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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 18:53:29 -0400
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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