Hi Ross
Solved - too much gain in the quant causing all the problems seen -
Need to work out why it had the effect it did but reducing it produces
a perfect spectrum.
We did see this when preparing for the last workshop. Tutorial 3 is set
up for a wideband noise input where the input power is spread across the
whole band. Putting in a sinusoid will cause saturation when
requantising. It is important to calibrate your system based on the real
input signal. Snapshot blocks at strategic places (post ADC, post FFT,
post requantiser etc) are useful in this regard.
Regards
Andrew
On Wed, May 15, 2013 at 6:58 PM, Ross Williamson
<[email protected]> wrote:
A couple of more questions regarding simulation of tutorial3 - The
setup has a 2^8 FFT (so 128 channels, 64 per vacc)
1) It looks as if the valid flag on Vacc is going high about 6 clock
cycles before it should. This was tested by putting in an f=0.5 sine
wave at the input to the adc (sample/period = 4, sample time = 1/4).
The output from the vacc1 is at 38 rather than 32 and a DC component
is also shifted by 6 (I'm not sure why there is a DC component). If
you look at the latencies from the valid flag to dout there is 4 (so
not quite 6) but I guess that gets us close - it is an error though
and a think there should be a delay on the vacc valid line.
2) Maybe this is to be expected but if I shift the frequency below
f=0.5, say f=0.25 then there are quite distinct harmonics present at
f= 0.5 (about 1/8 of main freq) 0.75 (about half of main freq). I'm
wondering if this is something to do with the discrete nature if the
input sine-wave but I haven't thought too much about it.
On Wed, May 15, 2013 at 1:04 PM, Ross Williamson
<[email protected]> wrote:
yes indeed a sync pulse does help significantly.... doh....
On Wed, May 15, 2013 at 12:36 PM, Jack Hickish <[email protected]> wrote:
Hi Ross,
Have you looked upstream at, eg, the FFT output, to check there is a single
spike coming out of that? Might also be worth making sure that you are
simulating a sync pulse, and waiting long enough for data after the sync to
propagate all the way through the design.
Cheers,
Jack
On 15 May 2013 20:28, Ross Williamson <[email protected]> wrote:
I'm trying to simulate tut3 by putting in a sine-wave at the input and
looking at the output of the accumulators - the acc_length is 2 and
FFT and pfb size is 12. Input parameters for the sine wave are:
Sample based, Simulation Time
Amplitude 0.1
Bias 0
Sample per period 100
number of offset samples 0
sample time 1/4
Interpret vector parameters as 1-D checked
I only look in the region where the output valid flag is high - The
problem is that I get a forest of lines out of both accumulators with
no obvious spike where the signal should be. It doesn't look like
there is clipping. I can post pictures of the scopes if needed but
I'm just wondering if I'm missing something obvious?
--
Ross Williamson
Research Scientist - Sub-mm Group
California Institute of Technology
626-395-2647 (office)
312-504-3051 (Cell)
--
Ross Williamson
Research Scientist - Sub-mm Group
California Institute of Technology
626-395-2647 (office)
312-504-3051 (Cell)
--
Ross Williamson
Research Scientist - Sub-mm Group
California Institute of Technology
626-395-2647 (office)
312-504-3051 (Cell)
--
Ross Williamson
Research Scientist - Sub-mm Group
California Institute of Technology
626-395-2647 (office)
312-504-3051 (Cell)
