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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Mon, 03 Jun 2024 07:43:41 -0400
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On Sun, 2 Jun 2024 23:45:43 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 6/2/24 21:44, Joe Gwinn wrote:
>> On Sun, 2 Jun 2024 20:18:50 +0200, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>> 
>>> On 6/2/24 18:19, Joe Gwinn wrote:
>>>> On Sun, 2 Jun 2024 11:31:33 -0000 (UTC), Cursitor Doom
>>>> <cd999666@notformail.com> wrote:
>>>>
>>>>> On Sun, 2 Jun 2024 11:17:58 -0000 (UTC), piglet wrote:
>>>>>
>>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>>>> On Sat, 1 Jun 2024 22:00:58 -0000 (UTC), piglet wrote:
>>>>>>>
>>>>>>>> Cursitor Doom <cd999666@notformail.com> 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:
>>>>
>>>> .<https://www.amazon.com/MWRF-Source-Male-Female-Attenuator/dp/B07MP9D9GC?th=1>
>>>>
>>>> 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.

I copied the wrong yandex.

Try this (29 May 2024):
..<https://disk.yandex.com/i/z6fYbeVfPRK7aA >

Joe Gwinn