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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sat, 01 Jun 2024 17:18:03 -0400
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On Sat, 1 Jun 2024 19:21:29 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>On Sat, 01 Jun 2024 14:28:35 -0400, Joe Gwinn wrote:
>
>> On Fri, 31 May 2024 22:17:37 -0000 (UTC), Cursitor Doom
>> <cd999666@notformail.com> wrote:
>> 
>>>On Fri, 31 May 2024 14:48:28 -0400, Joe Gwinn wrote:
>>>
>>>> On Fri, 31 May 2024 17:29:47 -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
>>>>>
>>>>>I don't believe the 10Mhz ref osc is the problem, John. The 2nd
>>>>>harmonic distortion goes away when the scope input impedance is set to
>>>>>50 ohms. There is some slight distortion on it, but not enough to
>>>>>cause an out-of- lock error. Furthermore, since that osc is the
>>>>>pace-setter for every other module in every other loop in this
>>>>>analyzer, its failure would give rise to way more error messages than
>>>>>a mere "YTO unlock" as it stands at present. The manual suggests the
>>>>>most likely areas where the fault is located are in one of the boards
>>>>>A19, A20, A21 or A11.  If A22 were the culprit, there would be over a
>>>>>dozen error messages.
>>>> 
>>>> What is the 10 MHz signal power level at the EXT input in these two
>>>> castes, 1 Mohm and 50 Ohm?  The expected range is 0 to +10 dBm.
>>>> 
>>>> Joe Gwinn
>>>
>>>7.68dBm on 50 Ohms 2.68V P-P on 1 Meg
>>>
>>>No issues there AFAIC.
>> 
>> Not so fast there.  It's the scope that is being set to 1 Meg or 50 ohm,
>> and at 1 Meg we are observing the drive into the 50 ohm input impedance
>> of the EXT ref input of the spectrum analyzer.
>
>Unless I misunderstand what you're saying, that's not the case, though. 
>What we are observing is the output of the analyzer's 10Mhz reference 
>oscillator taken from a BNC socket on the rear of it which HP have 
>thoughtfully provided and fed directly into a scope switchable between 1M 
>and 50 Ohms.
>
>> That 2.7 Vpp over 50 ohms is about +13 dBm, which exceeds +10 dBm.  So
>> the EXT input may be over-driven. Install a 5-dB inline attenuator and
>> see what happens.
>
>It's not an external input. They've put it there in order to make it easy 
>to check the frequency of the reference oscillator and adjust it to 
>precisely 10.000000Mhz if necessary (after a *minimum* 72 hour warm-up!)

Maybe.  But it's simpler to just try the attenuator.

Joe Gwinn