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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Sun, 2 Jun 2024 16:55:28 -0000 (UTC)
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On Sun, 02 Jun 2024 12:19:05 -0400, 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.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....