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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Wed, 5 Jun 2024 17:09:31 -0000 (UTC)
Organization: A noiseless patient Spider
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On Mon, 3 Jun 2024 22:44:19 +0100, John R Walliker wrote:

> On 02/06/2024 13:12, Cursitor Doom wrote:
>> On Sun, 2 Jun 2024 13:00:21 +0100, John R Walliker wrote:
>> 
>>> On 02/06/2024 12:31, Cursitor Doom wrote:
>>>> On Sun, 2 Jun 2024 11:17:58 -0000 (UTC), piglet wrote:
>>>>
>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>>> On Sat, 1 Jun 2024 22:00:58 -0000 (UTC), piglet wrote:
>>>>>>
>>>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>>>>> On Sat, 1 Jun 2024 15:44:17 +0200, Jeroen Belleman wrote:
>>>>>>>>
>>>>>>>>> On 6/1/24 14:07, Cursitor Doom wrote:
>>>>>>>>
>>>>>>>>>> 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!
>>>>>>>>>>
>>>>>>>>>> https://disk.yandex.com/i/7cuuBimDbOIBZw
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>>>
>>>>>>>> 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:
>>>>>>>> 850mV peak to peak is 425mV peak voltage. Average of that is
>>>>>>>> 0.425x0.636 =
>>>>>>>> 0.27V. Average power is average volts squared divided by the load
>>>>>>>> impedance of 50 ohms = 1.46mW = +1.65dBm.
>>>>>>>>
>>>>>>>> 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.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>> Use 0.71 for RMS instead of 0.636 ! I make that about 1.8mW or
>>>>>>> +2.6dBm ?
>>>>>>
>>>>>> 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!!)
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>> Since you have a power meter, a signal source, and an oscilloscope
>>>>> why not measure the peak to peak voltage on the scope and power on
>>>>> the power meter and see which calculation 0.636 vs 0.707 gives the
>>>>> closest agreement?
>>>>
>>>> It wouldn't prove anything one way or ther other, though, since that
>>>> power meter hasn't been calibrated for "quite a while" so to speak.
>>>> :)
>>>> It'll give a 'good enough' reading for my purposes, but won't be
>>>> accurate enough to meaningfully test your otherwise fine suggestion.
>>>
>>> I have an 8566B which is currently not working.  Both the status leds
>>> on the front panel at the bottom are red.  I haven't started to
>>> investigate yet.
>>> The fault developed slowly.  At first it would sometimes work, then
>>> progressively less often and now never.
>>> However, if the signal being discussed is available on the rear panel
>>> I could measure mine and see what it looks like and what voltage is
>>> delivered.
>>> John
>> 
>> Yes, that could be very helpful, John, since your fault is clearly
>> totally different to mine. Peak to peak volts into 50 ohms on a scope
>> will be fine if don't have access to an RF power meter.
> 
> I measured the 10MHz output of my unit with an HP 54542A digital storage
> 'scope and found the following:
> 1Mohm input via 5m coax 1.133 Vp-p 405.8 mVrms
> 
> 50ohm input via 5m coax 790.3 mVp-p 284.9 mVrms
> 
> The waveform with 1Mohm load looked like a perfect sine wave, whereas
> the 50 ohm loaded waveform showed some second harmonic distortion
> visible as a slight narrowing of the top of the sine and a slight
> flattening of the bottom of the sine.
> 
> The frequency measured with an HP 53131A frequency counter was
> 10.000199MHz.
> The spectrum analyzer had been on standby for a few weeks and fully
> powered for a few hours, so the crystal oven should have been at
> equilibrium.  I then substituted my rubidium oscillator which indicated
> 9.999997MHz on the counter.
> 
> The connector that I used was the one labelled 10MHz at bottom right,
> not the adjacent reference output which is connected to a reference
> input with a short coax jumper.  Is the same output that you measured?
> 
> John

Hi John, sorry for the delay in replying.
You got some interesting results there. Not what I'd expected, to be 
honest. We have to bear in mind that the 8566B is a different beast to 
yours, of course, so a bit of leeway can be allowed for that. 
Nevertheless, the fact you're getting a decent sine wave into 1Meg is 
curious, given what I experienced. I'm taking my 10Mhz from a BNC socket 
on the rear panel which HP placed there to make it easier to check and 
adjust the 10.0000000Mhz out as precisely as possible. I'm sure yours 
*will* be different, but that shouldn't matter. I also got the same 
results by popping off one of the 50 ohm interconnects from the board the 
A22 10Mhz ref feeds into.
Thanks for including the p-p voltage figures in your report; helps a lot. 
Many thanks again.