Synrad 32-1 - Page 37

SYNRAD® 32-1 Operator’s Manual Version 2.2
37
User I/O Connections
Technical Reference
Table 4-4 Tickle pulse specications.
Parameter Specication
Tickle Frequency 5 kHz
Pulse Length 1.0 µs ± 0.2 µs
Pulse Rise/Fall Time < 100 ns between +0.5 V to +3.5 VDC
The UC-2000 does not produce tickle pulses continuously, but generates them only when the
PWM signal is low. Tickle pulses are sent one tickle period, 200 µs, after the falling edge of a
PWM signal pulse. Figure below illustrates tickle pulse parameters.
5 VDC
0 VDC
1 µs200 µs
Figure 4-3 Tickle pulse waveform.
32-1 lasers are designed to operate at a tickle frequency of 5 kHz, which allows the laser to
meet published specications. Tickle frequencies lower than 4.5 kHz may compromise laser
performance, particularly optical rise times, and stress the RF electronics thereby reducing
long term reliability while tickle frequencies greater than 5 kHz may cause unintended lasing.
Special care must be taken to maintain plasma ionization without lasing at tickle frequencies
greater than 5 kHz. When sending 1 µs tickle pulses at 5 kHz, PWM signals can be sent at an
independent, higher frequency but must go to near zero (< 1%) duty cycle to ensure laser
turn-o.
Pulse width modulation (PWM)
Pulse Width Modulation, or PWM, controls laser power by varying the duty cycle of the laser’s
RF ampliers, which in turn control the time-averaged RF power applied to the laser. Because
laser output follows PWM input with a rise and fall time constant of ~100 µs, the laser cannot
precisely follow PWM signal frequencies over 5 kHz with duty cycles greater than 50%. Typi-
cally, the depth of modulation for a 50% duty cycle is 90 to 100% at 2 kHz and 60 to 80% at 5
kHz. The following gure shows 32-1 optical waveforms at two dierent modulation frequen-
cies.