Re: Distorted Sine Wave

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....