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I am building a general purpose LF/MF/HF/VHF/UHF "crystal calibrator" using a Progrock and a QLG1.? But I am not sure how to deal with the RF outputs from the Progrock.? I do have them AC-coupled through 0.1 microfarad capacitors.?

The Si5351a data sheet only specifies it for a 5 pF load.? I've read in other places that 50 ohm loads result in poor crosstalk between the outputs.? But this is of little concern in the current application.? I am a bit concerned about heating....the Progrock is drawing >50mA with some combinations of output frequencies and the current does seem to depend quite a bit on the load impedance (even a few inches of open-circuited coax has a noticeable effect).? I also wonder about how well the Si5351a would react to an AC short.

I'm thinking about putting in a series resistor between the Progrock/blocking capacitor and the output of my box, or an L network of resistors, to reduce the loading on the Si5351a while providing a low impedance source (50 ohms perhaps), since there seems to be more output power than I need for this specific purpose.? At some point I may add a multiplier circuit to produce microwave harmonics - that would benefit from lots of drive, so I don't want to attenuate too much and have to amplify back up again.

Any useful thoughts on this topic?

73,
Steve VE3SMA

I also wonder about how well the Si5351a would react to an AC short.

Steve,

I ran a U3S with a TX muting that simply shorted the output of the coupling capacitor to ground via a BS170. It is still working. But the consensus was that it is not a good idea.
I've no real knowledge but I do not see the loading being critical for use as a calibration tool. Maybe the 5pF is to maintain a sharp waveform. It is fed into BS170s in QRP Labs projects, much more than 5pF.

73 Alan G4ZFQ

I'm thinking about putting in a series resistor between the

Thanks, Alan.? At least there is some evidence that it won't self-destruct when AC-shorted!

73,
Steve VE3SMA

An AC short at the output would be less than desirable and does increase
the devices power needs and heating.

Better would be 10 to 50 ohm series and then short to ground.? Or turn
the oscillator off in software!

FYI the 5pf spec is for output rise time measurement.

Allison

Thanks, Allison.? I was thinking maybe 100 ohms in series with the blocking cap (to provide short circuit protection) and then 75 ohms to ground at the output of the box to recover a 50 ohm source impedance (if that really matters, which it probably doesn't).? I don't intend to intentionally short the output, but test equipment seems to get abused unintentionally at times!? But that does put a 130 ohm load on the Si5351a when feeding a 50 ohm load, which might still be a pretty low resistance for this chip to drive comfortably.

OK on the rise time thing....that makes sense.? I would like to retain short rise times as I intend to use the harmonics in the calibration application.

73,
Steve VE3SMA