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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 16:24:25 -0700
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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.