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updated 2001.05.21
Author Hiroshi Yoshida

Pull-in Effect on Two Blocking Oscillators

The circuit

It is widely known that two non-linear oscillators which have different characteristic frequency occasionally synchronise after interfering each other. It is called pull-in effect. Here those two blocking oscillators actually show the pull-in effect. The frequency of the blocking oscillator on the right side is a little higher than the left one. But after connecting these oscillators through the resistance 100k, we find these oscillators have synchronised oscillation which frequency is equal to the higher one. If you make the frequency of oscillator on the left side higher (practically reduce the resistance rba), you'll see suddenly the pull-in effect has disappeared, namely these two oscillators don't synchronise no longer. When you use this system as PFM (Pulse Frequency Modulation), intelligent signals are modulated through the change of resistance rba and the blocking oscillation is used as carrier wave. The tuning is done using the blocking oscillator on the right side. The right side of the oscillator is the simplest wave detector. You can get the demodulated intelligent signals as the voltage change at the capacitor cp. The band width depends on the strength of carrier wave but it is cut off sharply due to on/off of the pull-in effect.

The input file (pull_in_effect.cir)

two blocking oscillators (coupling)

* fo = 1 / (c1 rb)

.control   

  tran 1ms 60ms
  plot v(3)+20 v(13)-20 xlimit  0.055 0.060 
    + ylimit -40 40 xlabel time[s] ylabel 'V(3) V(13)' title
    + 'two blocking oscillators (coupling)'

.endc

v1 1 0 dc 3V

cp 6 0 0.02u
rp 5 6  0.1k
dp 2 5 diode
.model diode d

rb 3 1 25k
c1 2 3 0.05u
c2 4 1 0.20n
l1 4 1 5m
l2 0 2 5m
k12 l1 l2 0.995

q1 4 3 0 noname2

rcon 2 12 100k

va1 11 0 dc 3V

rba 13 11 20k
c1a 12 13 0.05u
c2a 14 11 0.20n
l1a 14 11 5m
l2a 0 12  5m
k12a l1a l2a 0.995

q1a 14 13 0 noname2

.model noname2 npn

.end

The results

Reference: non-coupling (i.e. line 2-12 is open)


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