This page is part of the [[optical:spy1|Spy1 construction tutorial]].



====== Control of acousto-optic deflection using Direct Digital Synthesis ======

Contributed by [[dejan@salk.edu|Dejan Vučinić]]

[[http://www.analog.com|Analog Devices]] sell a wide
variety of chips for
[[http://www.analog.com/en/subCat/0,2879,770%255F843%255F0%255F%255F0%255F,00.html|Direct Digital Synthesis (DDS)]]
of radio-frequency signals.
Two-dimensional acousto-optic deflection requires at least two
independent signal sources, but having more than two synchronized
generators greatly simplifies the synchronization with the data
acquisition system since an additional output can be used to
independently trigger the start of acquisition with
deflector-specific latencies.

The [[http://www.analog.com/en/prod/0%2C2877%2CAD9959%2C00.html|AD9959]]
device has four independent outputs that can generate frequencies
between zero and 250 MHz.  This frequency range is well
matched to the bandwidths of slow-shear TeO2 deflectors, which
typically operate under 100 MHz.
[[http://www.analog.com|Analog]] sells a USB-based evaluation board
with this chip, AD9959/PCBZ, that can be used to build a scan
controller in a hurry.

{{front_before.jpg?100}}{{back_before.jpg?100}}

The board, as shipped, requires several modifications before it
can be used.  Described here is one way to
do it, which requires basic tools and skill with rework of surface-mounted
electronic components.  (An alternative way that wouldn't require
soldering might be to use an external clock source and a lab power
supply; if you try this route please report your experience here.)

  1. Power supply

The AD9959/PCBZ board provides two 3.3 V inputs and one 1.8 V input.
One 3.3 V input is for the USB controller, the other is for the
AD9959 chip core, while the final 1.8 V input is used to power
the DAC.  A lab power supply with two or three outputs can be
used for this task, but is clearly an overkill.
[[http://www.ti.com|Texas Instruments]] make a small power
regulator meant for FPGAs that has three independent outputs which
can be set between 1.2 V and the input voltage.  This chip is sold
on an evaluation board
[[http://focus.ti.com/docs/toolsw/folders/print/tps75003evm-092.html|TPS75003EVM]]
which you can buy
[[http://www.ti-estore.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=TPS75003EVM-092|here]].
This power supply can be powered by a small wall wart that puts out
anywhere between 3.3 V and 6 V of DC at half an amp or so. (Note:
USB bus doesn't have enough juice for this, been tried.)

The TPS75003EVM evaluation board ships with the outputs set to 1.2 V, 2.5 V and 3.3 V.
To set the correct voltages for the DDS card replace the resistors in the
voltage dividers for outputs 1 and 3.  Output 1 is controlled by resistors
R10 (33 kΩ) and R11 (68 kΩ) and the capacitor C11 (10 pF).  Output 3 is
controlled by resistors R6 (68 kΩ) and R7 (12 kΩ). Consult the schematic on
page 11 of the user's guide that arrives with the module.

With a new board:
solder a power supply to the VIN and GND leads on the board;
orienting the board with the power source to your left
set jmp1 to "after vo2", or, the left position, set jmp2 to "after vo3", down position, and set jmp3 
with a 5 V power supply, Vout1, Vout2 and Vout 3 should be 1.2, 3.3 and 2.5 respectively;
Vout2 is already the right voltage for both the VCC_USB(3.3V) and the DVDD_IO(3.3V) prongs on the AD9959 



  2. Clock

The AD9959/PCBZ arrives configured for use with an external clock source,
such as an [[http://www.analog.com/en/prod/0,2877,AD9540,00.html|AD9540]].
Traces are provided on the board for a crystal to use with the built-in
oscillator, but several modifications must be made for the chip to
clock itself.  Consult the schematic on page 24 of the
[[http://www.analog.com/UploadedFiles/Evaluation_Boards_Tools/57418637811849AD9959_pcb_0.pdf|evaluation
board datasheet]].

     * remove the capacitors C21 and C22;

     * solder jumper wires across resistor locations R52 and R53;

     * add the crystal X2 (25 MHz) and capacitors C66 and C67 (39 pF);

     * supply power to the CLK_VDD pin, for instance by wiring the center pin of the connector J10 to the 1.8 V power supply pin;

     * move the jumper W11 from the REF_CLK default to the CRYSTAL position;

  3. Test

Here is what a finished scan controller could look like:

{{front_after.jpg?100}}{{back_after.jpg?100}}

In this example the power supply board is glued to the empty area of the DDS board.

Before testing the setup make sure jumper W7 is in the "PC"
position, jumpers W1, W2, W3 and W10 are shorted and jumpers
W4, W5 and W6 are open.

There are two ways to test the board: you can use the
device drivers and the software supplied with the AD9959/PCBZ
kit, or you can download the ''neurospy'' software and the
device drivers from [[http://neurospy.org]] and run a
test protocol, as shown here:

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