AN-5059| Application Note
AN-5059 LVDS Technology Solves Typical EMI Problems Associated with Cell Phone Cameras and Displays
AN-5059
Fairchild Semiconductor Application Note May 2005 Revised May 2005
LVDS Technology Solves Typical EMI Problems Associated with Cell Phone Cameras and Displays
Differential technologies such as Low Voltage Differential Signaling (LVDS) will be explained and compared to legacy single ended LVTTL. Through specific application examples, this article demonstrates the improved spectral content and advantages offered by LVDS technology. tions and 700MHz is allocated to fixed mobile broadcasting. In this case, a cell phone with a noisy clock will likely interfere with these mobile stations and nothing else. The realistic application of this cell phone clock may not be a perfect 100MHz. It may be off slightly, say 97MHz. In this case, the 9th harmonic winds up at 873MHz. The FCC frequency allocation table identifies 873MHz as nearly the middle of the cell phone frequency band. This means that the clock frequency of this cell phone can dramatically reduce the sensitivity of the cell phone receiver itself, potentially rendering it inoperative.
Today's Cell Phones
Today's cell phones, that are becoming continuously smaller and lighter, have an increasing possibility to adversely affect surrounding devices. This is because of the increased shear number and opportunity for close proximity to other ultra-portable electronic devices. Twenty years ago, a cell phone would not be in close proximity to an implanted defibrillator. Today it can be a common occurrence.
LVDS Technology
LVDS technology is a comparatively new technology that is rapidly replacing legacy TTL or LVTTL technologies. LVDS is a standards-based technology that utilizes two conductor paths rather than one as with TTL or LVTTL. At first glance, it may seem inefficient to utilize two conductors rather than one, however this two conductor system has the distinct advantage of operating at much higher speeds than its predecessor. It should be noted that with either TTL or LVTTL, a second conductor exists that is actually the power ground. The architecture of the LVDS technology is such that the two wires will utilize opposing polarities that will change at the same time based on a change with the data input. This means that the two wires (or other medium such as flex circuit wires, twisted pair of wires, etc.) will have opposing currents during the polarity change. The opposing currents in effect cancel each other so that the net current change is comparatively quite small. It is this advantage, combined with the fact th
at the voltage swing is typically 350mV, rather than 3.3V or 5.0V with TTL, that results in significantly reduced overall current change, ultimately resulting in less EMI.
Typical EMI Problems
EMI (Electromagnetic Interference) problems with cell phones usually fall into one of three categories: 1. Blatant EMI radiation that exceeds regulatory emissions limits during product qualification (FCC, and ETS/EN testing, etc.). 2. EMI that although meets regulatory requirements, continues to adversely affect devices in close proximity. 3. EMI that adversely affects the cell phone itself through harmonics and other spurious signals.
Products on the Edge
Cell phones, although usually designed by a single supplier, can be marketed in many countries. Unfortunately, regulatory requirements vary from country to country, and often one country does not recognize the standards or test results of another country. For example, a cell phone must undergo EMI testing for the requirements of each respective country. A device that "squeaks by" regulatory testing in one country, may barely fail in another country. Commonly, the same product design may be marketed under separate model numbers to reflect the different EMI testing, and can include minor circuit changes to allow regulatory compliance for a specific locale.
Compare and Contrast Typical Application Emissions with LVTTL to LVDS
As a means of directly comparing technologies, a test was designed to compare only the interface technologies. The remaining parameters, equipment, and test environment remain the same. In this case the parameters were a single bit, 100MHz, repetitive square wave, and the transmission medium was a 10cm flex circuit. 10cm represents a typical length found in many cell phones available today. Identical circuit boards were fabricated with the exception being 1 set utilized LVDS devices and the second set utilized LVTTL devices. Figure 1 shows the test set-up.
Harmonics and Why They are Bad in Cell Phone Applications
Harmonics are exact multiples of a fundamental frequency. As an example, a square wave clock operating at 100MHz in a cell phone can have visible harmonics on a spectrum analyzer at 300MHz, 500MHz, and 700MHz. Additional peaks are often seen with the spectrum analyzer but may represent additional spurious signals as a result of a local parasitic oscillation or signal reflection. Unfortunately, in this example of a 100MHz clock in a cell phone, a harmonic may exist at 700MHz and at 900MHz. According to the FCC Frequency Allocation Table dated April 13, 2004, 900MHz is the frequency utilized by fixed land mobile sta 2005 Fairchild Semiconductor Corporation AN500908
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