[sdiy] Trying to establish confidence in my test equipment

[Harry Bissell Jr]

I would make a precision voltage reference, and a string of
  0.1% resistors (or better), buffered with a really low offset
  voltage opamp.  Then find someone who has a REALLY
  good meter (maybe 5-1/2 to 6-1/2 digit)... and calibrate the
  voltage at the top of the string... so you get 0V, 1V, 2V etc.
  
  You could use this as a reference to calibrate your VCOs to 1V per octave
  (use a precise frequency source like some crystal controlled
  divider) and a scope to do that calibration.
  
  I built a unit like this, the best meter that I tried said 1.000V, 2.000V
  ...5.000V.
  
  Then just adjust the keyboard for best fit of the VCO.
  
  Your hearing is probably better than every meter you own...
  
  H^) harry

Chris Manders <wight446 at yahoo.com> wrote:  Hi Folks

Thanks to all of your input on my last question of
'VCO standards - CV to give what frequency' I have
decided on a range.

I built Ray Wilsons excellent Keyboard circuit
detailed here: (It works well).
http://www.musicfromouterspace.com/analogsynth/keybrdcontroller.html

Here's a poser for all of you.

I now come to calibrate this circuit.
I have FOUR meters. Two I obtained recently, and do
not know the calibration history.
I was trying to calibrate the output of this circuit
(connected to a three Octave keyboard) for an output
of 0.640 Volts Bottom C to 3.640 Volts top C. Here is
what I got:

Meter                  Low Reading    High Reading

Radio Spares M810      0.640          3.622
Beckman Ind. DM78      0.643          3.64
Fluke 8010A            0.64           3.59
Keighley 178           0.650          3.672

The last two Multimeters are of unknown calibration,
but are the highest quality meters I have (I think).
The top two meters are sealed units - Calibration
impossible.

My question now is . . . er . . . what is best?
I don't even know what the Keighley 178 meter is
worth, although it has a good 4 digit accuracy as
shown.

Clearly here one meter (the Fluke) says I could be
50mV under, and one meter (Keighley) says I am 22mV
over.

What would peoples opinions be of my results?
Do you think I have a 'bee in my bonnet', and these
results are actually within acceptable limits for this
circuit?
What would be the best thing to do? (I think it would
be too costly and difficult to get the bottom two
meters calibrated).

I am familiar with the quality of FLUKE meters, but
'Keighley'? Are they good quality? Worth the same as
Fluke?

Thanks for any advice
Chris


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