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Re: [Discuss-gnuradio] diminishing returns with increasing frequency off

From: tom x
Subject: Re: [Discuss-gnuradio] diminishing returns with increasing frequency offset
Date: Fri, 18 Mar 2016 20:01:58 +0100


This previous post by me should give more info: https://lists.gnu.org/archive/html/discuss-gnuradio/2016-02/msg00286.html
I've replaced the polyphase channelizer with one Frequency Xlating FIR filter for each channel.
The first problem (described above) is that centering the filter too far from the capture frequency means nothing gets through.
To solve this I would like to increase the sample rate but, as mentioned, the PHY block only output packets with samp_rate = 4MHz.
 I looked inside the 802.15.4 PHY block for some dependence on 4 MHz, and indeed, the samp_rate block is set to 4 MHz.
None of the other blocks use it though, here is a picture of the RX chain: http://i.imgur.com/0Ccwnr1.png?1
Maybe there is an implicit dependence on 4MHz, in the parameters set by the Clock Recovery block? I am not sure.
If there's any other relevant info I can give, let me know.


On Fri, Mar 18, 2016 at 7:33 PM, Martin Braun <address@hidden> wrote:

you'll need to tell us more about what you're doing and what you've tried.


On 03/17/2016 03:36 PM, tom x wrote:
> It's strange but the only sampling rate that works is 4 MHz; Packets are
> not decoded when the sampling rate is set above this rate.
> Thanks,
> Tom
> On Thu, Mar 17, 2016 at 7:33 PM, Martin Braun <address@hidden
> <mailto:address@hidden>> wrote:
>     What's your sampling rate? The further you go towards the edge of your
>     Nyquist zone, the more the anti-aliasing filter might be kicking in.
>     Cheers,
>     Martin
>     On 03/17/2016 10:23 AM, tom x wrote:
>     > Hi,
>     >
>     > My setup is a USRP N210 receiving from a transmitter sending a few
>     > packets a second. (The packets are decoded with Bloessl's 802.15.4 PHY
>     > block if it matters)
>     >
>     > Let's say the transmitter is sending data on frequency f_t.
>     >
>     > I set the USRP to receive on f_t plus an offset f_o.
>     > The next block is a frequency xlating FIR filter centered at -f_o.
>     > This is intended to correct the offset.
>     >
>     > I made a slider block for f_o and allowed it to vary between 0 and
>     3MHz.
>     > I need an offset of a few MHz for my application. However, the
>     number of
>     > packets received seems to drops as I increase f_o. I get almost no
>     > packets after f_o is increased over 0.5MHz.
>     >
>     > The overall purpose of this is to carve two channels out of the
>     received
>     > bandwidth with two frequency xlating FIR filter blocks, with the USRP
>     > tuned between the channels and the filter blocks centered at offsets
>     > that put them on the channel peaks. As explained above, if the
>     offset is
>     > too big, it stops working. Why do you think this is happening?
>     >
>     > Thanks,
>     > Tom
>     >
>     >
>     > _______________________________________________
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