Does the number of channels have to be a power of 2? On Wed, Nov 29, 2017 at 10:39 AM John Ackermann N8UR <j...@febo.com> wrote:
> I'm building a ridiculous flowgraph that breaks the AM broadcast band > (540 - 1700 kHz in the U.S.) into 117 10 kHz wide channels and measures > the energy in each. The thing is working but I see a frequency offset > in the output channels that is not present in the data before > channelizing. The output channels seem to have an offset in the range > of 400 to 700 Hz versus the unchannelized input. > > The signal chain is: > > 2.5 msps recording centered at 1.4e6 Hz -> xlating filter, decimation 2, > output centered at 1.12e6 -> PFB channelizer with 117 channels, yielding > a channel rate of 10,683.760683...... samples per second. > > Looking at the spectrum at the output of the xlating filter, the carrier > frequencies are correct. Looking at the output of a channel, the > carriers are offset by several hundred Hertz, always high. (Given the > absolute frequency is in the 1 MHz range, these offsets are parts in > 1e3, a pretty large amount.) > > I wonder if the large number of PFB channels is causing a rounding error > that results in these frequency offsets. Or is there something else > going on? > > I can probably fudge the xlating filter frequency a bit to move the > carriers closer to nominal, but would like to understand what's happening. > > I'm attaching the (absurdly huge) .grc file. The canvas is 4192 pixels > tall, so the flowgraph is smaller than the screenshot. :-) > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > -- Very Respectfully, Dan CaJacob
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