Cal Source

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Cal Source

Post by Sam » Fri Mar 04, 2011 1:37 pm

The "1MHz Cal Source" provides an output whose precise level can be determined with an ordinary voltmeter. It was originally intended to operate at 1 MHz (from an an-board oscillator or external source). But, due to the zero-response interfering with low level signals at 1 MHz, it is actually better to use 2 MHz. The circuit divides the input frequency in half to get a 50% duty cycle, so getting 2 MHz output actually requires a 4 MHz oscillator or input. I'm going to rename this board the "xMHz Cal Source". It should be precise with 1-5 MHz output, though it will be functional far beyond that. Just remember to use an oscillator at twice the frequency of the desired output.

You can use this directly as the signal source for path calibration, adding attenuation as necessary. Unfortunately, this won't work for phase calibration since you can't synchronize the cal source with the MSA phase. (You can use the TG to feed the cal source, but its frequency will get divided by two.)

If you have an external signal source, you can use this to calibrate its output level at 2 MHz. Just measure the cal source on a very coarsely calibrated MSA and then measure the signal source and adjust its output until it equals that of the cal source. The true output of the signal source then equals the true output of the cal source (determined by the voltmeter measurement--forget what the MSA is displaying.) Then use the signal source for mag calibration. Not phase calibration, unless you can phase lock the signal source to the TG.

Or, you can use the cal source to calibrate the output level of the TG, and use that for both mag and phase calibration. In this case, you will have to attenuate the cal source output to get it down to the TG level, or amplify the TG to get it up to the cal source level (about 0 dBm).

You can also do the comparison of signal levels with a scope or power meter instead of using the MSA.

Sam W.

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