RC Groups

RC Groups
    R/C Blogs
        Discussion 900Mhz evolution

#1 Jack Crossfire Nov 10, 2012 02:03 AM

900Mhz evolution
3 Attachment(s)
After 3 solid days, getting the 900Mhz chip radio to work with ARM was about as hard as getting it to work with PIC was, in 2009. It's the same chip, just a much higher clockspeed & a new driver.

Years of experience revealed the best way to drive the chips is not to use the hardware FIFO over SPI but to bit bang the SPI & connect the raw modulation pin to both UART pins. The UART on every microcontroller can be switched between tx only & rx only, allowing the pins to be ganged onto the single modulation pin.

This greatly shrinks the layout & allows the full 115200 baud with a very slow microcontroller. The great challenge was switching the radio between transmit & receive, so it would send telemetry. It needs a 5ms delay for the transition to finish, not a very easy time to measure on an 8 bit micro where all the timers are used up. So far, it's just using a hard coded counter.

It has to use a software start code & send the packet size just like the hardware FIFO. There's no other way to filter out the noise. Once the transition is finished, the latency in a 1 way stream is 1/115200 seconds. The voltage on 1 is the voltage on the other, delayed by a lowpass filter.

Over the years, the ability to detect a 1/115200 second time difference with a simple chip radio has led to the idea of measuring distance with low cost components. A pair of radios operating on different frequencies & hard wired so 1 directly transmitted the voltage received by the other could theoretically allow the propagation time of a voltage change to & from the aircraft to be measured. It would use a bank of staggered comparators, all timed by a slower clock, to detect very small differences in time.

10 staggered comparators timed at 100Mhz resolution would have 30cm accuracy. Averaging hundreds of samples might get it down to 3cm. It really depends on how noisy the signal is. Rough knowledge of RF communication says the lower the bandwidth, the less precisely the arrival time of a signal should be known. If it has 256kbit of bandwidth, the arrival time should only be known to 1/256000 seconds. But GPS has only 1megabit of bandwidth, so that shouldn't be a problem.

The Rigol & a set of 4 radios could do a simple proof of concept, up to 3 meter accuracy. Just a matter of time & money.

Now the great task is freeing up enough clockcycles to service the radio when the camera is running. The camera consumes so many clockcycles in large chunks, it causes outgoing radio packets to bunch up, causing the return packets to not have enough time to be sent.

So the MRF49XA arrived several months before M.M. & continues to improve, 1 year after M.M. departed this stage. She was truly, hardly a blip in the scheme of things.
