CMOS Shift Register Noise Generator - Strange Behaviour

Discussion in 'The Projects Forum' started by daviddeakin, Jan 21, 2015.

  1. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
    207
    27
    I have built a noise generator using a couple of CD4006 shift registers. The circuit is on page 9 of this PDF:
    http://schematicsforfree.com/archive/file/Oscillators and Generators/Pink-White & Other Noise Generators/Pink & White Noise Generators - Misc.pdf

    I used a CD4070 quad XOR instead of the older CD4030. I haven't implemented the pink filter or MPSA13 buffer, I'm just monitoring the output directly on the o'scope).

    It is on breadboard and all works perfectly until I start varying the supply voltage, between 5V and 15V. It appears that at certain critical supply voltages (e.g. 5.3V, 10.0V and others...), the sequence stops being random and somehow gets stuck in a repeating pulse train of a few kilohertz, even though the clock continues to run normally. In fact, at 6.8V the output stays permanently low, yet the clock is running! Can anyone shed any light on this??
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the timing changes with supply voltage, delay through the registers changes.
     
  3. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi david,
    A lot of the very old CD CMOS had unbuffered outputs, sometimes coded CD4070U. or UB
    The newer CD CMOS is buffered on the output, coded CD4070B.

    The buffering makes a big difference in performance of some circuits.

    The PDF posted is circa 1979
    E
     
  4. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
    207
    27
    The chips I'm using all appear to be the 'B' (buffered) type. I will try making use of the 4006's half-cycle delayed output latch and see if that helps.
     
  5. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
    207
    27
    The output latches didn't help at all -just stopped it working. However, it seems that a 100pF cap between the clock input and ground does the trick.
     
Loading...