Distorted Sine Wave

What is the 10 MHz signal power level at the EXT input in these two castes, 1 Mohm and 50 Ohm? The expected range is 0 to +10 dBm.

Joe Gwinn

xdevs.com/doc/HP_Agilent_Keysight/ HP%208566B%20Troubleshooting%20&%20Repair%20Vol.%201.pdf>

It's just a block diagram of the reference oscillator module. There's no detailed schematic of the oscillator itself. At least that's the case with the PDF version of the Service Manual I have.

It's probably in the book that I don't have, if it was ever released.

Joe Gwinn

HP actually published at least 5 manuals of various kinds about this analyzer, so it's possible it's in one of the others. However, the actual service manual is where I'd expect to find it if they published it at all. The other titles are:

HP 8566B Operator's Manual HP 8566B Installation and Verification Manual HP 8566B Test and Adjustments Manual HP 8566B Troubleshooting and Repair Manual

So if the answers are not in *any* of the above, it'd be jolly rotten luck indeed!

No issues there AFAIC.

Looking at that waveform again...

Could it be that the wavform you are seeing there is not a harmonic but just a partial cancelation due to reflections ?

If you look at one peak vs. the other, they look about the same time to me. About 1 scope division. Just the amplitude looks different on alternating cycles. A non-linearity I would think would make an even harmonic. 2nd harmonic is usually due to non 1/2 wave symetry (DC offset bascially)

What was tha amplitude after you loaded it down properly ? How about a picture ?

Where were you scoping this at again ?

boB

Except for some distortion. I'd have expected better spectral purity from a xtal reference oscillator. Perhaps I should try a different scope just to be sure it's not that that's at fault...

I thought I said the wrong thing ! I mean odd harmonic(s) from a non-linearity.

Here, I meant even harmonic(s) not just second harmonic.

It's rather hot today in Phoenix..

1. Everybody makes careless mistakes.
2. I don't need high temperatures to make them!

I've taken a shot of the waveform into the 50 ohm input. It's around 850mV peak-peak. Hopefully the slight distortion I spoke about is visible; the slightly more leisurely negative-going excursions WRT their positive-going counterparts. So it's not a pure sine wave as one would expect. Does it matter? I don't know!

The shape looks perfectly acceptable to me. This is +3dBm into 50 Ohms. Is that what it's supposed to be? Canned reference oscillators most often deliver +13dBm, sometimes +10dBm.

Jeroen Belleman

Is it? I only make it about half your figure: +1.65dBm. I admit I'm frequently prone to careless errors, so stand to be corrected, but here's my method:

I shall consult the manual to see what it ought to be - if I can find it, that is, as PDF manuals are a nightmare to navigate IME.

Not so fast there. It's the scope that is being set to 1 Meg or 50 ohm, and at 1 Meg we are observing the drive into the 50 ohm input impedance of the EXT ref input of the spectrum analyzer.

That 2.7 Vpp over 50 ohms is about +13 dBm, which exceeds +10 dBm. So the EXT input may be over-driven. Install a 5-dB inline attenuator and see what happens.

Joe Gwinn

Unless I misunderstand what you're saying, that's not the case, though. What we are observing is the output of the analyzer's 10Mhz reference oscillator taken from a BNC socket on the rear of it which HP have thoughtfully provided and fed directly into a scope switchable between 1M and 50 Ohms.

It's not an external input. They've put it there in order to make it easy to check the frequency of the reference oscillator and adjust it to precisely 10.000000Mhz if necessary (after a *minimum* 72 hour warm-up!)

Maybe. But it's simpler to just try the attenuator.

Joe Gwinn

If I can find a reverse attenuator, I'd be happy to try it.

Use 0.71 for RMS instead of 0.636 ! I make that about 1.8mW or +2.6dBm ?

I don't think you need such a thing. Inline attenuators (male-female) are symmetric, and don't care which way the signal travels.

Joe Gwinn

Or +2.9dBm if using the 0.88v pk-pk I think is shown in the scope pic rather than the 0.85v figure of your message.

Thanks, Erich. But there's no such thing as "RMS power" strictly speaking IIRC, so that's why I took the average figure; not that it makes much difference in practice. it does seem a bit on the low side, but despite reading through the most likely sources (the service manual and the trouble-shooting/repair manual) I can find nothing stated for what that signal level should be! This may be due to the user-unfriendliness of very large PDF manuals; I just don't know. Anyway, not very satisfactory! Later today I plan to do a direct power meter measurement of the ref osc (since none of us here seem to agree on what 850mV vs 50 ohms equates to!!)