Re: Distorted Sine Wave

On Fri, 31 May 2024 21:40:04 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

On Fri, 31 May 2024 15:10:32 -0400, Joe Gwinn wrote:
>

On Fri, 31 May 2024 18:54:06 -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

>
It's just a block diagram of the reference oscillator module. There's no
detailed schematic of the oscillator itself. At least that's the case
with the PDF version of the Service Manual I have.

 
It's probably in the book that I don't have, if it was ever released.
 
Joe Gwinn

>
HP actually published at least 5 manuals of various kinds about this
analyzer, so it's possible it's in one of the others. However, the actual
service manual is where I'd expect to find it if they published it at all.
The other titles are:
>
HP 8566B Operator's Manual
HP 8566B Installation and Verification Manual
HP 8566B Test and Adjustments Manual
HP 8566B Troubleshooting and Repair Manual
>
So if the answers are not in *any* of the above, it'd be jolly rotten luck
indeed!

Looking at that waveform again...

Could it be that the wavform you are seeing there is not a harmonic
but just a partial cancelation due to reflections ?

If you look at one peak vs. the other, they look about the same time
to me.  About 1 scope division.  Just the amplitude looks different on
alternating cycles.   A non-linearity I would think would make an even
harmonic.  2nd harmonic is usually due to non 1/2 wave symetry (DC
offset bascially)

What was tha amplitude after you loaded it down properly ?  How about
a picture ?

Where were you scoping this at again ?

boB