On 10/16/20 9:08 AM, Graham / KE9H wrote:
On Tue, Oct 13, 2020 at 12:19 PM paul swed <[email protected]> wrote:
Graham take a look earlier in the thread there are details about the
teensy. There is actually a lot of hardware out there today for little
money. Thats what makes the SDR DSP approach interesting and for me at
least the next thing to take a run at.
Regards
Paul
WB8TSL
Hi Paul:
I have looked at the Teensy 4.0 for use as a WWVB SDR receiver.
The problems I have are:
1.) PJRC (the designer/manufacturer) has gone out of their way to block
access to the standard ARM SWD programming interface, and you must go
through the unique USB loader executable and programming interface if you
want to program the board. It is awkward to use a normal development
environment such as NXP's MCUXpresso with a standard programming interface.
It is really intended for use with the provided Arduino programming
environment.
I'm not so sure about that. There's quite a few folks using ARM
toolchains and not using the USB loader. For that matter, if you can
generate an executable, in any fashion, you can use their loader.
I've done it with a 3.2, but not the 4.0, so maybe there's a hiccup..
But usually, the support forum has a wealth of information.
2.) If you use the provided Arduino IDE/environment, then the provided DSP
functions are restricted to the 16 bit versions. I prefer the ARM 32 bit
versions to go with my 19 to 24 bit data converter.
3.) I have found that the iMX RT1062 processor will only run at the
advertised 600 MHz speed, if it is exclusively executing from onboard
"tightly coupled RAM". If your program is large enough to require using the
OCRAM, then it slows down to 1/4 speed. If you need even more memory
space, and you try to use the provided external QSPI Flash, then it slows
down to about 1/32 speed. What it effectively does is run at some blend of
those speeds as it fetches cache and variously accesses the different
memories. Memory planning is critical for maximum performance.
Always the case with microcontrollers...
I don't know that you'll need that fast, though. I ran 32 point FFTs
in real time at 100kHz sample rate on a Teensy 3.2 at 72 MHz (actually,
it also ran on a Teensy 3.1 at 48 MHz), as well as a a FIR decimator.
For this application, you basically need to implement a Costas loop to
demodulate the PSK signal. So that's a few multiplies and adds per
input sample for the mixers, a table lookup or CORDIC for the sin/cos,
and then the loop filters.
The signal is narrow band too.. It might be easier to sample at
something like 240 kHz (4x input signal), then do a downconvert to
baseband I/Q (using ++--), a decimation with a CIC to a few kHz, then do
your Costas loop.
One trick used in the GPS world (and also in the "IF sampling" world) is
to sample at a rate which puts the aliased input signal at Fs/4. Some of
JPL's radios sample a 112.5 MHz IF with a 50 MHz sampling clock, for
instance. The GPS people like to pick a sample rate that makes sure
that even with maximum Doppler, the Doppler offset is always positive in
the aliased passband.
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