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[Discuss-gnuradio] diminishing returns with increasing frequency offset

From: tom x
Subject: [Discuss-gnuradio] diminishing returns with increasing frequency offset
Date: Thu, 17 Mar 2016 18:23:26 +0100


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?


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