Hello,
can you share also the Phase Difference trace?

cheers,
Mattia

2017-01-18 17:13 GMT+01:00 Anders Wallin anders.e.e.wallin@gmail.com:

Hi

Without seeing the circuit involved it’s a bit tough to guess all of the possible
things that might be happening. One branch leads off to things like the circuit
it’s self oscillating and creating the spur. Sub branches involve oscillation in
a regulator at low frequency vs RF oscillation somewhere else. Another branch
heads in the direction  of a loose cable running around the shop with a bunch
of 10 MHz on it. There are many branches ….

Bob

Hi,

Considering that you have a diode and a tuned LC-tank, the tank will
"ring" in it's own frequency, even if it is driven by the diode and 5
MHz. This creates a single side-band tone which then shows up both as AM
and PM. Well, that is my guess. 1.5 Hz is however very very close, which
is quite impressive for 10 MHz, so there should be something more going
on, I'm just speculating.

This gives some insight as one of many:
https://www.minicircuits.com/app/DOUB9-2.pdf
See the warning of over-power on last page.

MVH
Magnus

On 01/18/2017 06:01 PM, Bob Camp wrote:

You may find that the behavior varies quite a bit depending on what you put between the maser output and the doubler.  I've seen one case in particular that generates a ton of PM spurs, specifically an HP 11721A doubler driven by the 5 MHz output of a 5061A Cs standard.  The 5 MHz output uses a narrowband transformer with a parallel-tuned primary, so it looks highly reactive at frequencies other than 5 MHz.  I'm not sure what's really going on, but the 11721A (which is being run below its frequency spec in this case) doesn't seem happy at all ( http://www.ke5fx.com/11721a.png ).  Adding an SLP-5 lowpass filter in front of the 11721A makes the spurs go away, as does an isolation amp.

The 1.5 Hz spur in your plot looks like leakage from a nearby 10 MHz source, presumably a free-running OCXO that's a bit off frequency.  The rest of it looks like an open coax shield or something.  (If you're using a coaxial balun, try removing it.)  It's possible to get 2-3 dB of excess noise from a multiplier beyond the expected 20*log(N), depending on source and load characteristics, but I haven't seen anything that bad in the absence of other problems.

-- john, KE5FX
Miles Design LLC

I see spurs at 50 Hz and harmonics, which I assume are from the power
line at your location. This might be due to an oscillation in the power
supply regulator, leading to nonlinear regulator operation and
feedthrough of power line ripple. For example, low dropout regulators
can sometimes oscillate when an additional ceramic bypass capacitor is
added due to decreased phase margin in the feedback loop. It's also
possible that there is too much ripple before the regulator and you are
exceeding the dropout voltage, or that the regulator is going in and out
of an overcurrent condition. Many odd things may happen if the power
supply regular isn't working properly.

--

Bill Byrom N5BB

Thanks for all the comments so far.
I will try the doubler with another quieter source, and try removing
various potential noise-sources and exchanging cables...

I have now uploaded a few more images of the same data to the shared album
linked in my earlier post.

Anders

On Thu, Jan 19, 2017 at 10:39 PM, Bill Byrom time@radio.sent.com wrote:

I made some progress with this issue today.
It turns I was using a 75Ohm cable at some point (doh!) which caused a
'forest' of spurs far out. Possibly our other maser has a faulty/cut cable
which behaves similarly.
The final fix was to turn off our 25 MHz radio time-code transmitter which
was causing the strong close-in spur at around 1.5 Hz. It uses a modified
DCF77 code where it transmits full power AM-modulated 25MHz carrier for 0,
100ms or 200ms at the start of each second.

Here are PN plots of the 5MHz maser signal, same signal through 75ohm
reflective cable to the doubler, and through a 50ohm cable
to the doubler which solves the far-out spurs, and finally turning off the
radio transmitter. The result is now close to the +6dBc/Hz expected for a
doubler.
https://goo.gl/photos/qKKvg3SfE1XKxtq17
as a time-series of residual phase the switchoff of the time-code
transmitter looks like so:
https://goo.gl/photos/jNVJK2kj1kGUkSVd9

Finally I tried it with the transmitter on, but reduced coupling into the
lab by disconnecting a few monitoring-cables. Strangely this shifts the
spur even closer in (close to 1Hz now) and reduces the amplitude as expected
https://goo.gl/photos/jG6rxfuC8R2QKchM6

What makes frequency doublers especially sensitive to this kind of
interference? The 25MHz carrier is phase-locked to better than 1e-12 to our
masers, so there can't reasonably be a 1-1.5Hz offset in the carrier
frequency. What is the interaction? (5th harmonic of 5Mhz mixes with 25MHz?)

Anders

On Fri, Jan 20, 2017 at 11:06 AM, Anders Wallin <anders.e.e.wallin@gmail.com

Hi

I would bet that the spur moving is an indicator of either the 25 MHz transmitter carrier or
modulator drifting in frequency. My guess is that the Maser does not drift :)

Bob

Since the spur moved in frequency when the amplitude of the 25 MHz
interference changed, my guess is that you have some grounding or cable
leakage issues which are causing the measuring system to produce
erroneous results due to overloading. Do you have all instruments and
sources plugged into the same power line circuit with a common safety
ground? Have you tried moving the equipment cases close to each other
and connecting them with a large gauge wire or strap between the
chassis? A 2 M long RF cable is about a quarter wavelength at 25 MHz,
and it's easy to get ingress if the RF field is high, especially if you
aren't using double shielded cables. Also be sure the 25 MHz transmitter
and transmission line system is well shielded and properly grounded.

What type of cables and connectors are used in the signal path to the
phase noise analyzer?

Has someone performed a field intensity survey in conjunction with the
25 MHz transmitter for safety purposes? You may have a high RF field in
that area, and if the V/m is too great you will have many difficulties.

--

Bill Byrom N5BB

On Fri, Jan 20, 2017, at 01:21 PM, Bob Camp wrote: