Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Undersampling and its complement?
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Date: Sun, 3 Nov 2002 09:23:27 -0000
NNTP-Posting-Date: Sun, 03 Nov 2002 09:23:31 GMT
"James Meyer" wrote in message
> On Sat, 02 Nov 2002 00:40:46 GMT, James Meyer
> > My question is.... is there anything similar regarding generating an
> >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
> 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
> change my mind about exactly what I want and "only" have to rewrite
> 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
> those in any event.
Ahh...Now I know more on what you are doing, I can make a suggestion.
First, you may be interested in a program of mine for ultrasound
focusing http://www.anasoft.co.uk/focus.html. Its really a freebee for
By and large, without *very* expensive A/D D/A, you just aint ganna
extract the dopplar frequency with a direct digital conversion.
The main issue is that the reflected signal contains mostly fundamental,
the shifted frequency component being maybe 60db - 100db down. The
second issue is that looking for 100hz-10kz shifts in 10Mhz is a tad
tricky as well.
The way this has been done in the past in standard commercial medical
imaging equipment for colour flow dopplar, is to use a very high quality
oscillator and mix the signal down to backband. So, this is what I
suggest you do. The oscillator, is of course, the same one you use for
the transmit signal. If you need any over specific pointers, let me
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.