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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Fri, 31 May 2024 18:54:06 -0000 (UTC)
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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

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.