Re: Distorted Sine Wave

On 02/06/2024 13:12, Cursitor Doom wrote:

On Sun, 2 Jun 2024 13:00:21 +0100, John R Walliker wrote:
 

On 02/06/2024 12:31, Cursitor Doom 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.

>
I have an 8566B which is currently not working.  Both the status leds on
the front panel at the bottom are red.  I haven't started to investigate
yet.
The fault developed slowly.  At first it would sometimes work, then
progressively less often and now never.
However, if the signal being discussed is available on the rear panel I
could measure mine and see what it looks like and what voltage is
delivered.
John

 Yes, that could be very helpful, John, since your fault is clearly totally
different to mine. Peak to peak volts into 50 ohms on a scope will be fine
if don't have access to an RF power meter.

I measured the 10MHz output of my unit with an HP 54542A digital storage
'scope and found the following:
1Mohm input via 5m coax
1.133 Vp-p
405.8 mVrms
50ohm input via 5m coax
790.3 mVp-p
284.9 mVrms
The waveform with 1Mohm load looked like a perfect sine wave, whereas
the 50 ohm loaded waveform showed some second harmonic distortion
visible as a slight narrowing of the top of the sine and a slight
flattening of the bottom of the sine.
The frequency measured with an HP 53131A frequency counter
was 10.000199MHz.
The spectrum analyzer had been on standby for a few weeks and fully
powered for a few hours, so the crystal oven should have been
at equilibrium.  I then substituted my rubidium oscillator which indicated 9.999997MHz on the counter.
The connector that I used was the one labelled 10MHz at bottom right,
not the adjacent reference output which is connected to a reference
input with a short coax jumper.  Is the same output that you measured?
John