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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Mon, 03 Jun 2024 07:48:32 -0400
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On Sun, 2 Jun 2024 20:58:45 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>On Sun, 02 Jun 2024 14:08:48 -0400, Joe Gwinn wrote:
>
>> On Sun, 2 Jun 2024 16:55:28 -0000 (UTC), Cursitor Doom
>> <cd999666@notformail.com> wrote:
>> 
>>>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>
>>>>>>>>>> 

And <https://disk.yandex.com/i/z6fYbeVfPRK7aA>


>>>>>>>>>> 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).
>> 
>> What we don't know is exactly how you made the various measurements. If
>> you are observing the signal from the 10 MHz reference where it enters
>> the analyzer, I would expect that there is a T-connector with the scope
>> (set to 1 Mohm) listening in to passing signals.
>
>You did ask me this before and did post an answer. See Message-ID: 
><v3fsbp$2u0a6$1@dont-email.me>
>
>You also still appear to think that the 10Mhz signal is going into the 
>analyzer. It isn't. It's coming out. Again, see Message-ID: 
><v3fsbp$2u0a6$1@dont-email.me>

I did read that, but didn't know what to make of it.  I think an
annotated drawing is required.

On this drawing, where do the various scope traces mentioned up-thread
come from?

Joe Gwinn






>> In this case, the load seen by the incoming reference is that provided
>> by the input on the analyzer.  Which input is +10 dBm max.  If you set
>> the observing scope input to 50 ohm, the reference will see a 25 ohm
>> load, cutting the signal seen by the analyzer by 3 dB.  Which will take
>> +13 dBm down to +10 dBm, which is in range.
>> 
>> A 3dB attenuator in line will drop the signal to 10 dBm as well.
>> 
>> I've built lots of systems like that.  The 10 MHz reference is delivered
>> to everybody at +13 dBm, and it is the receivers' responsibility to
>> attenuate it to whatever they need.
>> 
>> 
>>>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....
>> 
>> To my eye, it does scream.
>> 
>> Joe Gwinn