Path: ...!local-3.nntp.ord.giganews.com!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sat, 01 Jun 2024 22:03:01 +0000 From: Joe Gwinn Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sat, 01 Jun 2024 18:03:01 -0400 Message-ID: References: <1n0i5jh257hiinlj2dhaatlo11s33m5n0e@4ax.com> <9k2i5jpfhu3ncfpm28ukusrok4hugal80s@4ax.com> <0v3n5jh1mrclpfh9jru7k4n54v9ovs03q5@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 177 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-STG2vV8FpxsOedvXtG717cQvE4DaRS03dSEaKyQmCbc8dxtwnGQ9weeM20I17461udsa31fPymqxhKe!XITlqWbugqvuGARwEC+210BuTm3tyUGpsYcD/BqKYbq9waF33Kxe3KcG28r+XMZOyKRNyS4= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 9147 On Sat, 1 Jun 2024 21:46:10 -0000 (UTC), Cursitor Doom 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 >> 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 >>>> 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 >>>>>> wrote: >>>>>> >>>>>>>On Fri, 31 May 2024 11:34:46 -0400, Joe Gwinn wrote: >>>>>>> >>>>>>>> On Thu, 30 May 2024 16:24:25 -0700, john larkin >>>>>>>> wrote: >>>>>>>> >>>>>>>>>On Thu, 30 May 2024 18:53:29 -0400, Joe Gwinn >>>>>>>>> >>>>>>>>>wrote: >>>>>>>>> >>>>>>>>>>On Thu, 30 May 2024 14:56:30 -0700, john larkin >>>>>>>>>>wrote: >>>>>>>>>> >>>>>>>>>>>On Thu, 30 May 2024 16:06:47 -0400, Joe Gwinn >>>>>>>>>>> >>>>>>>>>>>wrote: >>>>>>>>>>> >>>>>>>>>>>>On Thu, 30 May 2024 15:02:44 -0400, Phil Hobbs >>>>>>>>>>>> wrote: >>>>>>>>>>>> >>>>>>>>>>>>>On 2024-05-29 18:59, Joe Gwinn wrote: >>>>>>>>>>>>>> On Wed, 29 May 2024 22:11:47 -0000 (UTC), Phil Hobbs >>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> Cursitor Doom 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 >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>> >>>>>>>>>>>>.>>>>>>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