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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 14:56:30 -0700
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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?