7 Vital radio module design optimisation tips!

Radio module design confusion? Struggling with the idea of adding a module to your design or simply want some advice to confirm existing thoughts on your module design-in?

As radio module implementation becomes a more common task for design engineers we, at RF Solutions, are with increasing regularity asked for tips on designing with radio and for pit falls to avoid when using modules. So here are our top 7 tips for circuit design and board layout when using RF modules:

1. Choose your frequency carefully

It may seem obvious, but choosing the right module for the job is the first consideration and most important factor in getting your design right. You first must consider where your design will be used - ISM (Industrial Scientific and Medical) license free radio frequency bands differ all over the world and with them the characteristics of the module which use them. The most common frequencies are: 

In the EU: 433MHz - up to 10mW power, 868MHz: multiple channels with power output up to 500mW. 2.4GHz with outputs up to 10mW.

<pIn the US: 433MHz up to 1mW output, 315MHz up to 10mW output, 915MHz up to 500mW (with restrictions on protocol - spread spectrum). 

Most countries outside the EU or US will follow one model or the other, notable examples being Canada - who follow the US, Australia who follow the EU (mainly), Japan are similar to the US. 

When choosing a frequency consider your intended application and use the below "rules of thumb" for considerations. 

• The lower the frequency, the longer the range for a given power output. 
• The higher the frequency the more quickly you can send data.

NOTE: This is a very broad interpretation for overview only, for more info you are best off contacting the ITU, FCC or relevant EU department. 

2. Encoding

Radio modules come in various types, forms and with many levels of intelligence. You have a choice of either using a SMART radio module like our GAMMA and BRAVO modules, which will handle preamble, RF encoding. buffering etc or even offer you simple I/O lines to toggle for switching and telemetry. Alternatively, you may use a basic module like our ZETA and ALPHA modules, which will come at a lower cost but will need you to handle the RF data packet creation yourself. 

If encoding yourself then Manchester or PWM encoding is a must! Sending data over a wireless link is not the same as using a wire. Packets can easily be missed due to DC biasing on the receiver data-slicer caused by long sequences of 1's or 0's. Manchester encoding your data prevents this potential pitfall and increases effective range by decreasing your bit error rate. 

NOTE: If using SMART module then points 3 and 4 shouldn't apply. 

3. Data range

When sending data via an RF link, dropped packets will often mean re-transmission is required which could lead to slow response times or worse - corrupt data. As you increase the data rate of your RF link, the reliability will inevitably drop - therefore a rule of thumb is: The lower the data rate, the lower the bit error rate and therefore the longer the effective range. 

4. Preamble 

When sending raw data from point-to-point via any generic (not SMART) radio module it is essential to send a good deal of preamble before the actual data packet. This allows the receiver data-slicer to synchronise to the string, therefore when your data packet is actually received the chance of error is lower. Best results come from experimentation, finding a compromise for the minimum length of preamble for the required reliability over a given range. 

5. Antenna connection

It is very important to remember that any task from an RF module antenna pin is effectively part of the antenna. Therefore, it is important to try and make this track "invisible" to the RF connection, to do this: 

     1. Make it as short as possible.

     2. Impedance match it to the module - most are 50ohms (often layout software has an option for impedance matched tracks). 

     3. Avoid any bends, and certainly any right angles. 

6. Power supply and Decoupling

Radio modules need a stable power supply to work well. Make sure that your power supply lines to the module are decoupled and that the decoupling caps are as close to the module supply and GND pins as possible. We suggest using 100nF caps preferably low impedance tantalum types. 

7. Ground plane

The use of a double sided PTH board allows one side of the board to be used, principally as a ground plane with 'zero length' interconnections using plated though holes where necessary - the larger the better! We recommend standing a radio module upright and bringing the ground plane right up to it, however if you have to lay it flat bring the ground plane up to 1mm from the base and not under the module itself. 

You can check out our full range of radio modules on our website, along with full datasheet information on each product. Alternatively we are always happy to speak to designers on the phone via our Technical Support line +44(0)1444 227909 or via e-mail on support@rfsolutions.co.uk

Good Luck and Happy Designing!

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