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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sun, 2 Jun 2024 17:39:45 -0000 (UTC)
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On Wed, 29 May 2024 18:19:40 -0700, boB 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.
e 
I'm guessing there should have been a 50 ohm load screwed into the rear 
10Mhz reference oscillator output BNC socket when the analyzer's in use. 
There wasn't one but there is now. Unfortunately it hasn't cleared the PLL 
unlock issue.