AN-42041 5th-order s-video filter/driver reduces ntsc/pal system parts count www.fairchildsemi.com ? 1999 fairchild semiconductor corporation www.fairchildsemi.com AN-42041 rev. 1.0.1 introduction the fms6400 ic is an 8-pin, soic, 5th-order, dual video l- ter designed to replace bulky discrete passive lters (figure 1) or the handful of video integrated circuits commonly required to properly reconstruct dac-generated video signals. the fms6400 operates from a single +5v 5% supply. in addition to y/c ltering of dac outputs, the fms6400 contains circuitry for sync tip clamping and restoration. further, the lters internally sum the luma and chroma channels, provid- ing a third ltered output: the composite video signal. the fms6400 is designed for ac- or dc-coupled output applications, but requires capacitive coupling (0.1 f) of the input signals. all output channels are fully buffered, capable of driving 2v pp into a 150 load or 1v pp into a 75 load. the fms6400 also offers gain selections of 0db or 6db. this provides exibility to drive directly into an adc, limiting the number of discrete components. the fms6400 is fully pro- tected from load shorting, providing the maximum junction temperature of the part is not exceeded, and may drive two separate loads each. this application note contains performance results and design information collected from an fms6400 design utilizing a four-layer pc board. the schematic of the board used to eval- uate this device is shown in figure 9. application board testing test equipment one power supply: 5v 10%, 200ma maximum one 14-inch (or larger), high-resolution crt monitor: sony pvm-14m2u one y/c video signal source: jvc hr-s7100uv vcr set for ?live feed? from a local television station one hp 3577a network analyzer one tektronix tds640a oscilloscope one fms6400 demo board assorted video cables figure 1. s-video filter discrete solution 75 220f y in y out dac 75 v ref 75 * fms6400 also conatins sync tip clamp and feedback amplifier for ainti-clipping 220f cv out1 75 220f cv out2 75 x2 x2 75 220f c in c out dac 75 75 x2 may be ac or dc coupled
AN-42041 application note ? 1999 fairchild semiconductor corporation www.fairchildsemi.com AN-42041 rev. 1.0.1 2 procedure do not turn the power supply on until all connections shown in figure 2 are completed. 1. set the power supply to 0v. connect the power supply to the fms6400 demo board. note: use the shortest possible cables (50 or 75 ) for all the following video connections. 2. connect the video source ?s-video out? to the ?y in ? and ?c in ? connectors on the demo board. 3. connect ?y out ? and ?c out ? from the test board to s-video input of the monitor. 4. connect ?cv out ? from the test board to line a of the monitor. 5. adjust the input voltage to 5v. verify that the test board is not drawing excessive current ( 75ma). 6. apply the video test signal to the test board. a high- quality image should appear on the monitor screen. note: it may be necessary to connect the video test signal di- rectly into line c of the monitor to ensure the test signal is present and of high quality. 7. switch the monitor input to line a (y and c out ). 8. note the effect of the lter action on the video signal. 9. switch the monitor input to line b (cv out ). 10. note the effect of the lter action on the video signal. figure 2. connection diagram monitor fms6400 test board +5v gnd y out c out cv out2 cv out1 dc power supply + ? y signal source c in in c out y out line a results the bode plot in figure 3 is a non-subjective measure of the fms6400 ltering action. a network analyzer was connected to either input channel with a 200mv rms test signal and a plot was made of the frequency response of output vs. input. the resulting amplitude vs. frequency plot demonstrates the accuracy of the fms6400 regarding atness of response: 3db (cutoff) point at 7.1mhz and linear 50db/decade rolloff above cutoff. time domain measurements are shown in figures 4 and 5. note the ltering action performed on the ?choppy? dac input signal. the video signal was a standard color-bar pattern. the luma channel (figure 4) contains the video signal ampli- tude, while the chroma channel (figure 5) carries the color information. note that the d/a artifacts are reduced by at least a factor of 10. though not shown, the composite output is the ltered arithmetic sum of the luma and chroma chan- nels. figure 6 illustrates applications where the video signal is undersampled or requires a steeper rolloff. two lters are cascaded for a 160db/decade rolloff. note the use of 150 termination resistors on u1?s output (figure 7). this is done to reduce the loading on u1 and preserve the dc restoration capability of u2. do not increase the termination resistor val- ues above 240 . if the resistor values are increased, decrease the series capacitors proportionately. this cascading technique can be used with additional lters to obtain even steeper rolloffs with minimal effect on the -3db point.
AN-42041 application note ? 1999 fairchild semiconductor corporation www.fairchildsemi.com AN-42041 rev. 1.0.1 3 figure 3. fms6400 bode plot figure 4. luma channel filtering figure 5. chroma channel filtering figure 6. fms6400 bode plot (2 parts cascaded) 10k 100k 1m 10m 100m 10 amplitude (db) frequency (hz) 0 -10 -20 -30 -40 -60 -50 -70 -80 -90 -1db -3db 27mhz 10k 100k 1m 10m 100m 10 amplitude (db) frequency (hz) 0 -10 -20 -30 -40 -60 -50 -70 -80 -90 figure 7. cascading two fms6400s with 6db gain for increased filtering 75 0.1 f 0.1 f 1 f y in 73 2 8 1 5 4 5v 1 4 u1 fms6400 (6db option) u2 fms6400 (6db option) 150 100 f 150 75 220 f y out 75 220 f cv out1 75 220 f cv out2 0.1 f 1 f 73 2 8 5 0.1 f 75 0.1 f c in 150 100 f 150 75 220 f c out 0.1 f 6 may be ac- or dc-coupled
AN-42041 application note ? 1999 fairchild semiconductor corporation www.fairchildsemi.com AN-42041 rev. 1.0.1 4 figure 8. cascading two fms6400s, 1- 0db and 1 ? 6db gain for increased filtering figure 9. fms6400 typical application schematic 75 0.1 f 0.1 f 1 f y in 73 2 8 1 5 4 5v 1 4 u1 fms6400 (6db option) u2 fms6400 (6db option) 75 220 f y out 75 220 f cv out1 75 220 f cv out2 0.1 f 1 f 73 2 8 5 0.1 f 75 0.1 f c in 75 may be ac- or dc-coupled 220 f c out 0.1 f 6 + C ? r1 75 c1 0.1 f y in 1 4 5v fms6400 r3 75 c3 220 f y out r4 75 c4 220 f cv out1 r5 75 c5 220 f on channel modulator vcr or tv c7 0.1 f c8 1 f 7 v cco 3 gnd 2 v cc 8 5 r2 75 c2 0.1 f 5th-order filter c in r6 75 may be ac- or dc-coupled c6 220 f c out 6 5th-order filter
disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. fairchild does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights, nor the rights of others. life support policy fairchild?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor corporation. as used herein: 1.life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sus tain life, or (c) whose failure to perform when properly used in accor dance with instructions for use provided in the labeling, can be reasonably expected to result in signi cant injury to the user. 2.a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. ? 1999 fairchild semiconductor corporation www.fairchildsemi.com AN-42041 rev. 1.0.1 5 AN-42041 application note comments and precautions figure 9 is the fms6400 test board schematic. a pc board similar to the test board may be used for system evaluation, providing the size of the board allows for low-noise connections. the video and power connectors can be removed and direct solder connections made to the board. capacitors c3 through c6 were speci ed as tantalum because of their low-parasitic elements (esr, esl). high-grade, low- esr electrolytic capacitors may be substituted with no loss in performance. use care in choosing these capacitors to ensure the esr is both low and guaranteed. as with tantalums, observe polarity when installing in the circuit. the values of input capacitors c1 and c2 are optimized for the method in which the fms6400 restores the sync information. changing these values has an effect on the amount of ?tilt? on the luma channel only. conclusion video systems design engineers now have an inexpensive and space-ef cient solution for ltering video signals. the existing passive discrete and active solutions are no longer viable be- cause cost and physical space limitations top the list of priori- ties in new and revamped designs. the fms6400 p resents a turnkey solution to the challenge of designing a system that not only performs according to speci cation, but also comes in well below cost and space goals.
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