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From: James Meyer
Subject: Re: Undersampling and its complement?
X-Newsreader: Forte Agent 1.92/32.572
Date: Sun, 03 Nov 2002 02:54:22 GMT
NNTP-Posting-Date: Sat, 02 Nov 2002 21:54:22 EST
On Sat, 02 Nov 2002 00:40:46 GMT, James Meyer wroth:
> My question is.... is there anything similar regarding generating an 8
>MHz signal? Do I really need a 16 MHz or better DAC in order to generate 8 MHz
>signals if the signal will not have a very wide bandwidth?
I've been reading the replies and there's some good ideas there. To
elaborate on what I'm doing... I need to transmit and receive six channels of 8
MHz ultrasound. I'm interested in the Doppler return.
I'm presently using CW Doppler, the transmitter is always on, but I'd
like to try pulsed Doppler too.
The simplest way to get the job done with off-the-shelf equipment is to
buy a DSP board and connect it to some fast A/D and D/A converters and
synthesize the transmit and receive functions in software. That way I can
change my mind about exactly what I want and "only" have to rewrite the software
and not have to muck about with building new hardware. I'll need some
amplifiers to get the power for the transmit end and some more amplifiers to get
the receive levels up to reasonable levels for the A/D converters, but I'd need
those in any event.
Now reality rears its ugly head... A fast DSP board with 20 MHz or
faster converters is going to run me at least $7,500. I can save some bucks if
I can make do with a slower converter specification. Undersampling will get me
there for the receive end but I had never heard of anything like that for
generating signals. For pulsed Doppler I'd need to be able to turn the signal
on and off and if I wanted to play with phased arrays of transducers I'd need to
be able to adjust the phase of all the transmitted signals pretty quickly with a
fair degree of resolution. Just the thing that a DSP and a set of fast D/A
converters are good at.
My thinking now is to still do the functions all in software but on a
slower system at a lower intermediate frequency and use a local oscillator and
mixers to translate the signals up and down to and from the 8 MHz. Unless I win
the lottery, that is. 8-)
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