes the baseband LPF in the MAX2829 device to
>         cutoff at 7.5 MHz instead of 9.5 MHz when the board is
>         configured for Rx.
>
>         Referring to "LPF Register" on the MAX2829 datasheet, the old
>         code...
>
>         static void
>         set_reg_lpf(struct db_xcvr2450_dummy *db){
>         int reg_lpf = (
>         (db->common->d_rssi_hbw<<15) |
>         (db->common->d_txlpf_bw<<10) |
>         (db->common->d_rxlpf_bw<<9) |
>         (db->common->d_rxlpf_fine<<4) | 7);
>         send_reg(reg_lpf);
>         }
>
>         ...seems like it should read...
>
>         static void
>         set_reg_lpf(struct db_xcvr2450_dummy *db){
>         int reg_lpf = (
>         (db->common->d_rssi_hbw<<15) |
>         (db->common->d_txlpf_bw<<9) |
>         (db->common->d_rxlpf_bw<<7) |
>         (db->common->d_rxlpf_fine<<4) | 7);
>         send_reg(reg_lpf);
>         }
>
>         ...to be set correctly.
>
>         Has anybody else had problems with this? I'm using
>         txrx_edk10.1_r11370.bin. I don't have Xilinx/EDK tools installed
>         on my
>         setup, but if someone is willing to build a load with the fix, I can
>         test it out.
>
>         thanks,
>         Steve
>
>
>
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