RadioBanter - View Single Post

December 17th 04, 08:30 AM

On 16 Dec 2004 04:01:34 -0800, wrote:

I want to synthesize a SINC wave with main lobe width = 100ns,
Is it possible to use a LC filter response or
a DDS or a similar approach mey be?

An address counter, a lookup table (ROM/RAM) and a DAC should be able
to generate any arbitrary waveform.

Since the SINC waveform is symmetric relative to T=0, you just need to
store half of the waveform and reverse the up/down address counter at
T=0.

Since you are going to need a sample frequency of several hundred MHz
to get a clean wave shape, it might be a good idea to use a wide
memory system (say 64 bit wide RAM for 8 bit DAC) and run the RAM
address counter at fs/8 and either using digital multiplexers to feed
a fast DAC at fs or use 8 DACs in parallel at fs/8 and use a fast
analog multiplexer.

An other approach is to note that sinc(x) is sin(x)/x or 1/x * sin(x)
and generate 1/x and sin(x) separately and use an analog multiplier to
multiply them together. You also would have to ensure continuity at
x=0. The multiplier accuracy and dynamic range can be a limiting
factor in that region. Generating 1/x can be done with lookup tables
or with analog multiplier divider chips (which also could generate the
sin(x)/x directly) and there are of course multiple ways to generate
sin(x).

A DDS chip with AM capability might also be usable by feeding the 1/x
as the modulating waveform.

Paul OH3LWR