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.
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)
