On Fri, Mar 18, 2011 at 4:29 PM, Moeller
<address@hidden> wrote:
On 18.03.2011 18:10, devin kelly wrote:
> I have two problems with my data though. In the file attached is some TV spectrum (left half) and noise (right half).
> My first question is this: why isn't the spectrum for the TV signal flat, it seems to bob up and down. Note that each segment is from a different
> FFT, that is each FFT produces that oval shape. At first I thought this had something to do with the window I was using but I've tried hamming,
> blackman-harris, and rectangular windows and they all have this effect.
Why should it be flat? On the transmission side OFDM is quite flat.
Typically (NLOS conditions) you will receive multiple paths and they interfere.
The "frequency selective fading" effect is very strong for wide-band signals like DVB-T.
NB signals like GSM assume the "flat fading" model, by contrast.
I suppose flat is the wrong word. If you look at the image I provided in the first email you'll find that all the FFT outputs have this hump shape to them.
> My second question is why isn't the transition between TV spectrum and noise more smooth? It looks like it could be smooth but the gain seems to jump
> for an FFT right after 566 MHz and then settle down again. I thought this might be the AGC but I've tried different attack/decay rates, different
> reference levels, etc with the agc2_cc block but the problem remains.
Can you switch off the AGC? AGC can destroy the whole spectrum measurement.
Single spikes could be spurious mixing products from the receiver itself.
I've only started using the AGC block very recently. I started using because I wanted some way to control the AGC. This makes me think of the question, is the AGC on by default? That is, if I don't have an AGC block will the AGC be doing anything? Also, in the first two or three ICs in the WBX there is a digitally controlled attenuator and an LNA, those are part of the AGC correct?
A global filtering shape is expected from the analog RF filters.
Of course, on the borders of FFT blocks you have incorrect transitions,
depending on the shape within the reception bandwidth.
I assume this could be compensated by scaling the FFT values.
This is going to be the next thing I try.
To get the spectrum more smooth, you have to capture longer signals
within the blocks of measurement bandwidth, for averaging the PSD estimate.
> Any help with either of these questions would be appreciated,
> Devin
Do you want to publish the program? I think it would be very interesting
for the examples section of Gnuradio. Surely, you will get support from
the Gnuradio community to improve the sensing function.
Btw, did anybody try to write a spectrum sensor with the Funcube USB dongle?
This would be a low-cost alternative. The measurement bandwidth is not high,
but it can scan a broad range of the spectrum.
Possibly, after switching off AGC, compensating within the meas. bandwidth
and compensating the global filter shape, the result could be quite useful.
(if there are not too many spurious frequency components within the receiver itself)
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