Good news for 433mhz RF!

[espruino-discuss2]

Posted at 2015-05-27 by DrAzzy

Most of us are familiar with the issue of the Espruino getting bogged down dealing with random noise on RF receivers.

I was looking at the datasheet for the SYN470R, used in the good 433mhz receivers (the small yellow ones with the SOIC-16 package on them - these have at least twice the range as well, and are under $2/each on ebay), and came across this gem:

14.5. Data Squelching
During quiet periods (no signal) the data output (DO pin) transitions randomly with noise. Most
decoders can discriminate between this random noise and actual data but for some system it does
present a problem. There are three possible approaches to reducing this output noise:

1. Analog squelch to raise the demodulator threshold
2. Digital squelch to disable the output when data is not present
3. Output filter to filter the (high frequency) noise glitches on the data output pin.
   The simplest solution is add analog squelch by introducing a small offset, or squelch voltage, on
   the CTH pin so that noise does not trigger the internal comparator. Usually 20mV to 30mV is
   sufficient, and may be achieved by connecting a several-megohm resistor from the CTH pin to
   either VSS or VDD, depending on the desired offset polarity. Since the SYN470R has receiver AGC
   noise at the internal comparator input is always the same, set by the AGC. The squelch offset
   requirement does not change as the local noise strength changes from installation to installation.
   Introducing squelch will reduce sensitivity and also reduce range. Only introduce an amount of
   offset sufficient to quiet the output. Typical squelch resistor values range from 6.8MΩ to 10MΩ.

Sounds like we should be able to make 433mhz signals receivable directly on the Espruino with a decent receiver and a single resistor (soldering onto an SOIC package lead isn't too bad)!

All my good receivers are already in use, but I've got another 5 on order

[gfwilliams]

Posted at 2015-05-27 by @gfwilliams

That sounds great - it's a bit of a shame it's not done by default, but presumably at least the CTH pin is brought out on those more expensive modules?

I'll have to see about reverse engineering the cheap modules in case there is something similar on there.

[espruino-discuss2]

Posted at 2015-05-27 by DrAzzy

It's not broken out - don't be silly - but it's an SOIC-16 part without much clutter around it, so easy to get onto the pin. It's pin 7 on the 16 pin SYN470, by the way, or pin 4 on the 8-pin SYN-480.
I'm not even sure if the data squelching resistor isn't in place in the modules already - I've only used them after I'd decided to go with AVRs for handling 433mhz rf.

The cheap modules are superregenerative receivers, whereas the SYN4x0's are real superhet receivers. You may not find an analogous point to connect to on the cheap ones.

[espruino-discuss2]

Posted at 2015-05-28 by DrAzzy

Got 'em today.

I don't understand chinese shipping. Same shipping method, same seller, sometimes it takes months to arrive, other times days.

Curses!
They sent me 315mhz ones, the bastards. Luckily I have some transmitters to test with, but I;m back to not having any receivers for my next project.

[gfwilliams]

Posted at 2015-05-28 by @gfwilliams

:( Damn. I wonder what components are different to make them 315MHz.

Yes, good point about the cheap receivers. I checked the chip yesterday and it's just an op-amp. Figuring out whether adding squelch on that is possible is a bit beyond me :)

[espruino-discuss2]

Posted at 2015-05-28 by DrAzzy

Just a different crystal, I think. They use a reference just under 1/64th the target frequency