[time-nuts] HP5061B Square Wave Modulation Evalukation

Donald E. Pauly trojancowboy at gmail.com
Thu Apr 13 14:45:21 EDT 2017


We did some testing to evaluate the advantages of square wave
modulation instead of sine waves for locking to the atomic resonance
peak.  We drove J1 on the A3 board with a 2.8 Volt peak to peak
triangle at 20 cps from a signal generator.  The crystal oscillator
fine frequency was adjusted downward for below resonance. See
http://gonascent.com/papers/hp/hp5061/waveform/sqrmod.jpg .

The frequency out of the phase modulator is proportional to the
derivative of the varicap voltage drive.  For the triangle rising
slope, the varicap is linearly increasing in capacitance with an
constantly increasing phase lag.  For the falling slope it is linearly
decreasing in capacitance and a constantly increasing phase lead.
This results in a lower and higher frequency for the positive and
negative slope of the triangle respectively.   The frequency shifts
suddenly from about 300 cps below resonance to about 100 cps above
resonance.  We performed the same measurements with the fine frequency
centered and above resonance but did not photograph them.

Back of the envelope calculations predict a 10:1 improvement in jitter
with square wave lock for average measurement times of 1 second.  Beam
current was injected directly into the scope with its 1 Meg resistance
being the load for the electron multiplier.   The scope was inverted
to give a positive appearing beam current of 5 nA per division.  Peak
beam current is about 22 nA. (nA=Beam I meter reading)  Note that the
beam current is slightly delayed from the triangle and contains
positive spikes at both transitions.  This originates from the 1
millisecond or so travel time of the beam from the start of the
microwave path to its end.  During the rising portion of the triangle,
the frequency out of the phase modulator is lowest and slightly below
resonance.  During the falling portion, it is the highest and slightly
above resonance.

When the frequency suddenly shifts, the beam cannot react instantly
because it is only traveling at the speed of sound and must travel a
foot or so in the microwave path.  This takes on the order of a
millisecond. A positive spike is created  as the frequency passes thru
the resonance peak at 22 nA. This occurs at both the low to high
transition and the high to low transition.   The frequency changes so
fast that the voltage cannot reach 22 nA.  As the center frequency is
reduced, the negative half of the square wave becomes more negative
and the positive half becomes more positive.  As the center frequency
is increased the square wave negative half becomes less negative and
the positive half becomes less positive.

When the center frequency is at the resonance of the cesium line the
square wave disappears except for the spikes.  Above center frequency,
it reverses phase and gradually gets bigger.  This type of lock is
nearly 100% efficient and should be far lower noise.


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