Easing the challenge of RF design, Part 2: Avoiding Interference

Know your parameters
Designing a radio system requires knowledge of some fundamental parameters to understand what will influence link performance and reliability. In Part 1, I have already discussed perhaps the most fundamental parameters: output power and receiver sensitivity.

The others that are important are listed here:

• Receiver dynamic range
• Co-channel rejection
• Adjacent channel selectivity
• Reference frequency stability
• Mirror image attenuation
• Modulation principle

To answer that question, the designer must consider each of the parameters in turn. The receiver dynamic range is the maximum power variation at the receiver input pins that still results in a correctly demodulated signal. This means that to be received properly, the power of the signal of interest must be between the sensitivity limit and the sensitivity limit plus the dynamic range. The co-channel rejection (CCR) is a measure of the capability of the receiver to demodulate a "wanted" modulated signal without exceeding a given degradation due to the presence of an "unwanted" modulated signal, when both signals are at the same nominal frequency. This parameter is specified in decibels. For example, a co-channel rejection of 10dB would mean that if the wanted signal is 10dB (or higher) than the unwanted signal, then correct demodulation will occur (with a typical BER of less than 10^-3).

If this parameter is not specified in the datasheet, then a designer should assume 12 to 14dB co-channel rejection, the typical frequency shift keying (FSK) demodulator threshold (see below). As an example, consider the system in Figure 5, with two adjacent transmitters operating at the same frequency. Let's calculate how far away the unwanted signal source needs to be so as not to interfere with the demodulation of the wanted signal.

The equation for received power is :

(P_out1. G_ant1. G_{ant_RX})/Path_loss₁ ≤ ((P_out2. G_ant2. G_{ant_RX})/Path_loss₁). CCR

Can be reduced to:

P_out1 - P_out2 +20. log (range₂/range₁) ≤CCR

Assuming a CCR of 12dB, and that the transmitters have equal output power and antenna gain, the ratio between range2 and range1 must be at least 4 in order for the receiver to demodulate the wanted TX₁ signal correctly without interference from TX₂.

The adjacent channel selectivity (ACS) of the receiver is defined by the European Telecommunications Standards Institute (ETSI), for example, as the ability of a receiver to demodulate a received signal at the sensitivity limit, in the presence of a sine component centered in the adjacent channel (see Figure 6). (I.e. if the ACS for a 25kHz channel system is given as 30dB, demodulation of the wanted signal at the sensitivity limit may be performed with a sine component of 30dB higher power than the received signal present in the adjacent channel.)

Note that the ETSI definition is merely provided as a comparative figure. The system ACS is typically lower, as the signal in the adjacent channel is unlikely to be a sine component, rather a modulated spectrum.

Reference frequency stability is a parameter that influences ACS. Deviation from the ideal crystal reference frequency will result in a corresponding deviation of the transmitted frequency, and an intermediate frequency (IF) offset/deviation for a superheterodyne receiver (one that shifts the received signal to an IF) appearing as an offset in the IF-filter centre frequency (see Figure 7).

Receivers use the superheterodyne principle for its excellent channel filtering performance, but suppression is needed to avoid mirror image interference. Mirror image attenuation (MIA) is a measure of the extent of this suppression.

In Figure 7, the mirror image is shown positioned at the local oscillator (LO)-frequency minus the IF, but will also appear at the IF together with the wanted signal. Consequently the mirror image frequency must be attenuated to avoid destructive disturbance or loss of sensitivity.

MIA has traditionally been performed with an external filter at the antenna input, or more recently, with on-chip cancellation techniques. As the mirror image appears inside the IF-filter bandwidth after mixing, MIA minus CCR is the maximum allowable power difference between the two signals to ensure demodulation. For example, if the mirror image attenuation is 35dB and the co-channel rejection is 12dB, the received mirror image frequency power may not be higher than 23dB (i.e. 35dB -12dB) compared to the wanted signal.

The modulation principle is the final critical parameter to be considered for the transceiver. In the early days (particularly in the 433MHz band), amplitude shift keying (ASK, also known as on-off keying or OOK) dominated the license-free low power radio bands. Unfortunately, although ASK-based transceivers are simple and reasonably priced, they suffer from relatively poor reliability when under the influence of in-band interference. In ASK systems, logic '1' is represented by a carrier frequency, while logic '0' is represented by no carrier. Consequently, the presence of a very weak unwanted signal in the channel can be interpreted as logic '1' if the receiver is sufficiently sensitive.

Frequency Shift Keying (FSK) is a different approach in which each the two logic levels correspond to a frequency value:

DATA_FSK = "1" ' f'1' = center + Δf
DATA_FSK = "0" ' f'0' = center - Δf

The technique ensures that the receiver always sees a strong signal swamping any unwanted signal in the channel.

Gaussian Frequency Shift Keying (GFSK) and Gaussian Minimum Shift Keying (GMSK - the term used for a GFSK signal in which the bit rate is four times the frequency deviation) modulation are enhanced versions of FSK implemented to optimize modulation bandwidth efficiency (i.e. the maximum transmitted number of bits per Hertz of channel bandwidth).

GFSK applies Gaussian filtering to the modulated baseband signal before it is applied to the carrier. This results in a "dampened" or gentler frequency swing between the high ("1") and low ("0") levels. The result is a narrower and "cleaner" spectrum for the transmitted signal compared with the straightforward approach of FSK. Figure 8 shows the general principle.

Avoiding interference
As more companies produce products that use the license-free portion of the low power radio spectrum, designers have to deal with the possibility of interference from radio signals from other sources operating on an identical (or at least very close to) their own chosen operating frequency. In fact, regulations governing the ISM parts of the spectrum state "a device must expect interference". There are three basic techniques for minimizing the impact of interference for devices operating in the 2.4GHz band. These are time domain multiple access (TDMA), direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS).

TDMA works by subdividing narrowband allocations within the ISM portion of the spectrum into a number of timeslots allowing several users to share the same single frequency without danger of clashing. Each transceiver waits for its own clear timeslot before transmitting, thus avoiding interference. DSSS and FHSS both rely on modulation of the carrier signal. In addition, FHSS allocates a number of channels within the band, the exact width of which depends on the technology (see below).

In simple terms, DSSS transmissions multiply the data being transmitted by a "noise" component. This noise signal is a pseudorandom sequence at a frequency much higher than that of the original 2.4GHz signal, thereby spreading the energy of the original signal across a much wider band. The noise is filtered out at the receiving end to recover the original data, by again multiplying the same pseudorandom sequence by the received signal.