Press Backgrounders

Easing the challenge of RF design

The very mention of RF design is usually enough to scare all but the most confident designer. But it needn’t be a nightmare if the engineer follows the guidelines described in this white paper

 

RF design is not trivial. However, Nordic Semiconductor has worked hard to ensure it’s no longer solely the domain of the RF expert. Performance optimized transceivers, and the availability of development kits and reference layouts, makes it possible for a competent electronics design engineer to incorporate wireless hardware into their latest product.

 

Nonetheless, although the availability of commercial off-the-shelf (COTS) components and reference designs has made wireless system design a little easier, the engineer still needs to acquire some fundamental knowledge about the parameters that influence a wireless link’s performance.

 

A critical issue is communication reliability; how will parameters such as sensitivity, output power, adjacent channel selectivity, and operating frequency influence system performance? In other words, what is the probability of transmitting/receiving an error free data packet in the presence of other radio sources that could interfere with the signal.

 

A second equally critical issue is range. The designer has to ensure that the radio can operate over its stated range in a number of different operating environments. Given output power and sensitivity, what other parameters affect range? Environmental factors such as air humidity, obstacles such as people and furniture, building materials, and metal film sun-screened windows limit useful range and choice of antenna implementation.

 

The designer should also be very wary of taking the data sheets of some wireless component vendors as gospel. For example, if sensitivity is measured for a lower data-rate than the stated maximum then it’s worth asking why. Checking just a few fundamental characteristics such as these first may save much time and frustration later when realising that the circuit chosen does not comply with the system specifications.

 

Fundamentals of a wireless link

 

A wireless link comprises a transmitter with antenna, a transmission path and a receiver with antenna. The key performance parameters of this simple link are the output power (Pout) of the transmitter and the sensitivity of the receiver (see figure 1).

 

Figure 1: Schematic of a typical point-to-point wireless link

Figure 1: Schematic of a typical point-to-point wireless link

 

Sensitivity is the minimum received power that results in a satisfactory Bit Error Rate (BER, usually 10-3 or one bit error for every 1000 bits transferred at the received data output (i.e. correctly demodulated)) or Packet Error Rate (PER, see sidebar “The importance of Packet Error Rate”).

 

The difference between received signal power and the receiver sensitivity limit is the transmission link margin or “headroom”. Headroom is reduced by a number of factors such as transmission path length, antenna efficiency, carrier frequency, and obstructions in the transmission path (see figure 2).

 

Figure 2: Headroom is reduced by a number of factors such as transmission path length, antenna efficiency, and obstructions in the transmission path

Figure 2: Headroom is reduced by a number of factors such as transmission path length, antenna efficiency, and obstructions in the transmission path

 

 

Note that sensitivity and output power quoted in the RF-circuit datasheets are given for the load impedance optimised for the input low noise amplifier (LNA) and the output power amplifier. This means that the impedance of the antenna must be equal to the load stated in the datasheet, otherwise a mismatch and subsequent loss of headroom occurs. A typical matching network introduces around 1 to 3dB of attenuation.

 

The antenna transforms the transmitter output power into electromagnetic energy, which radiates from the antenna in a radiation pattern determined by the antenna geometry. For the licence free bands such as 915MHz in the US, 434 and 868MHz in Europe, and the global 2.4GHz band, the maximum output power is expressed as Effective Isotropic Radiated Power (EIRP).

 

An “isotropic radiator” is defined as a hypothetical lossless antenna radiating equally in all directions. This means the regulations governing the licence-free bands (issued by ETSI) do not allow transmission range to be boosted by using a directional antenna. If the antenna gain is larger than 1 (0dB) in any given direction, the output power has to be decreased accordingly.

 

For example, for a radio operating in the 2.4GHz band, the maximum transmission power is 25mW (14dBm) EIRP. A directional antenna that has 10dB gain in a given direction would seem as if it was transmitting 24dBm for a receiver positioned in this direction. Thus, the output power would have to be reduced to 4dBm to fulfil the ETSI requirements. Note that a directional antenna can be used for receivers without any penalty.

 

Calculating antenna gain and radiation pattern is generally quite complex, and its local environment heavily influences the resulting radiation pattern. Placing the antenna close to conducting surfaces is likely to distort the antenna’s radiation pattern and efficiency, but is virtually unavoidable for most practical applications.

 

...to continue reading this technology backgrounder, please download the PDF.

 

The complete document is 5,500 words and includes sections on: Fundamentals of a wireless link; Calculating antenna size; Multipath interference; Know your parameters; Avoiding interference; Interpreting the datasheet, and a sidebar on The importance of Packet Error Rate.