Receiver module follow-up

07.10.2011 22:15

Here are a few more notes about the 433 MHz super-regen receiver module I've been previously writing about.

I'm trying to make a multipurpose USB-connected receiver for packet transmissions from various sensing devices, like cheap weather stations. For this purpose I attempted to digitize the demodulated signal from the receiver with a USB connected audio interface and write some decoding software on the PC.

It turns out that my first, successful tests of this receiver module on a protoboard were just a lucky coincidence. While the sensitivity and range seem to be exceptional, that also brings a lot of trouble. It's incredibly difficult to avoid interference from near by devices. In one case, for instance, that turned out to be a nearby idle switch-mode phone charger. Running this receiver in the vicinity of (or even powered from) such a lively source of RF energy as a high-speed USB device proved to be especially challenging. The receiver is not very selective, so a very broad range of RF frequencies can trigger a spurious signal on the output. One particularly puzzling example is when I get a 50 Hz signal on the demodulator output when the device is powered from USB and my oscilloscope ground is connected somewhere in the circuit.

When trying to solve this problem I've also stumbled upon the apparently well known fact that multiple super-regen receivers will interfere with each other if they are close by. Apparently the oscillators in both receivers will affect each other and you get impulses on the outputs of both.

Anyway, this is prototype board I'm currently working on:

Prototype USB connected 433 MHz AM receiver

There's the USB interface on the left and the receiver module on the right. What would otherwise just be a couple of straightforward connections between two circuit boards is now a mesh of filter capacitors and resistors. PCB design guidelines for reduced EMI PDF document from Texas Instruments has a good overview of the techniques I used.

I took special care for proper grounding. The other side of the board is a solid grounded copper fill. +5V supply from the USB goes through two RC filters as close as possible to the connector to minimize any length of wire that could act as an antenna. There are additional blocking capacitors as close as possible to the RF and AF parts of the receiver module.

The demodulated signal line has similar RC terminations on both ends to prevent emissions.

The USB part will be completely covered by a grounded metal RF shield. It's the only part that still needs to be done. As it is the receiver still receives mostly interference from the USB bus and I'm hoping the shield will take care of that. If not and it turns out the interference is going out through the wires, I still have a few SMT chokes that might do the trick better than the RC filters I'm using now.

Posted by Tomaž | Categories: Analog

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