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A signal generator based on the Analog Devices AD9850 Direct Digital Synthesizer

A signal generator based on the Analog Devices AD9850 Direct Digital Synthesizer

0.1 – 40 MHz, Frequency entry through 4×4 keyboard in 1Hz resolution, Frequency increment/decrement with rotary encoder, Frequency step selectable from 1Hz, 10Hz, 100Hz, 1kHz, 10kHz, Output adjustable from 0dBm to +10dBm
I started working on using a Microchip 16F84 PIC to control the AD9850. Problem with the 16F84 is limited I/O, so I opted for using a separate 16F84 to read a 4×4 keyboard . This controller program reads the key-code and sends the code over one I/O pin to another 16F84. This controller reads the key-code and also reads a rotary encoder. It controls the AD9850 DDS , writes info to a 2×16 LCD and writes an eight-bit counter value to a DAC. See the block-diagram. The AD9850 is followed with a filter and an amplifier. Output of the amplifier is sampled and compared with a set level. The resulting error signal is used to control a Variable Gain Amplifier (VGA). The output is, as far as I can determine, constant within 1 dB over the generator range.

I used the National Semiconductor CLC5523 VGA. This is an excellent low-noise amplifier for the purpose. The VGA is followed with a Texas Instruments TI6002. This is a current feed-back amplifier designed for use in xDSL systems. It is a dual op-amp chip, I used only one amplifier. It has a very small heatsink and I have pushed the output to as much as half a watt without problems. In a dual amp configuration it could very well be used as a QRPp ‘PA’, were it not for the fact that it needs +12V and -12V. The output stage is not particularly critical, a dual-gate MOSFET as VGA and MAR/MAV type amplifier would also do nicely.

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