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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Wed, 29 May 2024 20:08:58 -0700
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On Wed, 29 May 2024 18:19:40 -0700, boB <boB@K7IQ.com> wrote:

>On Wed, 29 May 2024 20:49:27 -0000 (UTC), 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. :)
>>
>>
>>> Our boxes output a 10 MHz square wave. Our clock inputs have a 10 MHz
>>> bandpass filter, so they accept most anything.
>
>Weird but I'm not surprised that  4 feet if coax, unloaded at 10 MHz
>gives a strange waveform.  Can simulate this, I believe, in LTspice
>using the transmission line element(s).
>
>Learned something here though.
>
>boB
>AZ

No txline can create frequency components that are not in the source.

(Well, a NLTL can, but 4 feet of coax isn't a shock line.)

But the problem, as usual, is underspecified. Maybe some driver is
going nonlinear. A schematic would help.