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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Distorted Sine Wave
Date: Mon, 3 Jun 2024 19:38:21 +0200
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On 6/3/24 17:43, Cursitor Doom wrote:
> On Mon, 03 Jun 2024 07:48:32 -0400, Joe Gwinn wrote:
> 
>> 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
> 
> Joe, I appreciate you're only trying to help, but don't worry about it. I
> don't believe the oscillator this thread relates to is causing the PLL
> unlock error so we've all gone down a bit of a rabbit hole with this one.
> I really need to look elsewhere for the culprit.

For what it's worth, I found a manual on the web saying that the
10MHz output should deliver +5dBm into 50 Ohms. It's a bit low,
then, but I doubt that this is your problem.

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