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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Wed, 29 May 2024 22:11:47 -0000 (UTC)
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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. 

Cheers 

Phil Hobbs 


-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics