oscillator bank 1

[skelmonsta]

Hi all, I'm a software/DSP guy currently a bit over my head designing some hardware. I have a design in mind for my problem, but it's based on technology I would have used to do this twenty years ago. So I am writing in hopes of getting a "sanity check" on my solution.

Problem: build an 128-oscillator bank. Frequencies: 24000 Hz, 24093.75 Hz, 24187.5 Hz, ... 35906.25 Hz. Each oscillator must be on a different physical connection, for a capacitive sensor application. The spacing corresponds to a 1024-point FFT at 96kHz. Since they are all within one octave, the signals can be square waves (only the fundamental matters). Frequency accuracy needs to be 0.01% (+-4 Hz at 40kHz). Power needs I'm not too sure of yet, let's say a handful of mW at 5V for each oscillator.

This is a prototype that I will have to build. If it were a product, my solution would be: hire someone to design something, probably involving an ASIC...

Solution: The outputs are driven by TTL octal latches. A microprocessor (PIC) addresses the latches a byte at a time through some more TTL logic. The microcode updates a software counter for each oscillator and updates the latches when needed.

A vague solution, but hopefully precise enough to ask: is this at all the right technology to use? The correspondence of the frequencies to the bins of the FFT led me to think about solutions that would use this property to advantage. But it's hard to see how to do that given the need for separate outputs.

 

[sreid]

I'd like to add two thoughts to your plan (which is interesting). Any peroidic waveform will not have any harmonics below 2 times the fundamental. So a 24 kHz asymetrical square wave (say) cannot have any frequencies below 48 kHz. So none of the subchannel harmonics will be within your sub channel frequency range. You will have to low pass filter to avoid alising as always.

You could also collect a large numbere of samples (you have not said how big the FFT array would be) before doing the FFT. This increase the frequency resolution. What this would do for you is that if the subcarriers are not exactly on the center frequencies you want (but you know what they are) the FFT will bin these frequencies correctly but with a reduced but exactly known amplitude reduction. So you could correct for this.

 

[xray]

monsta, I worked with a guy during grad school that realized an application that is, i believe, a stone through away from your application. He realized a rather complex FFT in fpga hardware that was off the shelf. The gist of the unit is an optical carrier modulating 2 GHz of singal bandwidth maintained for ms duration, a time division multiplexing. The FFT had to chunk up the bandwidth in frequency bins over the ms duration. His name is Scott Becker and his thesis is on the Montana State University Site. I would have given you the web site directly but their homepage seems to be down. The thesis and dissertations after maybe 2000 are available for download. His was finished spring 2006. It is a big junk of data but I think there are some great parallels to what you are proposing.

Regards