Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sat, 01 Jun 2024 21:18:03 +0000 From: Joe Gwinn Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sat, 01 Jun 2024 17:18:03 -0400 Message-ID: <0v3n5jh1mrclpfh9jru7k4n54v9ovs03q5@4ax.com> References: <48bd78e1-7da8-3bba-2879-d22962203fa3@electrooptical.net> <9olh5j9al34fhrebr4grqq8h6c8javjpp1@4ax.com> <1n0i5jh257hiinlj2dhaatlo11s33m5n0e@4ax.com> <9k2i5jpfhu3ncfpm28ukusrok4hugal80s@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: 162 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-Bh0VLSTgwonCeOGPXesLoVG8VxlogmwRPrpzISwkL68kc4gl2PVZVKBnl5K5GETc3HScnWsTkhu5aTe!ZUtt51hmNIU/fqpuUsQjxZ4v909tbJaz+U4Ij/u98xhdo196jUNB4TX0RGaXlRTM/slcfM8= 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: 8453 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