Monday, March 7, 2011

Signal Generation

I have been watching Jeff's - KO7M Blog and his current QRSS/WSPR Transmitter project using the DDS-60 as part of the prototype. Jeff had previously done some work with the AD5330 8 bit DAC.

Jeff's efforts have inspired me to look at a previous purchased Si750 SDR-Kit, that I still had not built.

Without much difficulty, I now have put the Si570 SDR-Kit together (only the 3.3v Regulator went in backwards, which I fixed). Testing is still in progress.

While investigating; the kit's circuit and the Si570 datasheet, the DDS-60 and its AD9851 device, I have learned a lot more about micro-controlled frequency generators and DDS's in general.

This may be Old Stuff for some folks, but this is what I have learned, this info is saved here for my future use:
The Si570
The Si750 SDR-Kit is a real neat device with a very wide frequency range of 10MHz to >900MHz and can be controlled via a two wire computer connection (I²C). It's output is square-wave, which is ideal for a mixer LO of a receiver
But, the Si570 has some limitations for use as a general use signal source. The output (square-wave) would require difficult frequency sensitive filtering to obtain a clean sine wave over the entire frequency range.
Obviously for a single frequency, a filter can be easily created, but for that single frequency, a simpler crystal or oscillator circuit could be used instead. 
Once setup, the Si570 output is continuious, without assistance from the computer or micro controller. 
The DDS-60 
More information needed.
The AD5330 
The AD5330 8 bit DAC is a general purpose signal/wave-form source with a frequency range of 1kHz to 36kHz. It uses DDS processor to create the output wave form. Almost any shape of output can be created.
For use as a AF signal source, it's output is normally a stepped waveform, which normally needs a low pass filter to remove the steps.The output can be used with a mixer to create double sideband signals. 
A continuous use computer or micro controller is needed to generate the steps of the intended frequency or wave form.

I need to explore these circuits more, and will update as necessary.


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