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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 16:06:47 -0400
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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