Q-Switch Driver

Posted on Feb 7, 2014

The Q-Switch driver for an old Yag laser died. A replacement was >$1000 so we built our own for <$300 (picture and schematic below). The specs were >= 20W into a 50 ohm load at 68MHz. The rep rate had to be adjustable between 100Hz and 100KHz and have a TTL output to synchronize the output with other equipment. I had never made a RF amplifier befo

Q-Switch Driver
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re so for me that would be the tricky part. I started by ordering a 3. 3V CMOS oscillator, CPPC4-LT5RP ($8. 14), from and having them program it for 68. 000MHz. The square wave oscillator worked well but couldn`t drive a 50 ohm load. I AC coupled the oscillator to a MAX4258 video driver/multiplexer. The AC coupling centered the oscillator at GND and the driver can drive a 50 ohm load. The multiplexer will later be used to quickly turn off the oscillator for about 6us (to q-switch the laser). If I block all frequencies above the fundamental I can turn a square wave into a sine wave. An article on DDS IC`s in Circuit Cellar pointed me to The filter software was easy to use and free. I was able to make a 5th order low-pass to block everything above 72Mhz. The 68MHz fundamental will pass but all the harmonics will be attenuated (Ex: The third harmonic, 204MHz, is attenuated by 50db). Now I have a 68MHz sine wave that`s about 3Vpp (13dbm). Note: I need 20W (43dbm). We searched around and found the RA30H0608 from ($73). Its specs are 68-88MHz, 30W output, 12V power supply, and it has a gate pin to modulate the output (i. e. control the gain). I used a trim pot to set a DC voltage, buffered it with an op-amp configured as a voltage follower, and sent it to the gate pin on the amplifier to control the gain. Note: Turning the RF off via the gate pin was slow (about 10us) so I used the mux on the video driver IC to turn the RF on and...

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