High Accuracy, Low Amplitude RF Signal Generation in Receiver Sensitivity Testing

Transcription

1 High Accuracy, Low Amplitude RF Signal Generation in Receiver Sensitivity Testing Application Note Today's wireless communication technology adds many conveniences to everyday life. One of the foundations of communication anywhere at any time is wireless transceiver technology. Wireless transceiver technology enables sharing texts, pictures, videos, and other information whether it's between two handsets within close proximity to each other or via a satellite system to a spaceship thousands of miles from earth. The following diagrams show a typical wireless transceiver. The function of a wireless transmitter is to process the original information so it's suitable for wireless transmission. t includes blocks for coding, modulation, LO, up-converter, and amplifying subsystems. The function of a wireless receiver is to recover the original message from the signal it receives, it includes blocks for LNA, LO, down-converter, demodulation, decoding, etc. deally, the information recovered by the receiver is the same as the original information; however, some limits in the real physical world cause some deviations or errors. To receivers, the receiver sensitivity is one of the most important limits. A receiver sensitivity test measures the smallest possible signal power level at the input of a receiver that assures proper functioning of a wireless receiver and that it will decode data correctly. All receiver tests depend on a figure-of-merit. This is SNAD for analog radios and can be BER (bit error rate), MER (message error rate), FER (frame error rate), etc., depending Antenna BB Generator mage Filter DC 2 LO Shape Filter Figure 1. Diagram of a wireless transceiver. Shaping Filter Mod F Amp F BFP Mixer BPF Step Att RF Amp

2 Application Note Antenna LO DC 2 mage Filter BB Processor mage Filter BPF LNA Mixer mage Rejection Filter AGC Demod Matching Filter Figure 2. Diagram of a wireless receiver. on the communication standard. A wireless receiver can be receiver part of a mobile phone, base station, or any wireless standard compliant device such as WLAN, zigbee, Bluetooth, etc. Usually, there are three types of measurements for receiver sensitivity testing. Receiver Sensitivity Test with Out-of- Channel nterference (Receiver Blocking Performance Test) Receiver Sensitivity Test without nterference RF signal from RF SG A F signal for demodulation RF signal from RF SG A F signal for demodulation nterference signal from RF SG B Blocking signal nterference signal from RF SG B Receiving signal Noise floor Figure 3. Receiver sensitivity test without interference. Receiving signal Noise floor Receiver Sensitivity Test with nterference (Receiver Selectivity Test) RF signal from RF SG A nterference signal from RF SG B Figure 4. Receiver sensitivity test with interference. F signal for demodulation Receiving signal n-channel interference Noise floor Figure 5. Receiver sensitivity test with Adjacent-channel interference. One or two units of RF signal sources (RF SG) are needed for the receiver sensitivity test. RF SG A generates the small signal to the device under test (DUT), which stimulates the real signal received by the antenna of the receiver. The amplitude of the signal from source A is lower than -120dBm in some cases. RF SG B stimulates the interference signals in the receiving channel or out of the receiving channel but could impact the receiving signal, which is highly dynamic in amplitude. Absolute amplitude accuracy is one of the critical performances of an RF SG, which characterizes the accuracy of the amplitude generated by the instrument. The lower the difference is between the amplitude set by the user and the actual amplitude the SG generates, the better the absolute amplitude accuracy performance is. The performance is caused by some error factors, such as the environment's temperature, accuracy of the amplitude setting of the instrument, VSWR, etc. n the laboratory, engineers can minimize the impact to the absolute amplitude accuracy caused by the temperature and the VSWR mismatch error, but the carrier frequency is fixed once the requirement of a receiver is confirmed. So the absolute amplitude accuracy is determined by the RF SG itself, for 2

3 High Accurate Low Amplitude RF Signal Generation in Receiver Sensitivity Test example, the performance of the amplitude calibration algorithm and circuitry. Usually, the absolute amplitude accuracy of a RF SG is good when a signal in the >-60dBm range and becomes worse when a signal with very low amplitude is generated. Especially when generating a signal less than -100dBm, its absolute amplitude accuracy is much worse than that of a 0dBm signal. The following data, from the data sheet of a mainstream RF signal generators shows that the absolute amplitude accuracy of a signal in <-110dBm range is almost 2 times worse than that in the >-60dBm range when carrier frequency is set to 1.5 GHz. +23 to -60 dbm <-60 to -110 dbm <- 110 to -127 dbm 100 khz to 250 khz ±0.6 db ±1.0 db > 250 khz to 1 MHz ±0.6 db ±0.7 db ±1.7 db > 1 MHz to 1 GHz ±0.6 db ±0.7 db ±1.0 db > 1 to 3 GHz ±0.6 db ±0.8 db ±1.1 db > 3 to 4 GHz ±0.7 db ±0.8 db ±1.1 db > 4 to 6 GHz ±0.8 db ±1.1 db ±1.3 db Table 1. The amplitude accuracy of a signal in the <-110 dbm range is almost two times worse than that in the >-60 dbm range when carrier frequency is set to 1.5 GHz. The receiving sensitivity of a modern wireless communication device is usually less than -100dBm, for instance, the receiving sensitivity of a Node-B is less than -120dBm according to the TU protocol. And, the sensitivity of a wireless receiver will go even lower as technology develops in the future. t's becoming increasingly more challenging to design and test a receiver while the errors caused by the test instrument itself will bring further difficulties to the challenge. Here is an example. Engineers are designing a wireless receiver and the receiving sensitivity of the receiver specifies at -120dBm level. Two RF signal generators are being used. The absolute amplitude accuracy of SG A is ±3dB, while SG B is ±1dB. f the engineers choose SG A for their design and test, they need to set the output level of SG A at -123dBm because its actual output level may be positively deviated from -123dBm to -120dBm. With this setting, they avoid any risk that the receiver can t work well at -120dBm sensitivity. However, the positive and negative deviations exist in the same instrument, and either of the two deviations occurs randomly when the status of the instrument changes. So, the SG A output level is in the -120dBm to -126dBm range although the setting number in the instrument is at -123dBm. n order to meet the receiver sensitivity requirement, the receiver must receive and demodulate the signal from SG A correctly, which means that the actual receiving sensitivity of the receiver should be at the -126dBm level. The extra 6dB performance in receiving sensitivity is a significant improvement in cost, effort, and time. f the engineers choose SG B, they just need to design the receiver to receive and demodulate a -122dBm signal correctly (when setting SG B output level at -121dBm), according to the logic and calculation discussed above. The 4dB gap causes much more effort, much longer R&D time, as well as double, triple, or even higher cost. How can the absolute amplitude accuracy of the RF Signal Generator be increased in the test? An obvious way is to use a performance product whose absolute amplitude accuracy is usually less than 0.8dB at very low level; however, the price of a performance RF SG is expensive and can range from tens of thousands to several hundred thousand dollars. An effective, convenient, and much cheaper alternative is to set the external attention network between the signal generator and the DUTwith an attenuator. The external attenuator makes the signal generator output a higher level signal to compensate for the external attenuation, so the instrument works at the proper range where it performs the best absolute amplitude accuracy. The following process is recommended: 1. Choose the proper cables and attenuator(s) with an external attenuation network, then measure the attenuation. Assume the attenuation value is A (A>0 db), calculating the amplitude setting number to the signal generator in next step. Considering that the wide bandwidth receiving technology is more and more popular, and that signals have high peakto-average power characteristics, using the modulated wide bandwidth signal instead of the CW to measure the attenuation network is recommended. 2. Add the specified receiving sensitivity (S) to the attenuation A, so the result S + A = B is the rough level value setting to the signal generator. Then, check the data sheet of the instrument to get the absolute amplitude accuracy (±k) when generates the B level signal. Finally, calculate the exact level value (B-K) setting to the signal generator. For instance, the required receiving sensitivity of the DUT is -112dBm, and the attenuation of the external attenuation network is 60.4d, while the absolute amplitude accuracy of the signal generator is -60dBm to +15dBm is ±0.7dB, so the exact level value setting to the instrument is-52.3dbm (= ). 3. nput the signal to the DUT through the calibrated external attenuation network, and test the receiving sensitivity perfor- 3

4 Application Note This method is not limited to receiving sensitivity tests, but also for applications where an accurate low level signal is needed. By configuring a proper external attenuation network between the signal generator and DUT, the signal generator is able to generate signals in the range where its absolute amplitude accuracy is best. With mid-range performance at an entry-level RF signal generator price, Tektronix TSG4100A Series Signal Generators offer the best vector signal generator value on the market. CW, 50 Ohm Load, 18 C to 28 C (db, Typical) Figure 6. Receiving sensitivity test with external attenuator. mance. Then, change the frequency and repeat steps 1 3 to finish the sensitivity test in the receiving band. Other advantages of the method are: mproves VSWR performance between the signal generator and the DUT, which is one of the factors that impact measurement accuracy. Protects the signal generator to reverse power damages, e.g. test point is the antenna port of a BTS or RRU which receives and emits RF signal via the same port. Enhances the isolation between the two signal generators significantly, which is critical to correctly and accurately measuring the blocking performance of the receiver correctly and accurately. CW, 18 to 28 C >10dBm 10 to -30 dbm -30 to -60 dbm -60 to -100 dbm <-100 dbm 10M~0.1 GHz ±0.2 ±0.25 ±0.35 ±0.45 ± G~2 GHz ±0.15 ±0.15 ±0.25 ±0.35 ±0.6 2G~4 GHz ±0.3 ±0.2 ±0.35 ±0.6 ±0.8 4G~6 GHz NA ±0.3 ±0.4 ±0.75 ±1.25 Table 2. TSG4100A Series absolute amplitude accuracy. With the optional TSG4100A-ATT high accuracy 30 db RF attenuator (5 W, up to 6 GHz,) the absolute amplitude accuracy of a 1 GHz CW signal at >-130 dbm amplitude range is less than ±0.35 db. And, partner a TSG4100A RG Vector Signal Generator with Tektronix mixed doain oscilloscopes (MDO3000/4000B) and real-time spectrum analyzers (RSA306) for an unbeatable price/performance mid-rante RF solution for education, basic R&D, consumer electronics manufacturing, and commercial indusry application applications. 4

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