Xerox dual modulators

[mixedgas]

Ok, at Selem I was given one of the Xerox dual beam AOMs for dissection and reverse engineering.
The driver consists of a common RF oscillator, a power splitter, two doubly balanced modulators and two 800 mW (MAX) RF amplifiers. There are some other parts to adjust frequency and RF power levels, but the average user need not be concerned with what is in the box.

I had some time today to go after the driver mods if needed. Results, there is ONE TINY mod needed to the driver for it to work.

Powering up:

Simply connect +12V to 15 volts to the +15V input, connect -12 to -15 volts to the -15 input and connect +24-28 volts to the power pin for each channel. If your NOT going to use one channel, simply leave it un-powered. If all you want to do is get a crude idea of functionality, dropping the power input to the +28 inputs to +12-15 volts will not hurt anything.
You just will not get full power into the transmitted beam.

Modification:

Add a 330 ohm resistor in series with each "video" input. Each input has two diode drops to ground inside the doubly balanced modulator. The doubly balanced modulators are current driven, NOT voltage driven devices, so adding the 330 Ohm resistor converts your control signal from a voltage to a current. This protects the modulator and your DAC from destruction from a two diode drop to ground effective short. As you flow current into the modulator diodes, more RF is transmitted to the AOM cells. About 1-5 milliamperes results in full RF to the cell.

Pics and further explanation to come next week.

The attached block diagram shows one half of the circuit. Break the video cable for each channel and add the 330 Ohm resistor marked with the red "X" to the center conductor of the coax. This raises the impedance of the circuit to a high enough level that the DAC driving the cable will not short out. For those who do not know, the diodes in the balanced mixer are a special type that conduct RF when current is passed through them, so if the bridge circuit does not make sense to you, simply understand the purpose of a balanced modulator is to have adjustable conductance to RF. If you still don't understand this mod, you should have some one else do it who understands RF circuits. Depending on your DAC you may need to add a buffer circuit with offset and gain/loss to buffer the dac. I will post such a circuit next week.

Steve