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
psas-avionics mailing list

Reply via email to