Has anyone tested these 433MHz modules?

Hey,

I've ordered a bunch of those cheap 433MHz RF modules from ebay to play around with. They're basically split into transmitters and receivers, and are meant for small DIY projects for arduino and the like. Out of curiosity I'm interested to see, if anyone has done some testing on them and what results you've obtained. The modules look the same as the two on the following website:
**broken link removed**


I've attempted to transmit some data between two endpoints, and I've been unable to get them to work past the 2400bps while using ASK-OOK Differential Manchester encoding. Plugging the oscilloscope probe A into the transmitter input and probe B on the receiver output will show the initial and the resulting signal. While the transmitter is getting an ideal square wave signal on the input pin, the receiver will output quite a deformed waveform. It would seem that the output signal is having some 80 microsecond delay before it finally responds to the rising edge of the original signal. It is also trailing the falling edge by some 20µs, giving the resulting peaks a shorter duration for about 60µs on average (independent of the square waveform frequency). The resulting pauses are consequently 60µs longer.

This can be a problem for Manchester encoding where the circuit is supposed to be differing between short and long pulses/pauses. The signal is being transmitted as a series of digital/logical level changes (transitions). Every even transition represent a "clock pulse" and they are always present in the signal. The odd transitions represent data pulses, and are only present when transmitting a digital zero (0 = transition, 1 = no transition).
https://en.wikipedia.org/wiki/Differential_Manchester_encoding

The base time period (the time between a clock and a data pulse) is T and the period between two clock pulses is 2T. Setting the base time T to about 200µs gives us the frequency of about 5000Hz. While only every second pulse counts as data, it means the data rate will be about 5000 / 2 = 2500bps. The receiver now has a problem. Having the base period T of about 200µs and the window of margin of about +/-80µs makes some periods come dangerously close to one another. Now, if we try to increase the data rate even further, some peaks/pauses will begin to overlap, and the receiver will no longer be able to demodulate the signal properly.


For this reason I've increased the base period T to about 256µs, and the window of margin to +/-100µs (T = 256 +/- 100 µs, 2T = 512 +/- 100 µs) which seems to work quite well. I have yet to see how this progresses as the distance between tx/rx modules is increased, as I've only performed the current tests while the modules were close together.


Cheers!

I am not familiar with that particular module, but it looks like a standard Chinese manufacture module. These use an RF IC receiver that uses the AGC circuit as a crude power detector, which demodulates the ON/OFF keyed modulation into a digital serial output. The AGC circuit has external resistor/capacitor components to control the response time. So you can change those component values if you want a faster demodulation rate...but the agc circuit does have an upper speed limit. the quicker you make the response time, though, the more random spike noise you will get out of the digital output, since the R-C components act like a bandwidth limiting lowpass filter. if you do have a desired data rate, the R-C components can be optimized for it.

Yeah, it's one of those standard cheap chinese modules you get on ebay for a couple of bucks. I was hoping to squeeze some more performance out of it, but it appears it's already operating at its capacity. I'm not sure about modding the external components, but I can probably add some software filtering to eliminate short signal pulses. Although this will improve reception slightly, it will not however increase the data rate.

I am unsure which circuit is causing the deformed waveform - it may be the transmitter, it may be the receiver or maybe even both components are to blame. I've recorded the signal with a RTL-SDR dongle, and it already seems a bit deformed, but I didn't do any advanced analysis past a simple waveform visual check in an audio-editing program. Some people suggest that a receiver such as a Superheterodyne 3400RF is a much better choice, and I've already ordered some from ebay. Will test them when they arrive.

Cheers!

The issue for establishing communication during initial transmission is expected in these modules . The receiver needs to get at regular intervals data packets without effective information, what is called preamble, which actually aims to keep the AGC circuit of receiver with a suitable voltage of polarization.

Yes, in my tests I was already using the following preamble: 11001100-11001100 or 0xCC, 0xCC.
I'm also repeating the whole transmission 7 times (8 times total) with a short delay (guard time) of 10 ms in between.

i suggest you should transmit a preamble about 1ms before send data.and receive module is Super regenerative,may have much noise.

Not sure I understand.

I'm transmitting 2 bytes of preamble per message, so that's 16 bits of data. With Manchester's alternating signal pulses (clock/data/clock/data) it's twice the amount of transitions - 32 base time units. One base time unit (the time between a clock transition and a data transition) is 256 microseconds as set in the transmitter code, so the total preamble duration is about 256 * 32 = 8.192 milliseconds. And personally I still think it's a bit too sort, I'd increase it to twice the length.

Cheers!

That's right.The length of the antenna module for 433MHz, at about 17cm.

a preamble header, in Manchester encoding, might be wise. you 100% need that in a free running FSK system.

Are your tx and rx modules too close together? Those things have gawd awful dynamic range, and you might be overloading the receiver.