Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Jeroen Belleman Newsgroups: sci.electronics.design Subject: Re: Distorted Sine Wave Date: Sun, 2 Jun 2024 23:45:43 +0200 Organization: A noiseless patient Spider Lines: 110 Message-ID: References: <9tok5j9p388ookujrtbsofskjlbekfuhjb@4ax.com> <60rk5jti9l5154hqaqicohmj3u1lfd16g3@4ax.com> <4k6p5jhgmrigja3o0tdur5tvkfc7bsrd15@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sun, 02 Jun 2024 23:43:40 +0200 (CEST) Injection-Info: dont-email.me; posting-host="e56d717ab3cbbbe261a1c19c41844c4c"; logging-data="3709942"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18gP6qNS7I5zT7KaeajupwU" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Cancel-Lock: sha1:4ofiTt3/xWK0Jgv0pk4drKLXJYM= In-Reply-To: Content-Language: en-US Bytes: 6407 On 6/2/24 21:44, Joe Gwinn wrote: > On Sun, 2 Jun 2024 20:18:50 +0200, Jeroen Belleman > wrote: > >> On 6/2/24 18:19, 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. >>> >>> Joe Gwinn >> >> What scope picture are you looking at? I see only 0.88Vpp. > > This one, posted by CD on 1 June '24: > > .< https://disk.yandex.com/i/7cuuBimDbOIBZw> > > This is the one with the funny stuff at the bottom. If you look at > the upper waveshape, the peak amplitude to the inflection point near > the bottom is about 1.5 Vp, which implies 3 Vpp, which is +13 dBm into > 50 ohms. Why the inflection point? Because in a undistorted sine > wave, the zero crossing is linear, and does not flair. The scope > picture does not show where zero volts is, so had to use the > inflection point. > > Joe Gwinn I'm afraid you have lost me there... I see only a roughly sine-shaped wave framed with cursors along the peaks being 0.88V apart. I don't care about the DC level, only the 10MHz component matters. Its amplitude is only 0.44V. Jeroen Belleman