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NNTP-Posting-Date: Thu, 30 May 2024 18:55:55 +0000
From: boB <boB@K7IQ.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 11:55:55 -0700
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On Wed, 29 May 2024 20:08:58 -0700, john larkin <jl@650pot.com> wrote:

>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.
>
>

I was thinking the same thing.  Non-linearity.

But he said that when he loaded it with 50 Ohms, it went back to a
sinewave.   Maybe that non-linearity went away when it was loaded ?
Could be some kind of termination non-linearity or even coming from
the source with the higher levels of reflections.  Can't explain it
otherwise.

boB