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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Thu, 30 May 2024 15:02:44 -0400
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On 2024-05-29 18:59, Joe Gwinn wrote:
> 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
> 
Yeah, or an emitter follower.  Good point.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com