“As the wireless market continues to evolve, the next generation of mobile phones will have more features, such as multiple color screens (at least two color screens per phone) and high-resolution cameras with more than megapixels.
As the wireless market continues to evolve, the next generation of mobile phones will have more features, such as multiple color screens (at least two color screens per phone) and high-resolution cameras with more than megapixels.
Still driven by the trend toward compact designs, implementing high-resolution LCDs and cameras will present a number of challenges for designers, one of the main design considerations being the susceptibility of these new modules to electromagnetic interference (EMI).
For many mobile phones that are popular today (especially clamshell phones), the color LCD or camera CMOS sensor is connected to the baseband controller via a flexible or long trace PCB connected between the two main parts of the phone (upper and lower).
On the one hand, the connection line is disturbed by parasitic GSM/CDMA frequencies radiated by the antenna. On the other hand, due to the introduction of high-resolution CMOS sensors and TFT modules, digital signals operate at higher frequencies, so that the connecting line can generate EMI/RFI like an antenna or may cause ESD hazardous events.
In summary, in both cases, all of these EMI and ESD disturbances can destroy the integrity of the video signal and even damage the baseband controller circuitry.
To suppress these EMI emissions and ensure normal data transmission, several filter solutions can be considered, which can be achieved by using discrete RC filters or integrated EMI filters.
How to Suppress EMI and ESD Noise Interference from Cell Phone Cameras
EMI and ESD noise suppression methods
The currently known solutions are reaching their technical limits when considering design constraints such as board space, high filtering performance at the operating frequency of the handset, and preserving signal integrity.
Discrete filters do not provide any space savings for the solution, and they only provide limited filtering performance for narrowband attenuation, so most designers are now considering integrated EMI filters.
In handsets with high-resolution LCDs and embedded cameras, signals are transmitted from the baseband ASIC to the LCD and embedded cameras at specific frequencies (depending on the resolution).
The higher the video resolution, the higher the frequency of data work. To date, typical data has been operating at frequencies of around 6 to 20MHz, and the race for resolution is driving camera module makers to continue increasing this frequency to 40-60MHz.
To accommodate the increase in data rates without disrupting the video signal, the designer must choose a filter that takes into account the theoretically recommended low capacitance, ie: the filter cutoff frequency (1/2πRC) must be approximately 5 times the clock frequency.
In current wireless terminals, for 300,000 to 600,000-pixel camera modules, the clock frequency is approximately between 6 and 12 MHz. Therefore, it is recommended to select the cutoff frequency of the filter (upper and lower) in the range of 30 to 50MHz. Many filter solutions follow this theoretical recommendation, but as resolution increases and clock frequencies exceed 40MHz, the filter cutoff frequency must be in the 200MHz range. Therefore, it is foreseeable that some filter solutions are reaching their limits.
The experiment gave several filter capacitor values versus cutoff frequency, and clock compatibility. This shows that low capacitance filters are the most suitable solution for high frequency, high speed data signal transmission.
Designers know, however, that there is an insoluble trade-off between filter capacitor value and attenuation characteristics at GSM/CDMA frequencies. The low-capacitance structure affects the filter’s high-frequency performance, and most current low-capacitance filters cannot provide attenuation performance better than -25dB at 900MHz. Shows the effect of EMI filter capacitors on GSM frequency attenuation.
In addition to having an effect on filtering performance, low-capacitance filters can also affect ESD performance. Finding the best compromise between good attenuation, ESD performance, and a low-capacitance filter structure is challenging given that lower diode capacitance can significantly reduce ESD surge capability.
Improved low capacitance EMI filter
In order to meet the contradictory requirements of realizing a low-capacitance filter while maintaining high filtering performance, a semiconductor company has developed a new generation of EMI filters with high-frequency attenuation characteristics at 900MHz and an ultra-low-capacitance structure.
These new EMI filters based on IPAD technology (integrated active and passive components) use a standard PI filter structure with integrated ESD protection. The figure shows a basic filter cell configuration with series resistors and capacitors.
This new low-capacitance structure is used to provide cutoff frequencies in the 200MHz range, enabling data rates with clock frequencies in excess of 40MHz.
Although the diode capacitance has been greatly reduced to 8.5pF, it provides excellent filtering performance, ie better than -35dB attenuation in the frequency range around 900MHz.
The figure shows the S21 parameter specification using the basic unit architecture of this filter. The graph shows a 35dB attenuation characteristic at 900MHz, an unprecedented performance achieved by integrating an EMI filter with 17pF line capacitance.
In addition to the filtering function, the integrated input Zener diode also suppresses air discharge ESD strikes up to 15kV, achieving the performance level required by the IEC61000-4-2 Class 4 industry standard.
High-speed data compatibility
In order not to disturb the video signal, the new low-capacitance filter is designed with optimized line capacitance values to support chipsets with clock frequencies higher than 40MHz.
This structure has only a small impact on the rising and falling edges of the data signal, and there is almost no delay between the input and output of the device.
The input Rt (10-90% rising edge) and Ft (10-90% falling edge) are simulated with a maximum 2.8V, 1ns signal. The results show that the delay caused by the filter (the difference between the output and the input signal) no more than 1ns. Be sure to fully maintain data integrity even for high-resolution LCD or camera applications.
The figure shows the comparison of the transmission of a 3V video signal operating at a frequency of 40MHz through high and low capacitance filters. It can be found that the delay caused by the high capacitance structure is 5 to 6 times that of the low capacitance structure. In this case, the signal output voltage cannot be received correctly.
Highly integrated solution
Using an integrated EMI filter designed as a flip-chip package with stacked bumps simplifies PCB layout and saves up to 80% of board area compared to discrete designs.
The results show that the line integration ratio (PCB area/number of lines) is about 0.6. This means that these new filters can take up 0.6mm2 of PCB area per line to provide EMI functions and ESD protection.
4, 6 and 8 “PI” line configurations are recommended for this new filter family to provide design flexibility and meet most high-speed data line design requirements. Its PCB area is 2.4mm2, 3.7mm2 and 5.0mm2 respectively, so it can almost completely adopt the traditional SOT323 plastic package.
Semiconductor’s new low-capacitance EMI filters support 4-, 6-, and 8-wire configurations, each of which includes an RC filter network flanked by Zener diodes. A series resistance of 100 ohms with a line capacitance value of 17pF was used to achieve a minimum 30dB attenuation over the 0.8MHz to 2GHz range. The low capacitance of the devices means they can be used in next-generation LCD displays and camera sensors with clock frequencies in excess of 40MHz.