https://www.febo.com/pipermail/time-nuts/2017-April/thread.html
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. πθ°μΩω±√·Γλ WB0KVV _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.