# Troubled by the pfb channelizer center frequency location **URL:** \ **Category:** GNU Radio\ **Created:** [May 16, 2013, 1:16am UTC](https://www.ruby-forum.com/t/troubled-by-the-pfb-channelizer-center-frequency-location/229009 "2013-05-16T01:16:57Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [May 16, 2013, 1:16am UTC](https://www.ruby-forum.com/t/troubled-by-the-pfb-channelizer-center-frequency-location/229009/1 "2013-05-16T01:16:57Z")
Dear Group, I am learning to do the pfb channelizer using the now famous example on the documentation page at: [http://gnuradio.org/doc/doxygen/page\_pfb.html](http://gnuradio.org/doc/doxygen/page_pfb.html) The example is at the end of the page, where a 9-subchannels are pulled out of the original 9 kHz bandwidth signal. The original signals’ tones are at [freqs = [-4070, -3050, -2030, -1010, 10, 1020, 2040, 3060, 4080] The original signal bandwidth should be from -4500 to 4500 Hz. According to my understanding the first channel signal should be centered at -70 Hz, since the 9 channels’ centers should be at [-4000, -3000, …, 3000, 4000]. But the output plot from the example shows that the first tone is at slightly greater than 0 Hz frequency. I don’t know what is going on here? LD ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [May 16, 2013, 1:28am UTC](https://www.ruby-forum.com/t/troubled-by-the-pfb-channelizer-center-frequency-location/229009/2 "2013-05-16T01:28:29Z")
I guess I got it! The default channel map is such that the first channel is the UN-fftshifted frequency location which is at 10 Hz (the 5th one on the list)! Whoo! It would be nice to tell a newcomer about this. LD From: LD Zhang [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] Sent: Wednesday, May 15, 2013 4:16 PM To: discuss-gnuradio Discussion Group Subject: Troubled by the pfb channelizer center frequency location Dear Group, I am learning to do the pfb channelizer using the now famous example on the documentation page at: [http://gnuradio.org/doc/doxygen/page\_pfb.html](http://gnuradio.org/doc/doxygen/page_pfb.html) The example is at the end of the page, where a 9-subchannels are pulled out of the original 9 kHz bandwidth signal. The original signals’ tones are at [freqs = [-4070, -3050, -2030, -1010, 10, 1020, 2040, 3060, 4080] The original signal bandwidth should be from -4500 to 4500 Hz. According to my understanding the first channel signal should be centered at -70 Hz, since the 9 channels’ centers should be at [-4000, -3000, …, 3000, 4000]. But the output plot from the example shows that the first tone is at slightly greater than 0 Hz frequency. I don’t know what is going on here? LD ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [May 19, 2013, 12:27pm UTC](https://www.ruby-forum.com/t/troubled-by-the-pfb-channelizer-center-frequency-location/229009/3 "2013-05-19T12:27:54Z")
On Thu, May 16, 2013 at 12:26 AM, LD Zhang [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I guess I got it! The default channel map is such that the first channel is > the UN-fftshifted frequency location which is at 10 Hz (the 5th one on the > list)! Whoo! It would be nice to tell a newcomer about this. > > LD Glad you got it working. You could have read the documentation on the block, though: > **[GNU Radio Manual and C++ API Reference: gr::filter::pfb\_channelizer\_ccf Class...](https://www.gnuradio.org/doc/doxygen/classgr_1_1filter_1_1pfb __channelizer__ ccf.html)** Look at the ‘set\_channel\_map’ function. It explains the channel mapping; and how to change it. Tom