Budgeting Harmonics for ZigBee Front-End Modules

Transcription

1 APPLICATION NOTE Budgeting Harmonics for ZigBee Front-End Modules Introduction The growth of low-power, cost-effective wireless radio systems is driving more applications to use the ZigBee communication standard. Some of these applications require a signal to propagate over a short range. When installed in a large building or operating outdoors over a wide area, such a system may benefit from transmitting more power. A ZigBee radio system powered by a +20 dbm Power Amplifier (PA) significantly increases its range by 400 percent from 133 to 543 meters compared to a low power 0 dbm PA (Wloczysiak [1]). However, a transmit PA may generate higher spurious emissions, particularly harmonic spurs. Additional filtering would be required to satisfy the radiated emission requirements of the different regulatory standards listed in Table 1. This Application Note details how to specify harmonics for a ZigBee Front-End Module (FEM). Requirements from the different regulatory standards are reviewed and analyzed using the simulation harmonic content of Offset Quadrature Phase-Shift Keying (OQPSK) spread spectrum modulation. The effects of the measurement bandwidth and detector on the harmonics level of both sine and ZigBee waveforms are also evaluated. Finally, this document provides measurement data that illustrates the simulation results. Radiated Emission Regulatory Requirements In the United States, radiated emission requirements are provided under Federal Communications Commission regulation FCC CFR47, part 15; in Europe, ETSI EN /EN ; and ARIB STD-T66 in Japan. Annex F of IEEE Std [2] summarizes the requirements that are also listed in Table 1 for 2.4 GHz operation. Note that spurious requirements in Table 1 are relevant for the harmonic frequencies. Beside the absolute dbm limit, test conditions that include the measurement bandwidth and detector type also differ between the different standards, which may relax the actual requirement. IEEE Std GHz Transmission Link Waveform Characteristics The IEEE standard 2.4 GHz link uses OQPSK modulation with Direct Sequence Spread Spectrum (DSSS) techniques. A test bench was set up using ADS Electronics Design Automation (EDA) application software. ZigBee half sine wave pulse shape OQPSK 2Mchip/s modulation I/Q waveforms were created, up converted to RF, and passed through a non-linear PA for which the 2 nd Order Intercept point (IP2), 3 rd Order Intercept point (IP3), and gain were specified. The polar and time domain in-phase (I) and quadrature (Q) waveforms are shown in Figure 1. The polar plot demonstrates that the modulation has a constant envelope. Therefore, a high efficiency, saturated PA can be used in the system without degrading the performance of the link, which minimizes the current consumption. However, this is true only if the transceiver does not produce an unwanted amplitude modulation component, which may happen with a quadrature modulator. The carrier frequency spectrum is shown in Figure 2. Second and third harmonics are shown in Figure 3 and 4, respectively. The harmonic spectrums are plotted with a 10 khz resolution bandwidth to identify clearly where the maximum of the signal energy is located. The second harmonic peaks at a 1 MHz frequency offset while the third harmonic peaks at 1.5 MHz A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

2 Table 1. Regulatory Standard Requirements (2.4 GHz, ) Radiated Emission FCC CFR47, Part 15 ETSI EN /EN ARIB STD-T66 Requirements Detector Limit Detector Limit Detector Limit Maximum power Peak +30 dbm RMS/peak (compensated for duty cycle) +20/+23 dbm Peak 10 mw/mhz (+10 dbm/mhz) Peak power density Average +8 dbm/3 khz Peak +10 dbm/mhz Narrow band spurious emission Wide band emission Note 1: Restricted band only. Peak Average Peak 20 dbc/100 khz < average + 20 db 41.5 dbm/mhz (Note 1) < average + 20 db Peak 30 dbm/1 MHz Peak dbm Peak dbm/hz OQPSK (Q Component) OQPSK (I Component) Time (μs) OQPSK (V) m m4 Time = 5.0 μs OQPSK (I Component) = 0 V m5 m5 Time = 6.0 μs OQPSK (I Component) = 0 V I Component Q Component Figure 1. ZigBee 2MChip/s OQPSK I/Q Polar and Time Domain Waveforms PA Input/Output Power PA Input PA Output RF Frequency (MHz) RBW = 100 khz Total PA Output Power = +20 dbm Figure 2. ZigBee 2MChip/s OQPSK Fundamental Spectrum 2 March 24, 2010 Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice A

3 APPLICATION NOTE BUDGETING HARMONICS FOR ZIGBEE FEMS PA 2nd Harmonic m6 Frequency = 4901 MHz PA 2nd Harmonic = dbm m6 RBW = 10 khz PA Output Power = +20 dbm RF Frequency (MHz) Figure 3. ZigBee 2MChip/s OQPSK Second Harmonic Spectrum PA 3rd Harmonic m7 Frequency = MHz PA 3rd Harmonic = dbm RBW = 10 khz PA Output Power = +20 dbm m RF Frequency (MHz) Figure 4. ZigBee 2MChip/s OQPSK Third Harmonic Spectrum A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

4 PA 2nd Harmonic m1 Frequency = MHz PA 2nd Harmonic = dbm m RF Frequency (MHz) Figure 5. ZigBee Second Harmonic Spectrum: Unfiltered and in a 1 MHz RBW PA 3rd Harmonic RF Frequency (MHz) Figure 6. ZigBee Third Harmonic Spectrum Unfiltered and in 1 MHz RBW Harmonics Level and Detector Bandwidth It is reasonable to imagine that a constant envelope signal produces constant envelope harmonics when passing through a non linear element. This is true if the measurement bandwidth is higher than the bandwidth of the signal, itself. From Table 1, test conditions call for a 1 MHz measurement (detector) bandwidth. The dark plot lines in Figures 5 and 6 show the second and third harmonics, respectively, measured with a 1 MHz filter. The energy contained in the unfiltered signals represented by the gray plot lines is clearly higher. Effect of Band Limiting on Harmonics Level and Detector When ZigBee harmonic waveforms pass through a 1 MHz filter, one of the effects is that the waveform varies in the time domain as demonstrated in Figures 7 and 8. While the unfiltered version of the harmonics shown as a dotted line has a constant envelope, the filtered version shown in black does show variations over time. Spectrum Analyzers/Detectors Modern spectrum analyzers digitize the IF signal and the maximum bandwidth depicts the bandwidth of the analog-todigital (ADC) converter [3, 4]. The available signal is a collection of samples that represent the envelope of the original RF signal. The number of samples (N) depends on the ADC sampling rate and sweep time according to the following equation: N ADC Sweep Time (1) SAMPLERATE For instance, if the sample rate is 30 M samples/s and the sweep time is 100 ms, the number of samples is 3 million. Obviously, the spectrum analyzer does not display them all. First, the Intermediate Frequency (IF) may vary during the sweep time. Indeed, the sweep time corresponds to the entire time the Local Oscillator (LO) requires to cover a certain frequency span within the IF frequency step. When the span is set to zero, the LO runs continuously at the same frequency, and the entire sweep time is dedicated to one single IF frequency. 4 March 24, 2010 Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice A

5 APPLICATION NOTE BUDGETING HARMONICS FOR ZIGBEE FEMS nd Harmonic Power Time (μs) Figure 7. ZigBee Second Harmonic Power vs Time, Unfiltered and in 1 MHz RBW 40 3rd Harmonic Power Time (μs) Figure 8. ZigBee Third Harmonic Power vs Time, Unfiltered and in 1 MHz RBW The signal may also be filtered by the Resolution Bandwidth (RBW) and decimation filters to produce a number of samples sufficient to extract the useful information. The samples are finally grouped into equal, time-spaced intervals and processed depending on the type of detector selected, peak, average, or RMS defined by the following equations: Peak V max(v i ) n (2) 1 V Vi (3) n 2 V rms V (4) V Vi n Where: V i = Voltage of sample i n = Total number of samples for one time interval For instance, the peak detector records the peak value of each interval, eventually represented by a pixel on the display. Figure 9 shows a time domain signal represented four different ways for a 20 μs sweep time. The Figure illustrates a 1 MHz sampled envelope with 20 samples. The three curves represent the same set of data displayed for three different peak detectors, average, and RMS. Figure 9 demonstrates that for a time variant signal, the measurement results depend on the detector type and are likely to be different between them. Bin 2, for which the envelope is constant, shows a particular case in which all results are equal. Therefore, the type of detector chosen to measure the timevariant, filtered ZigBee harmonic signal level affects the measurement results A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

6 3.5 Bin1 Bin2 Bin3 Bin Enveloppe (V) Time (μs) Samples Average Peak RMS Figure 9. Comparison of a Time Domain Waveform Displayed with Peak, Average, and RMS Detectors Table 2. Sine and ZigBee Waveform Harmonics Results for the Peak, RMS, and Average Detectors Harmonic Level Sine Peak/RMS/Average Peak ZigBee RMS Average Margin to FCC Specification (db) (Note 1) Margin to ETSI Specification (db) (Note 2) 2 nd harmonic rd harmonic Note 1: 41.5 dbm/mhz average detector. Note 2: 30 dbm/mhz peak detector. Simulation Results Applying a 1 MHz detector bandwidth, the harmonics simulation was run for the sine and ZigBee waveforms over a 150 μs time period. The results for the peak, RMS, and average detectors are summarized in Table 2 with the corresponding margin to the FCC and ETSI requirements. The peak, RMS, and average values of the constant envelope sine waveform harmonics are the same. Although the peak value of both second and third harmonics for the ZigBee waveform is the same as the sine waveform, it is interesting to note that the RMS and average values are significantly lower by about 5 to 6 db. The harmonics performance of the system for the peak detector was purposely set to be exactly at the FCC specification level ( 41.5 dbm/mhz). This demonstrates that while the system would have no margin to the FCC specification with a sinewave signal, it does outperform by 6 db with the ZigBee waveform. In addition, second harmonic performance for the different detectors was simulated over a range of Second Order Output Intercept Points (OIP2s). Figure 10 shows that there is a constant offset between the peak, RMS, and average detectors for the ZigBee waveform. Similarly, third harmonic performance for the different detectors was simulated over a range of Third Order Output Intercept Points (OIP3s). Figure 11 shows that there is a constant offset between the peak, RMS, and average detectors for the ZigBee waveform. 6 March 24, 2010 Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice A

7 2nd Harmonic Power Peak Average RMS OIP2 Figure 10. Second Harmonic Level vs OIP Peak Average RMS 42 3rd Harmonic Power OIP3 Figure 11. Third Harmonic Level vs OIP3 Measurement Results Hardware Test Set Up The test set up is shown in Figure 12. From the ADS test bench used in the previous section, the I/Q waveforms are collected into a text file and loaded into an Agilent ESG4438C signal generator featured with an arbitrary waveform generator. The RF output of the generator is sent to the Device Under Test (DUT), Skyworks SKY ZigBee FEM [5] that has a rated transmit power of +20 dbm. The DUT output is eventually measured with an Agilent Power Spectrum Analyzer (PSA) E4445A. ZigBee Spectrum and Time Domain Waveforms Measurements The fundamental ZigBee second and third harmonics are shown in Figure 13. The plot displays the 50 percent occupied bandwidth for each tone. The fundamental 50 percent bandwidth is about 750 khz. The second and third harmonic 50 percent bandwidths are about 2 MHz and 2.8 MHz, respectively. The second and third harmonic time domain waveforms are shown with a 1 MHz RBW and 150 μs sweep time in Figure 14. As demonstrated in the previous section, the envelope for both filtered harmonics varies over time. Harmonics Measurements Data Sine wave and modulated signal harmonics are measured for the same +20 dbm transmit output power for different detector types, RMS and average, and an RBW of 1 MHz. Sine wave and modulated signal second harmonic spectrum measurements are shown in Figure 15 for average and RMS detectors A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

8 Trigger source for duty cycle test SKY65344 Evaluation Board Figure 12. ZigBee Hardware Test Set Up Figure 13. ZigBee Spectrum Measurements: Fundamental, Second, and Third Harmonics 8 March 24, 2010 Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice A

9 Figure 14. ZigBee Harmonics Power vs Time Measurements With a 1 MHz RBW Figure 15. Second Harmonic Spectrum Measurement for Sinewave and ZigBee Modulated Signals: Average vs RMS Detectors A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

10 Figure 16. Third Harmonic Spectrum Measurement for Sinewave and ZigBee Modulated Signals: Average vs RMS Detectors Table 3. Harmonic Measurement Results for +20 dbm Sine Wave and ZigBee Signals Harmonic Level Sine Wave (RMS = Average = Peak) Modulated RMS Modulated Average Difference, Sine/ZigBee Average (db) 2 nd harmonic rd harmonic The second harmonic of the sine waveform is about 45.7 dbm regardless of the detector type. For the ZigBee modulated waveform, the levels are dbm and dbm for the RMS and average detectors, respectively. Figure 16 represents the same measurement results for the third harmonic. Again, the level of the sine waveform harmonic is the same regardless of the detector type. For the ZigBee modulated waveform, the data show a difference of about 2 db ( 49.9 dbm vs 52.0 dbm). The measurement data summarized in Table 3 show that the difference between the sine and ZigBee waveform harmonic level is about 7 db, which is comparable to the simulation results of Table 2 (6 db). Effect of Duty Cycle on Harmonics Transmitting only a percentage of the time can significantly reduce spurious emissions. As evaluated in the previous section, the reduction in power for a time variant signal depends on the type of detector used for the measurement. For example, assume a sine waveform transmitting at +20 dbm 50 percent of the time. Equations 1 through 3 determine that the peak value is +20 dbm and the RMS value is +17 dbm. However, the average value is +14 dbm. It is very interesting to note that the transmit power is half the peak power, which is very intuitive. However, the average voltage is also half the peak voltage. When converted into dbm, this is a 6 db reduction from the peak power. This result is also illustrated by Equation F.5 of IEEE Std TM-2003, Annex F [2], which is valid only for the average detector. For the RMS detector, the denominator of the fraction changes to Dc rather Dc^2 or a 3 db power reduction rather than 6 db for a 50 percent duty cycle. Figure 17 shows the spectrum measurements of a 100 percent and 50 percent duty cycle sine waveform measured with average and RMS detectors. The transmit power is dbm for a continuous transmission. For a 50 percent duty cycle, the output power drops to dbm and dbm using the RMS and average detectors, respectively. 10 March 24, 2010 Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice A

11 Figure 17. Sine Waveform Fundamental Spectrum Measurements for 100% and 50% Duty Cycles, Average vs RMS Detectors Figure 18. Harmonics Spectrum Measurements vs Duty Cycle Figure 18 represents the harmonic spectrum measurements when the ZigBee waveform is pulsed. For the second harmonic shown on the left hand side, the blue and red curves represent the harmonics of the sine and ZigBee-modulated waveforms, respectively, both running continuously. As noted in Table 2, the modulated waveform harmonic is significantly lower (~7 db from 45.6 down to 52.9 dbm) than the sine waveform. In addition, the green curve representing the harmonic of the ZigBee-modulated waveform operating at a 50 percent duty cycle has another 5 db reduction in power from 52.9 dbm down to 57.5 dbm. For the third harmonic, similar measurements are shown on the right hand side of Figure 18 with a duty cycle of 20 percent. Eventually, the level of the third harmonic of the ZigBee waveform is down to 61.8 dbm. (20 percent duty cycle should have produced about 14 db reduction in power from 51.9 dbm continuous data point of Table 2, the level is actually limited by the spectrum analyzer noise floor ( 68 dbm). Conclusions Budgeting harmonics for FEMs should be a fairly straightforward task. Since the FEM includes all of the harmonic filtering, the device must meet the regulatory standard limits for the specific frequency band of interest and area A Skyworks Proprietary Information Products and Product Information are Subject to Change Without Notice March 24,

12 However, for wideband signals such as spread sprectum Zigbee, the measurement results strongly depend on the measurement bandwidth and the type of detector. For example, Table 2 shows that second and third harmonics of a ZigBee waveform using an average detector (FCC CFR47, part 15 regulation) are 6 to 7 db smaller than with a peak detector (measured to the ETSI EN /EN regulation). A narrow band sine waveform is mostly used to test FEMs, but it also produces higher harmonics than a ZigBee signal (refer to Table 3). To comply with the FCC standard when pulsed operation is used, the averaging effect of the detector can reduce the level of unwanted emission by a factor as high as 20 db (10 percent duty cycle). Eventually, the goals for ZigBee FEM harmonic performance should be set, taking into account the parameters listed previously, to ensure that the system complies with the regulatory standard and with the expected margins. References 1. Wloczysiak, Stephane. Extending 2.4 GHz ZigBee Short Range Radio Performance with Skyworks Front-End Modules, Microwave Journal, August 2009 ( 2. IEEE Std TM-2003, Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs). 3. Agilent Spectrum Analysis Basics. Agilent Technologies Application Note 150, 2 August 2006 ( 4. Spectrum Analyzer Operating Manual, Rohde & Schwarz, pp and SKY GHz Transmit/Receive Front-End Module with Low-Noise Amplifier Data Sheet, Skyworks Solutions, Inc., document number Copyright 2010 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. ( Skyworks ) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. 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