Re: Distorted Sine Wave

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:
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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