Re: Distorted Sine Wave

On Fri, 31 May 2024 22:17:37 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

On Fri, 31 May 2024 14:48:28 -0400, Joe Gwinn wrote:
>

On Fri, 31 May 2024 17:29:47 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:
 

On Fri, 31 May 2024 11:34:46 -0400, Joe Gwinn wrote:
>

On Thu, 30 May 2024 16:24:25 -0700, john larkin <jl@650pot.com> wrote:
 

On Thu, 30 May 2024 18:53:29 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
>

On Thu, 30 May 2024 14:56:30 -0700, john larkin <jl@650pot.com>
wrote:
>

On Thu, 30 May 2024 16:06:47 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
>

On Thu, 30 May 2024 15:02:44 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2024-05-29 18:59, Joe Gwinn wrote:

On Wed, 29 May 2024 22:11:47 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

Cursitor Doom <cd999666@notformail.com> wrote:

On Wed, 29 May 2024 13:42:13 -0700, john larkin wrote:
>

On Wed, 29 May 2024 21:43:54 +0200, Arie de Muijnck
<noreply@ademu.com>
wrote:
>

On 2024-05-29 19:07, Cursitor Doom wrote:

Gentlemen,
>
Whilst fault-finding on my HP 8566B spectrum analyzer, I've
found the 10Mhz reference oscillator is generating an
'unsatisfactory waveform'
which may be causing the device to be unable to lock it's
main PLL. I've come across this waveshape before, but
mostly with oscillators I was building and in the process
of trying to iron out the wrinkles of and certainly NOT a
critical reference oscillator from a respected
manufacturer. Can anyone tell what's most likely going on
here?
>
https://disk.yandex.com/i/z6fYbeVfPRK7aA

>
>
Looks like reflections in the cable. Try the 50 Ohm
termination.
>
Arie

>
If the drive is a sine wave, a cable can't generate that 2nd
harmonic.

>
I don't understand how a reflection can account for it either.
THe cable's only 4' long! However, with the 50 ohm input
enabled, the 2nd harmonic disappears. It's just one of those
inexplicable mysteries that no one knows the answer to. :)

>
That?s pretty diagnostic. There must be an LC filter on the
output?mis-terminating it will cause all sorts of
frequency-response whoopdedoos.

 
It also occurs to me that if there is a diode in series with a
resistor somewhere, the impedance presented to the feed coax may
be 50 ohms for positive input voltage, and say 10 Kohm for
negative.  At the very least one could get an inverted
reflection on negative.
 
Joe Gwinn
 

Yeah, or an emitter follower.  Good point.

>
Plus some LC filter wiggles, to distort and smooth things.
>
I dug around and found a copy of the HP 8566B spectrum analyzer
service manual.  The 10 MHz ref input is an amplifier driving a
mixer,
with not hint of for instance a TTL input.  So, the problem must be
elsewhere.  Or, it's just busted.
>
.<chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://

xdevs.com/doc/HP_Agilent_Keysight/
HP%208566B%20Troubleshooting%20&%20Repair%20Vol.%201.pdf>

>
Joe Gwinn

>
334 pages! Where is the issue?

>
The pagination was unclear, so I didn't see a way to say.  How I
found it was to look at the schematics.  Look for module A22.  Don't
think search works on such images, but visual search didn't take that
long.
>
Joe Gwinn

>
Too much work for free consulting.

 
I took another look.  There is a page reference on the right edge,
near the bottom, which is not marked as a page X of Y, but is.
 
Anyway, look at pages 59 and 77.  At 77, look for A22 in the lower
left region.  Just above, look for INT and EXT jacks.  The EXT jack is
where an external reference enters.
 
Joe Gwinn

>
I don't believe the 10Mhz ref osc is the problem, John. The 2nd harmonic
distortion goes away when the scope input impedance is set to 50 ohms.
There is some slight distortion on it, but not enough to cause an
out-of- lock error. Furthermore, since that osc is the pace-setter for
every other module in every other loop in this analyzer, its failure
would give rise to way more error messages than a mere "YTO unlock" as
it stands at present. The manual suggests the most likely areas where
the fault is located are in one of the boards A19, A20, A21 or A11.  If
A22 were the culprit, there would be over a dozen error messages.

 
What is the 10 MHz signal power level at the EXT input in these two
castes, 1 Mohm and 50 Ohm?  The expected range is 0 to +10 dBm.
 
Joe Gwinn

>
7.68dBm on 50 Ohms
2.68V P-P on 1 Meg
>
No issues there AFAIC.

Not so fast there.  It's the scope that is being set to 1 Meg or 50
ohm, and at 1 Meg we are observing the drive into the 50 ohm input
impedance of the EXT ref input of the spectrum analyzer.

That 2.7 Vpp over 50 ohms is about +13 dBm, which exceeds +10 dBm.  So
the EXT input may be over-driven. Install a 5-dB inline attenuator and
see what happens.

Joe Gwinn