[time-nuts] PN/AM and 1.5Hz spur from frequency doubling?
kb8tq at n1k.org
Fri Jan 20 14:21:50 EST 2017
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 :)
> 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
> 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
>> 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.
>> 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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