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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sat, 01 Jun 2024 18:03:01 -0400
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On Sat, 1 Jun 2024 21:46:10 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>On Sat, 01 Jun 2024 17:18:03 -0400, Joe Gwinn wrote:
>
>> 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
>
>If I can find a reverse attenuator, I'd be happy to try it.

I don't think you need such a thing.  Inline attenuators (male-female)
are symmetric, and don't care which way the signal travels.

Joe Gwinn