GSM mobile phone electromagnetic compatibility design

1 Electromagnetic compatibility

(EMC) Definition EMC performance is expressed as the size of a device or unit and other devices or units within a certain range of time, frequency, electromagnetic space within the scope of "peaceful coexistence", in other words: The adverse effects of equipment or units on other equipment or units, the interference is small, and in this electromagnetic environment, the equipment or unit can work stably and reliably, and has certain anti-interference ability.

2 electromagnetic interference mode

There are four types of interference in the frequency domain: co-channel interference, adjacent channel interference, intermodulation interference, and spurs. Adjacent channel interference and spurs can be solved by filtering techniques. Co-channel interference can be solved by reasonable frequency management and allocation. Intermodulation interference can be solved by improving linear dynamic range and rationally selecting devices and operating points. • Interference between different levels of magnitude has strong signal blocking and near-far effects. These two types of interference can be solved by reasonable selection of power levels and automatic power control (APC). Conducted interference: Interference generated by the transmission line as a medium, for example, interference generated by power lines and ground lines, which can be solved by filtering and rational PCB design. • High-frequency radiation interference: This interference can be solved by shielding to reduce the energy that interferes with the electric field and magnetic field propagating in the “sensitive” space.

3 GSM mobile phone electromagnetic compatibility design

There are three techniques available to improve and improve EMC performance: grounding, shielding, and filtering. For each technology, there are relatively rich content for different categories of specific products, which makes EMC design difficult and requires experience.

3.1 GSM mobile phone EMC design requirements

Receive sensitivity: better than -102dBm / RBER (residualBER) < 20% (condition: class II, static, 900MHz band); · Transmit frequency error: <1 & TImes; 10-7, phase error RMS: ≤ 5 °, peak ≤ 20 °; · RF output power (four types of mobile phones): 5dBm (3.22mW) - 33dBm (2W). Receiver co-channel, adjacent channel, blocking, intermodulation indicators, transmitter modulation spectrum, switching spectrum, spurious radiation and other indicators must meet the requirements of ETSGSM11.10 and 05.05 (European Telecommunications Standards) technical specifications.

3.2 Characteristics of mobile phone EMC design

EMC design plays a very important role in communication systems, communication equipment, household appliances and other equipment. For the home communication electronic products such as GSM mobile phones, the performance of the mobile phone EMC has direct and reliable operation on the GSM network system, the mobile phone RF index, the human body sensing level, the reliability and stability of the mobile phone hardware and software. influences. The GSM mobile phone EMC design has the following characteristics: 1 The overall size of the whole machine is small, and the power density per unit volume is large. The overall dimensions of GSM mobile phones are generally 100mm long, 40mm wide and 25mm thick. For four-class machines, the maximum output RF power is 2W (33dBm) compared with other communication devices (eg 100W paging transmission). Machine, 25W FM radio, 20mW cordless phone), the mobile phone's unit volume power density is the largest, which puts higher requirements on EMC design. 2 light weight, high choice of shielding materials. Because GSM mobile phones have weight requirements (generally less than 100 grams), it is impractical to use too much, too thick shielding material in the mobile phone, which increases the difficulty of EMC design. 3TDMA burst burst mode of operation. The GSM mobile phone adopts the TDMA communication working mode (8 time slots per frame, 4.615 ms per frame, 577 μs per time slot), which puts high requirements on the output switching spectrum and power supply filtering. 4 The problem of human body induction is outstanding. Handheld communication terminals all have human body sensing problems. Since GSM mobile phones operate in the 900MHz and 1800MHz frequency bands, the operating frequency is higher than that of general handheld communication terminals (eg 150MHz, 450MHz FM radio, 48-74 MHz home cordless phone). So the problem is more prominent. rWh safety and electromagnetic compatibility network

4 GSM mobile phone EMC design technology

In the GSM mobile phone EMC design work, like other communication devices, the three technologies of grounding, shielding and filtering are adopted. For mobile phones, these technologies have their own characteristics. network

4.1 Grounding

From a circuit analysis perspective, grounding is a technique that provides a low impedance path to the power supply or signal source over the operating frequency range. From this point of view, it is a basic principle to use large-area grounding, short and thick wires to be grounded nearby. Grounding is closely related to shielding and filtering. Good and reliable grounding is a prerequisite for high-quality shielding and filtering. The grounding method has a little grounding, multi-point grounding, large-area grounding, bus bar grounding, etc. Most of the mobile phones use PCB large-scale grounding layer grounding and PCB peripheral busbars to ground.

4.2 Shielding

Because mobile phones are high-performance and cost-effective communication household appliances, and have limitations in size and weight, when designing mobile phone EMC shielding technology, careful consideration must be given to the shielding method and material selection as well as the processing technology and surface treatment. In the mobile phone, the materials and features that can be selected are as follows: • Engineering plastic with metal coating or coating: It is characterized by lowering the production cost of the mobile phone, easily matching the structure and shape tolerance of the outer casing, and reducing the mobile phone. Weight, but due to the thickness and density of the coating, the shielding effect is generally not as good as the all-metal shielding structure. Since the design and processing of domestic plastic molds and the plating of metal layers are generally completed by different manufacturers, it is difficult for EMC designers to propose specific electromagnetic shielding effects. The quality and stability of the products are difficult to guarantee, and plating is likely to occur. Problems such as peeling off of the coating, mechanical deformation and surface oxidation, which seriously affect the shielding effect. The author believes that unless the plastic parts using this method have a stable process guarantee in terms of physical, chemical, and electrical characteristics, it is generally inappropriate to use this method in domestically developed mobile phones. · Conductive rubber: This material is a high-conductivity metal powder (such as silver, nickel, BalloTIni) added to silicone rubber. Its shape and cross-section can be selected according to needs (for example: O, U shape). The basic basis for the selection is that the conductivity index is high and has sufficient mechanical strength, elasticity and stability. The characteristics of this material are: 1 elasticity and electrical conductivity will deteriorate with time (aging); 2 poor seismic performance, After being shocked, the conductivity index will be deteriorated; 3 the dimensionality is poor and the deformation is easy to fall off. To form a high quality, low impedance ground plane, there are technical difficulties. For the above reasons, this material is less used in new mobile phones. ·Metal stamping and tensile shielding structure: In theory, the best shielding effect can be obtained by this method. The following factors should be considered in the design: (a) Select materials with high electrical conductivity and magnetic permeability. Metal conductors have two kinds of losses, absorption loss and reflection loss on electromagnetic waves. When the operating frequency is greater than 100MHz, the loss is mainly due to absorption loss. (b) After the stamping and drawing and surface treatment, the shielding material requires high dimensional accuracy, the surface is not easily oxidized, and has certain mechanical strength to meet the requirements of production and maintenance. (c) Low manufacturing costs. (d) The galvanic corrosion caused by the Galvani battery effect is small. (e) Processing and processing of closed metal shielded gold holes that need to be welded: From the perspective of EMC, it is desirable that the number of holes and the diameter of the holes are smaller and smaller, from the quality of the welding process of infrared reflow soldering. Considering that it is desirable to have more and more holes and apertures, this requires EMC designers to make trade-offs between the two. (f) The size, shape, shape tolerance, finish, and material of the shield near the antenna of the mobile phone will have a great influence on the distribution parameters of the antenna, thereby further affecting the output power and power consumption of the PA (power amplifier). RF indicators such as phase error and receiving sensitivity require sufficient attention from the designer. In view of the specific conditions and technological level in China, the author believes that the shielding material can be nickel-plated or beryllium-plated nickel or nickel-based stainless steel with a thickness of about 0.2mm (it is necessary to fully notice the rebound deformation of the material after demolding). If the shield needs to be welded, the surface treatment is changed to tin plating.

4.3 filtering

Filtering is a technique that provides a transmission pole for signals in certain frequency points and a transmission zero for signals in other frequency points. In the GSM mobile phone, a variety of different filtering techniques are adopted: 1 from the cutoff frequency of the filter, there are low pass (LPF), high pass (HPF), band pass (BPF) filters; 2 from the operating frequency of the action Look, there are low, medium and high frequency filters; 3 from the way of signal transmission, there are transmission type and bypass type filters; 4 from the material used, there are SAW (surface acous TIc wave), transmission line, cavity Body, ferrite, ceramic filter; 5 from the device used, there are passive and active filters.

The following describes some of the special filtering techniques used in GSM phones.

(1) One-way power (battery) input, linear active filtering technology for multi-channel power output. Because GSM mobile phone works in TDMA mode, it works only in certain time slots during one frame period, so it is in the entire power circuit. There are TDMA pulses, especially in the transmission time slot. For mobile phones with 3.6V power supply, the peak current can reach 2.5A in the high power range during the transmission time slot. With this in mind, this technology is widely used in GSM mobile phones, namely, receiver, transmitter driver, PA, quite synthesizer, CPU, Keno processing, SIM card, audio processing, LCD unit with active filter output powered by. This filtering technology ensures that the interference between the output power supplies is small, thus ensuring EMC performance. If passive filtering technology is used, not only a large filter capacitor is required, but also it is difficult to meet the technical requirements. In addition, GSM mobile phones are very particular about power consumption indicators (the length of standby continuous talk time). From this point of view, the use of power supply filter management technology is conducive to separate control and management of each unit circuit, so that the relevant unit circuit is only in the corresponding The work is started in the time slot, and the sleep state is waited in the remaining time slots, thereby achieving the purpose of further reducing power consumption and saving power.

(2) Ferrite filter This device can be classified into several types according to the use conditions: standard signal, high-speed signal, ultra-high-speed signal, and power supply. Using the impedance-frequency characteristics of ferrite (determined by material formulation and process), it is inductive at low frequencies, capacitive at high frequencies, and has a parallel resonance point at some intermediate disadvantage. The ferrite filter is connected in series in the loop to form a low-pass filter, which has the characteristics of small size, low cost and large operating current (compared with the LC filter), and is especially suitable for the large part of the PA power supply. Current pulse filtering has a high performance-price ratio.

(3) The transmission line wave filter on the PCB uses a transmission line at the input end of the receiver and the output end of the PA. The filter is more cost effective than the traditional lumped-parameter LC filter. Since the equivalent inductance parameter of the transmission line is difficult to accurately calculate in the design (related to the dielectric constant of the PCB, the thickness of the PCB, the length of the wire on the PCB, the operating frequency, and the edge effect), and the dielectric constant of the PCB material may follow The production batch changes and is placed in the GSM mobile phone. The capacitor's under-arm capacitance (generally using high-Q high-frequency ceramic capacitors) requires "sliding" grounding on the transmission line for more accurate power matching and impedance matching, which improves receiver sensitivity. It is advantageous to reduce the transmit phase error, increase the RF output power, and reduce the power consumption of the power supply.