Noise impact
When testing a mobile phone, you should first understand the structure of the mobile phone before considering noise. There are both a transmitter (TX) and a receiver (RX) in the mobile phone. The transmitter power is between + 30dBm and -55dBm, and the receiver signal reception range is between -20dBm and -108dBm. The dynamic range of the transmitter and receiver of a CDMA mobile phone is very large, exceeding 80dB, and the maximum power (+ 23dBm for a class III mobile phone) and the receiver sensitivity (-104dBm) are the most important indicators to be understood.
CDMA mobile phone testing requires multiple tests to ensure that the mobile phone meets specific standards. For example, testing the power of the transmitter of the mobile phone to ensure that the transmission power range is accurate in a wide dynamic range. In particular, the maximum transmission power of the mobile phone should be tested to ensure that the radiated power of the mobile phone is close to the minimum value specified by the maximum EIRP (effective isotropic radiated power), Ⅲ The transmission power range of the class mobile phone is + 23dBm ～ + 30dBm. Compared with noise, because the power measured by the instrument is larger, the noise is generally only an insignificant factor in the entire power measured. In addition, the mobile phone transmitter should also be tested to see if it meets the minimum transmission requirements. The CDMA minimum power transmission requires that the mobile phone transmission must be less than -50dBm. Even in this case, the transmission channel noise impact is usually negligible and will not affect the minimum power measurement. The problem of measuring the lowest power of CDMA is generally caused by the lowest noise limit of the power meter used.
On the mobile phone receiving channel, the receiver sensitivity measurement will be affected by noise. The CDMA mobile phone receiver sensitivity is specified at -104dBm, and the mobile phone must be able to demodulate the forward link signal transmitted at -104dBm at a frame error rate of less than 0.5%. Eb / Nt is a parameter indicating the ability of the mobile phone to accurately receive and demodulate the forward link signal, where Eb is the energy per bit of the communication channel, and Nt is the total noise on the reception bandwidth, which is a bit like the signal-to-noise ratio in analog circuits ( S / N). When the Eb / Nt ratio increases, the receiver can better demodulate the signal better; and as Eb / Nt decreases, the mobile phone is likely to erroneously demodulate the forward link signal. The actual bit energy of the forward link communication channel is 15.6dB lower than the specified value of the total forward link power of -104dBm. In other words, the actual signal received by the mobile phone during the measurement is -119.6dBm. From this point, we will mix Refer to the forward link signal in the content of the Walsh code.
According to the different tolerances of the mobile phone receiver design, its sensitivity to noise is also different. Generally, when the mobile phone performance is at the edge value, there will generally be noise problems on the forward link channel. There are many factors that affect Nt in Eb / Nt .
Noise source on the receive channel
The essence of KTB bottom noise Nt is thermal noise, which is always present in the environment. Thermal noise is also called KTB bottom noise, where:
K = Boltzmann constant (1.38 & TImes; 10-23)
T = reference temperature (Kelvin)
B = receiver effective noise bandwidth
For a CDMA system operating at a bandwidth of 1.23MHz, the thermal noise is about -113dBm. You may ask, how does the receiver demodulate the -119dBm communication signal through the -113dBm noise floor? This is because the CDMA processing gain is nearly 21dB, which can transfer 14.4kbps / 9.60kbps to 1.228Mcps rate.
â—† Component noise
The noise of the front-end components of the receiver (downconverter and amplifier) ​​will also produce Nt, affecting the sensitivity of the mobile phone, which is the specified sensitivity level of -104dBm reached by the forward link power test stage. In addition, all other noise factors will increase Nt and affect the success of the sensitivity test. Compared with mobile phones on the edge of performance indicators, mobile phones with a certain tolerance have more room to accommodate the increased noise.
â—† Environmental noise
There are many noises from unknown sources that will reduce the Eb / Nt of the forward communication channel. If any circuit with transmit power will produce spectral noise, the size depends on whether the signal itself or the intermodulation of two interfering signals has higher power falling on the measured bandwidth, but also depends on whether it appears with the sensitivity test. External units can also generate interference noise, especially for AMPS systems when testing 800MHz CDMA mobile phones. There are even reports that microwave ovens can interfere with mobile phone sensitivity testing. There are many production factories where lunch locations are close to the production line. If you find a lot of sensitivity problems at noon or during breaks, you should know where to find the cause.
â—† Test under close contact
In a production environment, there are many test benches that test mobile phones at various stages according to the test plan, which means that a mobile phone being tested at a certain stage will interfere with another mobile phone being tested at a different stage. Generally speaking, it is sensitivity that the interfered mobile phone tests. Remember that the forward link is set at -104dBm, the main interference may be that the neighboring mobile phone is receiving a large forward link signal, and just one mobile phone is performing the sensitivity test, and its forward link signal is set at -104dBm Or below. Mobile phone tests that set the forward link at a higher level include dynamic range, minimum transmit power, and open loop range. Generally speaking, the forward link for these tests is -25dBm.
Receiver and transmitter coupling
Another source of noise that affects the size of Nt is the cross-coupling between the transmitter and receiver of the mobile phone. This is a problem in the design of the mobile phone, due to the lack of proper isolation or matching between the receiving and transmitting channels. Because it is a design problem, the solution cost is high and difficult. The forward and reverse links are separated by 45 or 80MHz, so the interference between the unit band and the PCS band is highly isolated between the links, and crosstalk from one link to another link will be found at the front end of the mobile phone Phenomenon, this is the problem of the front-end design of the mobile phone.
Receiver channel sensitivity
Now that the main sources of spectral noise are listed above, let's take a look at how these spectral noises enter the mobile phone receiving channel.
Usually we use two methods to test the mobile phone, the most commonly used is to test through the physical RF connection, this connection is often referred to as "over-current" connection, the less commonly used test connection method is directly through the mobile phone antenna. We assume that all the tests discussed here are conducted through physical "over-current" connectors, and whether to connect the antenna is determined according to the mobile phone production process. In order to find the worst case, we assume that the antenna has been assembled into the mobile phone.
In most cases, mobile phones are easily affected by noise due to poorly shielded RF cables or unconnected antenna ports. Generally speaking, the antenna port is the main source of noise, especially if the antenna is connected during testing. Figure 3 shows the possible impact of a mobile phone undergoing a sensitivity test on another mobile phone with a forward link power of -25dBm. Mobile phone 1 is a mobile phone under test and has a higher forward link power of -25dBm. Since the transmission channel power of mobile phone 1 is -25dBm, assuming that the attenuation of the unconnected switch is 20dB, the signal leaked through the unconnected port of the antenna may be as high as -45dBm.
The size of the attenuation value varies depending on the design. Since the signal leaking through the antenna is propagated through the air, its propagation attenuation can be calculated by the Friis conversion equation:
among them:
Pr = Received power
Pt = transmission power
Gr = antenna transmission gain
Gt = antenna receiving gain
λ = wavelength
d = distance between Tx and Rx
Assuming that in the worst case, mobile phone 1 is 1 meter away from mobile phone 21 (assuming only far-field effects, near-field factors are difficult to consider), align the two mobile phones in parallel and there is no attenuation material between the mobile phones, then the antenna of mobile phone 2 You will receive a signal up to -83.92dBm. In this way, the receiver of mobile phone 2 will have an interference signal of -103.92 dBm, because it is assumed that the attenuation from the antenna to the overcurrent switch is 20 dB. This example illustrates the general reason why a mobile phone generates noise on the receiving channel of another mobile phone. According to the assumptions made in the example, there are many cases in the actual implementation that will cause different results. The mobile phone provides the basis for entering another mobile phone channel. Azimuth, distance, switch attenuation, shielding, antenna design and implementation, etc., these factors all play a role in the overall conversion that introduces noise.
A few notes
• If the connection test is over-powered, the antenna is not connected better than the connected one. Because the design of the mobile phone is based on the effective transmission and reception of signals in the required frequency band, if the test is carried out through the physical electrical connection, the antenna can still receive external signals and be added to the front of the transmitter and receiver. After connecting the antenna, it may also transmit large signals to the outside with higher power when measuring the transmitter of the mobile phone. After eliminating the antenna port matching (removing the antenna from the antenna connector) at the desired test frequency, the mobile phone can receive and transmit signals at a higher attenuation level.
• Because external interference can enter the connection between the RF cable under test and the mobile phone, it is important to use a properly shielded cable, preferably an N-type connector with triple shielded cable.
• The shielding of the mobile phone during the test will greatly attenuate the external interference from other mobile phones or unknown spectrum noise sources. The previous examples of mobile phone interference to the mobile phone are useful in determining the shielding attenuation when specific testing is required. Shielding mobile phones from external interference is not the only way to eliminate noise. Careful handling of all tested mobile phone frequencies can also effectively avoid interference. This requires more complex control software to initialize the number of channels and frequencies to eliminate frequency conflicts.
• Paying close attention to the distance between two mobile phones using the same frequency can also achieve frequency reuse. The example of mobile phone interference with mobile phones is very useful in determining the safety and full reuse distance.
Conclusion of this article
When dealing with receiver interference, especially in the production environment, it is not an easy task to find out the reason for the decrease in output due to the failure of the sensitivity test. The location of the factory, working hours, test methods and other seemingly unrelated factors Will cause random sensitivity to fail. Take a step back to understand the main sources of interference noise and how these noise sources affect the front end of the receiver, so that it will be easier to solve the problem of low sensitivity test pass rate.
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