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

Anders Wallin anders.e.e.wallin at gmail.com
Fri Jan 20 12:22:39 EST 2017

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
as a time-series of residual phase the switchoff of the time-code
transmitter looks like so:

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

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?)


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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