Re: 7904A readout bounce


NigelP
 

OK, I think this is now the final input from my side on this topic!

I studied the schematic and noted something that I thought odd in comparison to the test schedule F5. I can see that the pot R130 is said to adjust the 1MHz jitter with 1uS/Div. Now, without a detailed analysis it seems to me that R130 is adjusting a "DC" or LF component since it does not have the capacitor associated with all the other compensation pots (except for C200 connected around the op-amp). All other adjustment pots are associated with a capacitor value which tracks the rep-rate value you are trying to compensate (higher value for lower frequency and vice versa). Admittedly all pots do have some level of DC adjustment but R130 circuitry seems to be a bit more DC-aggressive (maybe just my weird way of thinking about things :().

Now when I first tried this F5 procedure, noting that 1MHz did not need any adjustment (there was no jitter) I didn't bother to even try adjusting it, and just concentrated on R132 & R131 which is targeted at 10Hz which is typically where all the problem exhibits itself.

I decided to try out my theory that R130 ought really to be responsible for DC/LF jitter/bouncing correction; and indeed it does! I was able to tune out the 10/100Hz bounce far better than with the supposed correct pots R132/131 :). Re-evaluating the 1MHz rep rate results showed no loss of readout jitter compensation, and indeed I did not even bother to check the C200 adjustment. So simply speaking, first adjust R130 at 10Hz and then follow the procedure as defined in Table5-5.

So I now have a quite satisfactorily compensated readout across the full range of test offered by the 067-0587-01 and 067-0587-10 Signal Standardisers. I also found that using the Gain setting on the Standardiser (a full screen height staircase waveform) rather than the Step Response setting gave a far more aggressive signal for the compensation to be adjusted against; it was more visible and in effect the rep rate of the test signal is then one-eighth of the Step test. I can just observe a marginal vertical movement but nothing that is going to bother me.

I did also notice that adjusting R130 against 10/100Hz did actually vary the overall top-to-bottom magnitude of the readout display versus the Y-channel waveform; perhaps to be expected?

So reviewing the F5 test schedule and Table 5-5, firstly (in my manual at least) there appears to be a typo error...... step-5 indicates a rep rate of 100KHz (which is actually step-2) and should read 100Hz. Also in my personal experience the first step should be to adjust R130 at 10Hz and then check at 1MHz (perhaps adjusting only C200 where necessary) before moving on to the other steps in the table.

Comments on the above discoveries from those more knowledgeable/experienced are surely welcomed :).

Nigel

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