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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Fri, 31 May 2024 22:17:37 -0000 (UTC)
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On Fri, 31 May 2024 14:48:28 -0400, Joe Gwinn wrote:

> On Fri, 31 May 2024 17:29:47 -0000 (UTC), Cursitor Doom
> <cd999666@notformail.com> wrote:
> 
>>On Fri, 31 May 2024 11:34:46 -0400, Joe Gwinn wrote:
>>
>>> On Thu, 30 May 2024 16:24:25 -0700, john larkin <jl@650pot.com> wrote:
>>> 
>>>>On Thu, 30 May 2024 18:53:29 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 30 May 2024 14:56:30 -0700, john larkin <jl@650pot.com>
>>>>>wrote:
>>>>>
>>>>>>On Thu, 30 May 2024 16:06:47 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>>wrote:
>>>>>>
>>>>>>>On Thu, 30 May 2024 15:02:44 -0400, Phil Hobbs
>>>>>>><pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>>
>>>>>>>>On 2024-05-29 18:59, Joe Gwinn wrote:
>>>>>>>>> On Wed, 29 May 2024 22:11:47 -0000 (UTC), Phil Hobbs
>>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>>>> 
>>>>>>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>>>>>>>> On Wed, 29 May 2024 13:42:13 -0700, john larkin wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On Wed, 29 May 2024 21:43:54 +0200, Arie de Muijnck
>>>>>>>>>>>> <noreply@ademu.com>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> On 2024-05-29 19:07, Cursitor Doom wrote:
>>>>>>>>>>>>>> Gentlemen,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Whilst fault-finding on my HP 8566B spectrum analyzer, I've
>>>>>>>>>>>>>> found the 10Mhz reference oscillator is generating an
>>>>>>>>>>>>>> 'unsatisfactory waveform'
>>>>>>>>>>>>>> which may be causing the device to be unable to lock it's
>>>>>>>>>>>>>> main PLL. I've come across this waveshape before, but
>>>>>>>>>>>>>> mostly with oscillators I was building and in the process
>>>>>>>>>>>>>> of trying to iron out the wrinkles of and certainly NOT a
>>>>>>>>>>>>>> critical reference oscillator from a respected
>>>>>>>>>>>>>> manufacturer. Can anyone tell what's most likely going on
>>>>>>>>>>>>>> here?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> https://disk.yandex.com/i/z6fYbeVfPRK7aA
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Looks like reflections in the cable. Try the 50 Ohm
>>>>>>>>>>>>> termination.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Arie
>>>>>>>>>>>>
>>>>>>>>>>>> If the drive is a sine wave, a cable can't generate that 2nd
>>>>>>>>>>>> harmonic.
>>>>>>>>>>>
>>>>>>>>>>> I don't understand how a reflection can account for it either.
>>>>>>>>>>> THe cable's only 4' long! However, with the 50 ohm input
>>>>>>>>>>> enabled, the 2nd harmonic disappears. It's just one of those
>>>>>>>>>>> inexplicable mysteries that no one knows the answer to. :)
>>>>>>>>>>
>>>>>>>>>> That?s pretty diagnostic. There must be an LC filter on the
>>>>>>>>>> output?mis-terminating it will cause all sorts of
>>>>>>>>>> frequency-response whoopdedoos.
>>>>>>>>> 
>>>>>>>>> It also occurs to me that if there is a diode in series with a
>>>>>>>>> resistor somewhere, the impedance presented to the feed coax may
>>>>>>>>> be 50 ohms for positive input voltage, and say 10 Kohm for
>>>>>>>>> negative.  At the very least one could get an inverted
>>>>>>>>> reflection on negative.
>>>>>>>>> 
>>>>>>>>> Joe Gwinn
>>>>>>>>> 
>>>>>>>>Yeah, or an emitter follower.  Good point.
>>>>>>>
>>>>>>>Plus some LC filter wiggles, to distort and smooth things.
>>>>>>>
>>>>>>>I dug around and found a copy of the HP 8566B spectrum analyzer
>>>>>>>service manual.  The 10 MHz ref input is an amplifier driving a
>>>>>>>mixer,
>>>>>>>with not hint of for instance a TTL input.  So, the problem must be
>>>>>>>elsewhere.  Or, it's just busted.
>>>>>>>
>>>>>>>.<chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://
>>xdevs.com/doc/HP_Agilent_Keysight/
>>HP%208566B%20Troubleshooting%20&%20Repair%20Vol.%201.pdf>
>>>>>>>
>>>>>>>Joe Gwinn
>>>>>>
>>>>>>334 pages! Where is the issue?
>>>>>
>>>>>The pagination was unclear, so I didn't see a way to say.  How I
>>>>>found it was to look at the schematics.  Look for module A22.  Don't
>>>>>think search works on such images, but visual search didn't take that
>>>>>long.
>>>>>
>>>>>Joe Gwinn
>>>>
>>>>Too much work for free consulting.
>>> 
>>> I took another look.  There is a page reference on the right edge,
>>> near the bottom, which is not marked as a page X of Y, but is.
>>> 
>>> Anyway, look at pages 59 and 77.  At 77, look for A22 in the lower
>>> left region.  Just above, look for INT and EXT jacks.  The EXT jack is
>>> where an external reference enters.
>>> 
>>> Joe Gwinn
>>
>>I don't believe the 10Mhz ref osc is the problem, John. The 2nd harmonic
>>distortion goes away when the scope input impedance is set to 50 ohms.
>>There is some slight distortion on it, but not enough to cause an
>>out-of- lock error. Furthermore, since that osc is the pace-setter for
>>every other module in every other loop in this analyzer, its failure
>>would give rise to way more error messages than a mere "YTO unlock" as
>>it stands at present. The manual suggests the most likely areas where
>>the fault is located are in one of the boards A19, A20, A21 or A11.  If
>>A22 were the culprit, there would be over a dozen error messages.
> 
> 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

7.68dBm on 50 Ohms
2.68V P-P on 1 Meg

No issues there AFAIC.