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I'm a complete newbie. I don't want to bother this fantastic resource with my ignorance, but I'm starting to have strange thoughts.

How do I go from a bitstream (say, the output of an USB port) to a RF signal (say, to a QPSK chip like the AD8346)? A computer GPIO connection would probably be too slow for a bandwidth of say, 20MHz.

Are there some specific chips usable in the process or is the standard solution to use a FPGA as a buffer in between, reading the USB data and driving the QPSK modulator? (or maybe including the QPSK modulator in the FPGA itself?)

Thank you. I find the topic fascinating.

cedivad
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    I'm no expert on USB data so maybe you could help potential answerer's by describing roughly the data from USB - is it packetized? Is the raw data on USB suitable for transmitting on RF or does it need a preamble? Does it need re-packetizing etc.? – Andy aka Mar 21 '15 at 11:25

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20mhz qpsk is easy.

you can easily accomplish a 20MHz baseband with 2 GPIOs from a modern ARM processor and an IQ modulator.

essentially, GPIO A sends 0 or 1 for the I channel. GPIO B, sends 0 or 1 for the Q channel. The IQ modulator will combine this to give you the 4 symbols for QPSK. your bandwidth is directly proportional to your symbol rate. something like 18 MSymb/s will probably give you your 20MHz BW.

Generating 18MHz signals on an ARM M4's GPIOs is easy. the info can be fed directly from a USB 2.0 (480mbps) transceiver to the ARM.

of coarse, this is just baseband.. then you will need a mixer and amplifier...but thats another question

what gets difficult to do, are modulations like SOQPSK. which require DACs, DDS, or RF circuity to shape symbol transitions. Another super hard one would be something like multi-h cpm, where 20mhz really needs some massive processing behind it.

hassan789
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