Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sun, 2 Jun 2024 16:55:28 -0000 (UTC) Organization: A noiseless patient Spider Lines: 100 Message-ID: References: <9tok5j9p388ookujrtbsofskjlbekfuhjb@4ax.com> <60rk5jti9l5154hqaqicohmj3u1lfd16g3@4ax.com> <4k6p5jhgmrigja3o0tdur5tvkfc7bsrd15@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sun, 02 Jun 2024 18:55:29 +0200 (CEST) Injection-Info: dont-email.me; posting-host="40a1d383888994c45f6bff6cd4b36ba9"; logging-data="3615124"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+3BpBBdUYfSvihFj8Ig/c4zMzecpFO6eY=" User-Agent: Pan/0.149 (Bellevue; 4c157ba) Cancel-Lock: sha1:LN9zJlycXGlMLb/c8yh4M1Fapss= Bytes: 5980 On Sun, 02 Jun 2024 12:19:05 -0400, Joe Gwinn wrote: > On Sun, 2 Jun 2024 11:31:33 -0000 (UTC), Cursitor Doom > wrote: > >>On Sun, 2 Jun 2024 11:17:58 -0000 (UTC), piglet wrote: >> >>> Cursitor Doom wrote: >>>> On Sat, 1 Jun 2024 22:00:58 -0000 (UTC), piglet wrote: >>>> >>>>> Cursitor Doom wrote: >>>>>> On Sat, 1 Jun 2024 15:44:17 +0200, Jeroen Belleman wrote: >>>>>> >>>>>>> On 6/1/24 14:07, Cursitor Doom wrote: >>>>>> >>>>>>>> I've taken a shot of the waveform into the 50 ohm input. It's >>>>>>>> around 850mV peak-peak. Hopefully the slight distortion I spoke >>>>>>>> about is visible; the slightly more leisurely negative-going >>>>>>>> excursions WRT their positive-going counterparts. So it's not a >>>>>>>> pure sine wave as one would expect. Does it matter? I don't know! >>>>>>>> >>>>>>>> https://disk.yandex.com/i/7cuuBimDbOIBZw >>>>>>> >>>>>>> The shape looks perfectly acceptable to me. This is +3dBm into 50 >>>>>>> Ohms. >>>>>>> Is that what it's supposed to be? Canned reference oscillators >>>>>>> most often deliver +13dBm, sometimes +10dBm. >>>>>> >>>>>> Is it? I only make it about half your figure: +1.65dBm. >>>>>> I admit I'm frequently prone to careless errors, so stand to be >>>>>> corrected, >>>>>> but here's my method: >>>>>> 850mV peak to peak is 425mV peak voltage. Average of that is >>>>>> 0.425x0.636 = >>>>>> 0.27V. Average power is average volts squared divided by the load >>>>>> impedance of 50 ohms = 1.46mW = +1.65dBm. >>>>>> >>>>>> I shall consult the manual to see what it ought to be - if I can >>>>>> find it, that is, as PDF manuals are a nightmare to navigate IME. >>>>>> >>>>>> >>>>>> >>>>> Use 0.71 for RMS instead of 0.636 ! I make that about 1.8mW or >>>>> +2.6dBm ? >>>> >>>> Thanks, Erich. But there's no such thing as "RMS power" strictly >>>> speaking IIRC, so that's why I took the average figure; not that it >>>> makes much difference in practice. it does seem a bit on the low >>>> side, but despite reading through the most likely sources (the >>>> service manual and the trouble-shooting/repair manual) I can find >>>> nothing stated for what that signal level should be! This may be due >>>> to the user-unfriendliness of very large PDF manuals; I just don't >>>> know. Anyway, not very satisfactory! Later today I plan to do a >>>> direct power meter measurement of the ref osc (since none of us here >>>> seem to agree on what 850mV vs 50 ohms equates to!!) >>>> >>>> >>>> >>>> >>>> >>> Since you have a power meter, a signal source, and an oscilloscope why >>> not measure the peak to peak voltage on the scope and power on the >>> power meter and see which calculation 0.636 vs 0.707 gives the closest >>> agreement? >> >>It wouldn't prove anything one way or ther other, though, since that >>power meter hasn't been calibrated for "quite a while" so to speak. :) >>It'll give a 'good enough' reading for my purposes, but won't be >>accurate enough to meaningfully test your otherwise fine suggestion. > > > The 0 to +10 dBm range I mentioned came from the service manual. > > Looking at your scope picture, it looks like a 3 Vpp signal, which is > +13 dBm, a very common distribution level, but one that exceeds the > analyzer's allowed range. All that's needed to fix this is a 3dB inline > attenuator. Here is one for SMA connectors: > > . > > Just buying a few of these and doing some experiments will be far > cheaper and faster than the various alternatives discussed.t > > Joe Gwinn I think you're looking at the first picture with the signal into the scope's 1 Meg input. The 50 ohm trace is only 850mV peak-to-peak or thereabouts and when I measured it with an actual power meter, came out at about +2.5dBm so within the range you stated; no attenuation needed (thanks for the range, by the way; I needed to know that). I've now measured the 100Mhz oscillator and that seems fine, although I only saw 0.61V p-p into 50 ohms, so somewhat less than the 10Mhz oscillator's output. So far, I've not measured anything which screams "the fault's here!" as all the expected signals are present - although admittedly I have many more to test. But certainly all the *major* signals within this complex beast are present. It's looking like it could be an issue with one of the phase detectors or LPFs. Sigh....