Troubled by the pfb channelizer center frequency location

GNU Radio
1

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

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

2

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:[email protected]]
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

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

3

On Thu, May 16, 2013 at 12:26 AM, LD Zhang [email protected] 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:
http://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