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Re: [Discuss-gnuradio] FFT Baselines curved?
From: |
Lamar Owen |
Subject: |
Re: [Discuss-gnuradio] FFT Baselines curved? |
Date: |
Wed, 20 Apr 2005 17:09:52 -0400 |
User-agent: |
KMail/1.8 |
On Tuesday 19 April 2005 10:54, n4hy wrote:
> Cascaded Integrator Comb filters are pretty simple filters that are very
> fast to compute
> but produce those slow roll offs. There are more complex things that
> could be done and
> hopefully now that the USRP is really flowing out the door,
> daughtercards and all, and
> as we come up to speed, we can work on these subtleties in the core code.
Yeah, we're trying to apply these as spectrometers at PARI, and getting a flat
baseline is pretty much required. I personally am working on spec-an stuff
for AM broadcast (to do 47CFR73.44 occupied bandwidth measurements, where 12
bit resolution is fine).
For the spectrometer project, we don't actually want an FFT. We want a swept
filter analyzer instead, for integration purposes. FFT is acceptable, but
most radio astronomy spectrometers are swept filter. Getting swept filter in
software isn't too hard, either, but pre-filtering with the FPGA would be
nice, then set up the software filters as fixed-frequency.
The skirts next to signals are quite curved, too, and make the box not as
useful as it could be in my application. Straight A/D at a low enough sample
rate to get over the USB with no filtering at all I could deal with, but I've
not dug into the code deep enough as yet to see how that works.
I can choose basically any IF I want, and 10.7MHz is fairly standard in the RA
world, which falls in the passband of the USRP nicely.
My personal USRP I ordered with a TVRX (plus a BasicTX/RX pair), and I must
say the WFM demod is nice for UHF NTSC TV audio. I see the BTSC stereo
subcarriers properly, and can even see some hsync leakage in the audio.
I may also write an NTSC block; I do enough video work that having a
vectorscope and a waveform monitor is required. Currently I drag around an
SGI O2 with the analog video card; the SGI diagnostic software for that card
has vectorscope and waveform monitor capabilities.
Does anyone (Matt?) have a good characterization of the frequency response
curves of the BasicRX? If not, I'll take our calibrated noise sources and
generators here and produce one; but it will be offset by the curvature
already noted.
--
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC 28772
(828)862-5554
www.pari.edu