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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Wed, 29 May 2024 23:10:56 +0200
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On 5/29/24 22:49, Cursitor Doom 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. :)

If this oscillator is made to drive 50 Ohms and you don't
provide that, internal buffer stages may saturate or do other
weird things. If it works OK *with* the 50 Ohm load, then
your problem is solved, no?

Jeroen Belleman