On Fri, Feb 6, 2015 at 3:15 AM, Andrew Greenberg <a...@ece.pdx.edu> wrote:
> While laying out the third version of the jGPS board, it suddenly > occurred to me: we chose the MAX2769 because we were goaded into > thinking that the serial output was actually... well, serial. And not > some weird strange bizarre useless thing, as it really is. Didn't seem *too* bad? 4 x 16 bit serial streams with all the I0's, I1's, Q0's, and finally Q1's? A few timing signals too? > Since it's > not usefully serial, we're switching the MAX2769 to parallel mode, > throwing the output at a CPLD, and then putting SPI'ing it into the > STMF407. > This should be doable, too. > > ... So that's 32.768 Mbps, which is a whole lot of data, and is > > 25 Mbps of the usual SPI limits. On the other hand, at those rates we > get real bandwidth. > So sample slower, and alias down into another nyquist zone. > > Currently on the MAX2769 [2], our IF is 2.048 MHz, the sample rate is > 4.096 MHz at 4 bits, so our bitrate is 16.384 Mbps. > OK, hold it. IF at 2.048 MHz? What happens when you sample a 2.048 MHz sine wave at 4.096 MHz (draw a diagram to help visualize)? Now what does it look like if you vary the sine wave +- 5 kHz? Your doppler offset is going to be all messed up. In fact, frequencies above *or* below the center will always appear *below* center because your frequency plan straddles two Nyquist zones. Pick your sample frequency to be at least 2 x (IF freq + 1 sided bandwidth). > > Anyone have anything to say about this? I'd appreciate thoughts on this! > Another possibility... Just use the 2 "I" bits at twice the sample frequency and do the I-Q split in software with two local oscillators. You'll get better IQ phase accuracy anyway. That doesn't eliminate your serializer, but it simplifies it some. > > Thanks! > > Andrew > > [1] http://www.skyworksinc.com/Products.aspx?CategoryID=562 > [2] > http://www.maximintegrated.com/en/products/comms/wireless-rf/MAX2769.html > > -- > ------------------------------------------------------- > Andrew Greenberg > > Electrical and Computer Engineering > Portland State University > http://www.ece.pdx.edu/ > a...@ece.pdx.edu C: 503.708.7711 > ------------------------------------------------------- > > _______________________________________________ > psas-avionics mailing list > psas-avionics@lists.psas.pdx.edu > http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics >
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