Re: Distorted Sine Wave

On Sat, 01 Jun 2024 14:28:35 -0400, Joe Gwinn wrote:

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.

Unless I misunderstand what you're saying, that's not the case, though.
What we are observing is the output of the analyzer's 10Mhz reference
oscillator taken from a BNC socket on the rear of it which HP have
thoughtfully provided and fed directly into a scope switchable between 1M
and 50 Ohms.

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.

It's not an external input. They've put it there in order to make it easy
to check the frequency of the reference oscillator and adjust it to
precisely 10.000000Mhz if necessary (after a *minimum* 72 hour warm-up!)