Re: 2445A calibration

Chuck Harris
 

Ack!

A rule of thumb is that when calibrating, is you need to use references
that are a minimum of 10x more precise and accurate than what you are
calibrating.

The scope you are using to measure the Arbitrary Waveform Generator, is
likely no better than +/-1%, assuming it was recently calibrated.

To that end, the PG506 is specified to be within +/-0.25%, which is
not quite 10x more precise and accurate than the 2445A is capable, but
certainly a lot better than what you are doing.

Further, the PG506's output impedance, when in voltage calibration mode,
is not 50 ohms, it is about as close to zero ohms as was humanly possible
in its day. To wit, its specified accuracy is +/-.25% +/-1uV for loads
of 1M *and* of 50 ohms.

The AWG most likely has a near zero ohm source with an internal 50 ohm
series resistance, put there to prevent reflections, from reflections
at the load end of the cable.

If 0.5Vp-p is showing as 0.55Vp-p, something is very wrong.

As to the DAC swing, it is not supposed to be symmetric about ground.
It must be 2.500V +/-1mv swing when the DELTA control is turned from
the fully CW position to the fully CCW position.

-Chuck Harris



maxim.vlasov@... wrote:

Hello Chuck,

I have started from the beginning. Replaced 4 aluminum caps on the A5 board (checked the ESR and capacity and they were fine). Then borrowed two 6.5 digit Agilent multimeters (to make sure about the readings) and set the 10V reference voltage to 10.0001V (can't do any better, the resistor is crusty). To measure things I use the ground hole under the calibrator output for the banana plug and pointy probe to measure things on J119.
Then I've continued to set the DAC reference voltage. The whole band is 2.501 volt, however it's not super symmetric: -1,236 for CCW and -1.265 for CW.
Then without touching anything else and re-running the CAL01 I've jumped on CAL02. Unfortunately CAL02 fails in the same fashion -> LIMIT ERROR.
Also noticed, that the gain of the scope is a little bit higher. I.e. instead of getting 0.5V p-p for 500mV calibration signal I get 0.55V. I wonder if it needs some fixing before going to CAL02.

For the vertical calibration I don't have the calibrator module like PG506. I substitute for that the AWG. The signal is 1KHz 50% duty cycle. Low level is 0V, HIGH level is what menu asks for into 50 Ohm, i.e. for 500mV required the AWG is set to provide LO=0V HI=0.5V into 50 Ohm. The output was tested on the "reference" scopes terminated to 50 Ohm.

I'm a bit lost in what could go wrong. I'll try checking the voltage ripple at the output of the power rails.

I just wonder if CAL02 would be affected by bad CAL01?

Thank you!

Best wishes,

Maxim



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