[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

[Simon Marsh]

Many thanks to Bob D, Bob C, Bruce and Magnus for the links, references 
and being patient.

I've spent a bit of time looking at the glitching with the idea of 
evaluating a few different algorithms to deal with it. I also looked a 
bit at the hardware and instead of very simply having a single D-flop 
doing the sampling, I now have the 2 D-flops in a 74AC74 wired in series 
so that one does the sampling and the other acts as a shift register 
before the output is sent on to the BBB.

I got so far with this before realising that one of the D-flops was 
being much more noisy than the other and indeed it was only a single 
output that was particularly noisy. Switching to the inverted output 
reduced the noise considerably.

After a bit of head scratching I swapped the part with the result that 
_all_ the glitches vanished. Completely. Even at small beat frequencies 
(<5hz).

So, I've managed to go from one extreme to the other. I believe I should 
be seeing _some_ glitching, so would appreciate any pointers as to what 
now might be hiding it or how I could diagnose what is going on ?

The discussion on slew rates was interesting, I still have this knocked 
up on some pluggable breadboard, the slew rate is going to be poor and 
could easily be a contributing factor. Is it possible that poor slew 
rates mean I have quite a large 'dead zone' when the clock and data 
edges are co-incident (and where the flip flop is unable to effectively 
sample) and this is large enough to be masking any intrinsic oscillator 
noise ?

Cheers


Simon