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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sat, 1 Jun 2024 15:44:17 +0200
Organization: A noiseless patient Spider
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On 6/1/24 14:07, Cursitor Doom wrote:
> On Fri, 31 May 2024 17:33:51 -0700, boB wrote:
> 
>> On Fri, 31 May 2024 17:02:13 -0700, boB <boB@K7IQ.com> wrote:
>>
>>> On Fri, 31 May 2024 21:40:04 -0000 (UTC), Cursitor Doom
>>> <cd999666@notformail.com> wrote:
>>>
>>>> On Fri, 31 May 2024 15:10:32 -0400, Joe Gwinn wrote:
>>>>
>>>>> On Fri, 31 May 2024 18:54:06 -0000 (UTC), Cursitor Doom
>>>>> <cd999666@notformail.com> wrote:
>>>>>
>>>>>> On Fri, 31 May 2024 11:34:46 -0400, Joe Gwinn wrote:
>>>>>>
>>>>>>> On Thu, 30 May 2024 16:24:25 -0700, john larkin <jl@650pot.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> On Thu, 30 May 2024 18:53:29 -0400, Joe Gwinn
>>>>>>>> <joegwinn@comcast.net>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> On Thu, 30 May 2024 14:56:30 -0700, john larkin <jl@650pot.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> On Thu, 30 May 2024 16:06:47 -0400, Joe Gwinn
>>>>>>>>>> <joegwinn@comcast.net>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> On Thu, 30 May 2024 15:02:44 -0400, Phil Hobbs
>>>>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> 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.
>>>>>>>>>>>
>>>>>>>>>>> Plus some LC filter wiggles, to distort and smooth things.
>>>>>>>>>>>
>>>>>>>>>>> I dug around and found a copy of the HP 8566B spectrum analyzer
>>>>>>>>>>> service manual.  The 10 MHz ref input is an amplifier driving a
>>>>>>>>>>> mixer,
>>>>>>>>>>> with not hint of for instance a TTL input.  So, the problem must
>>>>>>>>>>> be elsewhere.  Or, it's just busted.
>>>>>>>>>>>
>>>>>>>>>>> .<chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://
>>>>>> xdevs.com/doc/HP_Agilent_Keysight/
>>>>>> HP%208566B%20Troubleshooting%20&%20Repair%20Vol.%201.pdf>
>>>>>>>>>>>
>>>>>>>>>>> Joe Gwinn
>>>>>>>>>>
>>>>>>>>>> 334 pages! Where is the issue?
>>>>>>>>>
>>>>>>>>> The pagination was unclear, so I didn't see a way to say.  How I
>>>>>>>>> found it was to look at the schematics.  Look for module A22.
>>>>>>>>> Don't think search works on such images, but visual search didn't
>>>>>>>>> take that long.
>>>>>>>>>
>>>>>>>>> Joe Gwinn
>>>>>>>>
>>>>>>>> Too much work for free consulting.
>>>>>>>
>>>>>>> I took another look.  There is a page reference on the right edge,
>>>>>>> near the bottom, which is not marked as a page X of Y, but is.
>>>>>>>
>>>>>>> Anyway, look at pages 59 and 77.  At 77, look for A22 in the lower
>>>>>>> left region.  Just above, look for INT and EXT jacks.  The EXT jack
>>>>>>> is where an external reference enters.
>>>>>>>
>>>>>>> Joe Gwinn
>>>>>>
>>>>>> It's just a block diagram of the reference oscillator module. There's
>>>>>> no detailed schematic of the oscillator itself. At least that's the
>>>>>> case with the PDF version of the Service Manual I have.
>>>>>
>>>>> It's probably in the book that I don't have, if it was ever released.
>>>>>
>>>>> Joe Gwinn
>>>>
>>>> HP actually published at least 5 manuals of various kinds about this
>>>> analyzer, so it's possible it's in one of the others. However, the
>>>> actual service manual is where I'd expect to find it if they published
>>>> it at all.
>>>> The other titles are:
>>>>
>>>> HP 8566B Operator's Manual HP 8566B Installation and Verification
>>>> Manual HP 8566B Test and Adjustments Manual HP 8566B Troubleshooting
>>>> and Repair Manual
>>>>
>>>> So if the answers are not in *any* of the above, it'd be jolly rotten
>>>> luck indeed!
>>>
>>>
>>>
>>> Looking at that waveform again...
>>>
>>> Could it be that the wavform you are seeing there is not a harmonic but
>>> just a partial cancelation due to reflections ?
>>>
>>> If you look at one peak vs. the other, they look about the same time to
>>> me.  About 1 scope division.  Just the amplitude looks different on
>>> alternating cycles.   A non-linearity I would think would make an even
>>> harmonic.
>>
>> I thought I said the wrong thing !  I mean odd harmonic(s) from a
>> non-linearity.
>>
>>
>>> 2nd harmonic is usually due to non 1/2 wave symetry (DC
>>> offset bascially)
>>>
>>>
>> Here, I meant even harmonic(s)  not just second harmonic.
>>
>> It's rather hot today in Phoenix..
> 
> 1. Everybody makes careless mistakes.
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