[time-nuts] PN/AM and 1.5Hz spur from frequency doubling?

[Bill Byrom]

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:

> 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

> 

>> On Jan 20, 2017, at 12:22 PM, Anders Wallin
>> <anders.e.e.wallin at gmail.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 at gmail.com
>>> wrote:

>> 

>>> 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 at radio.sent.com>
>>> wrote:
>>> 

>>>> 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

>>>> 

>>>> 

>>>> 

>>>> 

>>>> 

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