Interferer waveforms

AN3218

2/6

Doc ID 17543 Rev 2

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 in-

band. 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.

Table 1.

Minimum receiver jamming resistance requirements for 2450 MHz PHY

Adjacent channel rejection

Alternate channel rejection

0 dB

30 dB