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NNTP-Posting-Date: Wed, 29 May 2024 22:59:39 +0000
From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Wed, 29 May 2024 18:59:39 -0400
Message-ID: <3lcf5jd7li0a3c0fgddt7o8lnfocvls2pr@4ax.com>
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On Wed, 29 May 2024 22:11:47 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>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. 

It also occurs to me that if there is a diode in series with a
resistor somewhere, the impedance presented to the feed coax may be 50
ohms for positive input voltage, and say 10 Kohm for negative.  At the
very least one could get an inverted reflection on negative.

Joe Gwinn