ST AN3218 Application note

1 Introduction

This application note describes a method which could be used to characterize adjacent
channel rejection (ACR) on RF chips. It also compares different methods used by other
manufacturers when quoting ACR performance.

Adjacent channel rejection (ACR) is an important parameter for any radio receiver. It is a
measure of how well a receiver performs on its frequency channel when there is an
interfering system in the vicinity operating on a nearby channel.

ACR is generally one of the parameters that is used to compare the performance of different
RF ICs. However, different silicon vendors use different methods for measuring ACR, which
may distort performance figures.

This application note presents a method used to measure ACR on its IEEE 802.15.4-2003
compliant ICs, and compares it against other methods.

AN3218

Application note

Adjacent channel rejection measurements

for the STM32W108 platform

1.1 Supported STM32W108xx kits

This document is applicable to the following STM32W108xx kits:

● STM32W108xx starter kit (part number: STM32W-SK)

● STM32W108xx extension kit (part number: STM32W-EXT)

● STM32W108xx low-cost RF control kit (part number: STM32W-RFCKIT)

2 Requirements

The IEEE 802.15.4-2003 standard specifies a minimum level of ACR that chips must meet.

It is defined as follows:

6.5.3.4 Receiver jamming resistance

The minimum jamming resistance levels are given in Table 22. The adjacent channel is
one on either side of the desired channel that is closest in frequency to the desired
channel, and the alternate channel is one more removed from the adjacent channel.
For example, when channel 13 is the desired channel, channel 12 and channel 14 are
the adjacent channels, and channel 11 and channel 15 are the alternate channels.

March 2011 Doc ID 17543 Rev 2 1/6

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Interferer waveforms AN3218

Table 1. Minimum receiver jamming resistance requirements for 2450 MHz PHY

Adjacent channel rejection Alternate channel rejection

0 dB 30 dB

The adjacent channel rejection shall be measured as follows. The desired signal shall
be a compliant 2450MHz IEEE 802.15.4 signal of pseudo-random data. The desired
signal is input to the receiver at a level 3 dB above the maximum allowed receiver
sensitivity given in 6.5.3.3. In either the adjacent or the alternate channel, an IEEE

802.15.4 signal is input at the relative level specified in Table 22. The test shall be
performed for only one interfering signal at a time. The receiver shall meet the error rate
criteria defined in 6.1.6 under these conditions.

Most 802.15.4 ICs exceed the standard's requirements by a long way.

The standard does not specify the filtering of the interferer signal, it only states that it should
be 802.15.4 compliant, which means it must meet the spectral mask and error vector
magnitude (EVM) specifications.

3 Interferer waveforms

For the ACR figures quoted in datasheets, the interferer signal is generated by using the
arbitrary waveform generator mode of a signal generator, and constructing a near ideal

802.15.4 O-QPSK waveform containing pseudo-random symbols.

Other manufacturers use a heavily filtered IEEE 802.15.4-2003 signal to measure ACR.
This has the result of removing all energy from the interferer's sidelobes that would fall in­band. This method creates such a signal by filtering the ideal signal prior to loading into a
signal generator. The filter uses a 100 tap FIR with cutoff frequency at 3.5 MHz so that the
2nd (3 MHz) sidelobe is not attenuated, but the 3rd one (4 MHz) is almost completely
removed. While this signal is IEEE 802.15.4-2003 compliant (it meets the EVM specified in
the standard), it is not representative of any real implementation since this degree of filtering
is not practical in real silicon.

Figure 1 and Figure 2 show a comparison of 802.15.4 spectra produced by signal

generators and 802.15.4 silicon.

Using an ideal signal source, ACR performance is ultimately limited by energy from an
interfering signal that falls into the wanted channel bandwidth. Figure 2 shows that at 5 MHz
offset, the ideal and real silicon spectra are 42 dB below the wanted signal level in a
100 kHz bandwidth. A good receiver will have a 1.1 MHz bandwidth, and the integrated
power in this bandwidth is –38 dBc at 5 MHz (1.1 MHz is the bandwidth of the matched filter
for optimum signal reception, different receivers may have wider bandwidths than this).
Therefore, if a receiver has an SNR requirement of 3 dB, then it cannot achieve an ACR of
better than 35 dB. Any datasheet that quotes more than 35 dB for ACR is not using an ideal
or even a representative 802.15.4 interferer signal. While some chips may be capable of
higher rejection of the main signal lobe at 5 MHz, this is of little value since the in-band
sidelobe level limits real system performance.

At 10 MHz, the receiver cannot achieve a rejection of better than 48 dB for an ideal 15.4
signal.

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