Very helpful. Thanks,
m
> Hi Mandana
>
>>> It is important to ensure that sufficient signal is coming out after
>>> requanisation to ensure
>>> reasonable correlator efficiency while avoiding clipping. This
>>> requires a calibration step that
>>> can be done using autocorrelation products.
>>>
>> Thanks for the useful comments.
>> Is there any suggestions on the calibration strategy and how to monitor
>> for
>> clipping. Right now, I am going to re-adjust the gain on bins that are
>> below
>> 1000 auto-corr units and try to make the spectrum as flat as I can ( by
>> looking at the spectrum every second). Once this is settled (or maybe it
>> won't), I will re-adjust gains only once every so-many minutes maybe.
>> Does
>> this sound reasonable?
>
> Our calibration strategy basically involves the following steps;
> 1. Take the autocorrelation products for the input being calibrated.
> 2. Divide each product by the number of accumulations. This gets you back
> to the average power of one output value of the requantiser for each
> frequency
> channel of your input.
> 3. The real part of these autocorrelation values (XX*) contains
> real(x)^2+imag(x)^2
> where x is average output of the requantiser for each channel. real(x)
> and imag(x)
> are 4 bit values so real(x)^2+imag(x)^2 is a 9 bit value. We try to
> get this 9 bit
> value into the range 20-30 in the band of interest. This depends on the
> dynamic
> range of your signal of interest, more head room may be needed if you
> don't want
> clipping.
>
> These steps will need to be repeated a few times as quantisation is not a
> linear
> process and should be done using an input that matches the final
> signal of interest
> as closely as possible (power levels especially).
>
> The process can be automated using a script. The amount of times you
> need to redo
> it depends on how much your signal changes, for a similar power input
> it should not
> be necessary to do it very often (our RF input level is held at a
> certain level so we
> can pretty much calibrate once).
>
> The DC channel output of the FFT often has a higher power than other
> channels
> so this may cause clipping after requantisation even though the rest
> of the signal is fine.
> It may be useful to multiply this channel with 0 to remove it as it
> generally is not useful. We have a snap
> block connected to the output of the requantiser that can be used to
> take occasional
> snapshots of the data stream to check for clipping and a bit in a
> status register that
> can be monitored. It would be more important to monitor the ADC and
> FFT for clipping
> as these affect multiple channels.
>
> Hope this helps
> Regards
> Andrew
>
