Measuring Improving Differential Gain Differential Phase Video Ar

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Measuring Improving Differential Gain Differential Phase Video
Arne Buck Riphahn
March 1994
Measuring Differential Gain Differential Phase Video Op-Amps Differential gain (DG) differential phase (DP) specifications that designers composite video systems everyday. will define them here just sure speaking same language, ensure understanding rationale test technique used National. Composite video encodes brightness (luminance), timing (sync), colour (chrominance) into channel. Luminance voltage offset from reference, "black", level. Sync appears level defined "blacker than black." Chrominance encoded highfrequency (with respect luminance signal) subcarrier. average value (mid-point) chrominance luminance. color "dimensions": amplitude which determines saturation, phase relative reference chrominance burst which encodes hue. example, pink same relative phase red, lower amplitude, hence less saturated red. NTSC shifted 103.7° from reference, green 241.3°. measured chrominance subcarrier frequencies. NTSC (National Television Systems Committee, 1953) uses 3.579545MHz color subcarrier. Phase Alternation Line (PAL) alternates phase reference burst with every scan line. subcarrier frequency 4.433619MHz. Differential phase change chrominance (highfrequency) phase with luminance level. This manifests itself picture color shift illumination changes. blue parrot indoors does become purple parrot sun. Differential gain change chrominance gain with luminance level. saturation changes viewed scene brightness varies. shirt noon must turn pink night. Both distortions which, sufficiently large, perceived eye. Another think error signal channel amplifier magnitude variation high-frequency sinusoid (the subcarrier) offset changes. This offset simplest crudest, offset. also take form low-frequency sinusoid ramp. Similarly, alternative view change carrier phase shift channel over range offset simulating luminance (see Figure Figure traditional measuring broadcast video equipment such switcher distribution amplifier, with vectorscope. distribution have several integrated discrete amplifiers between input output. Vectorscopes measure such high-level systems <1°DP. Newer video test equipment based digital technology, such Tektronix ©VM700, make measurements <0.03% <0.03°. This resolution results after errors video test signal generator have been calibrated out. This equipment adequate measure complete video system, cannot measure individual operational amplifier required resolution. Video designers usually take worst-case approach when selecting video op-amp. Consider, example, design with five amplifiers between input output. desired overall system specification this board 0.05% 0.05° then assumes each five op-amps will contribute "+0.01%" "+0.01°" respectively. These errors will then phase" total "+0.05." There usually sign associated with rather, absolute value used. VM700 measures signed each step modulated staircase. overall however, displayed unsigned peak-to-peak magnitude. Modern video test equipment measure final cannot measure individual op-amps with required resolution. Imagine, well, design system with better than 0.05 numbers. were possible select op-amp with +0.01% another with -0.01%, would cancel zero. Some state-of-the-art video test equipment built this way. Another concern, especially with advanced video test equipment, squeezing most op-amp.
1994 National Semiconductor Corporation
Printed U.S.A.
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Refer Figure positive video waveform. video information voltages above (positive video); sync information below (negative sync). Video equipment must conform standard such this input output. What happens signal between these external ports only designer's business. choose invert video (negative video, positive sync) gamma correction with inverting summing circuit. Another compelling reason work with inverted video that particular amplifier have better both, with negative video. "polarity" change predictable between negative positive video. This feature (not bug) then exploited improve system specifications. easy, using current industry-standard test equipment, measure negative video Comlinear uses HP4195 network analyzer make measurements devices customer circuits. There many good reasons choose this particular machine. network analyzer gain phase measuring capability resolve significantly less than 0.01% 0.01° target specifications device under test, DUT. This particular analyzer built-in source which used independent sweep variable; single-frequency, gain phase measured displayed parametrically source voltage. This allows behavior amplifier over entire luminance range, both positive negative video, observed characterized detail. qualitative, well quantitative, measurement made. Trends amplifiers easily seen. This possible with which only delivers numbers. Since network analyzers measure gain trace mathematics HP4195 enable conversion percentages. Finally internal programming capability this analyzer yields self-contained measurement system, which does require external computer interface hardware/software still used other purposes. machine needs ancillary items make this specialized measurement. CLC400 with supplies boosts HP4195's sourcing ability. source limited drive current. measurement unity gain buffer requires twice input signal level, relative with gain two, order maintain correct output amplitudes. Operating CLC400 with supplies increases maximum output voltage device ±3.5V. lowpass filter, with CLC400, guarantees output impedance over frequency level from source into power combiner. power splitter sums swept source (luminance) oscillator (chrominance) passively, introduced into input test signal. test needed controlled impedance throughout
test system. Comlinear uses test environment. video designer would most likely have test disposal. this case resistor output CLC400 would power combiner filter would etc. DUT, system, would have series resistor between output 14dB attenuator. Thus total driven load proper environment. With test set, series resistor 14dB would used instead. Multiple video loads simulated adding additional resistors between output earth. test signal analyzer input capacitively coupled isolate from source oscillator from analyzer receiver. 14dB attenuator ensures that test signal measured sufficiently amplitude cause overload distortions analyzer front end. software straightforward. Previously, Application Note OA-08 [Comlinear 1993-1994 data book, pages 11-27], through calibration subtracted this from subsequent measurement. This proved unnecessary once lowpass filter, described above, added. change gain phase being measured here. absolute gain phase point sweep irrelevant. measurement made S21. straight amplitude-ratio measurement format required here, decibels. Averaging converting percent problematic. mathematics easy understand program non-dB measurements made. source mode, frequency that desired color subcarrier system requirements. amplitudes sources set. Once instrument been measurement begin. sweep triggered over source sweep range. necessary have large number measurement points over sweep. measure total twenty-one points clarity plotted data- negative video, positive video, reference black level. required resolution number measurements averaged. have found average sweeps good compromise between measurement time resolution most op-amps. Some, like CLC400 CLC410 with 0.01, require average measurements. There comes point where further averaging does enhance resolution. After data collected, they scaled displayed. gain data converted percentages. percent gain phase data scaled screen. trace mathematics built into programming language find maximum deviation, peak-topeak change gain phase, over luminance sweep. Minimum maximum found along trace, displayed does
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VM700. final numbers displayed addition sweep graph. vectorscope also gives peak-to-peak magnitude. This accordance with NTSC standards. There small problem with programming language this machine. Oscillator source levels must hard-coded cannot stored variables easy experimentation with different circuit gains attenuations. This isn't inconvenient, most op-amps will tested gain two. exact hardware chosen will have differing insertion losses which must taken into account. coupling necessitate changing oscillator level slightly between NTSC subcarrier frequencies. current practice, engineers 4.43MHz frequency, considered more demanding test condition. Also, piece equip- ment often sold countries with different broadcast standards. product meets more stringent specification, size fits all. Often current feedback amplifier with 200MHz bandwidth gain displays little difference between NTSC frequencies. practical upshot this that must determine appropriate oscillator amplitude sweep range each individual hardware assemblage empirically, with oscilloscope. NTSC levels found follows:
example, test setups results 0.95V maximum luminance level. When HP4195 source 0.95V, gain output 1.424V. oscillator, begin manually resetting source From front panel, adjust oscillator amplitude yield 286mVpp equivalent load, 572mVpp output. Note oscillator setting edit this into test program (see Figure Look Carefully Specified Datasheets
much easier make measurements than know exactly what measuring. J.W.N. Sullivan, physicist, 1928
Figure shows measurement results this test system. CLC406 shown. measured both positive negative sync (negative positive video, respectively). marked points show specified (peak-to-peak) that quoted Comlinear's data sheet. Notice that would better numbers just took delta between endpoints sweep range, essentially test only luminance levels. This neither what video standards delineate, what vectorscopes VM700 report. Another interpretation these standards implies value error. Neither Comlinear Tektronix this. VM700 display shows DP/DG largest peak-to-peak delta over luminance range. Simply giving endpoint delta between black level white level misleading. behave linearly with luminance. VM700 gives signed numbers each step modulated staircase, similar HP4195 program. Both pieces equipment used device characterization screening best system performance. final number worst-case, peak-to-peak magnitude measured over entire, continuous, luminance range.
oscillator frequency 3.58MHz. find necessary sweep range,
source oscillator amplitude first manually 1mV, zero allowed HP4195 software. 0.714V equivalent load, 1.424V output.
Adjust level from front panel yield When sweep range numbers have been
found, these then entered into program. programmed start- stop-sweep voltages which displayed HP4195 screen.
Figure Test
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Some manufacturers measure gain phase 0.714V then tell difference. With elan, they call this data sheet discussion. This would only make sense amplifier's errors were perfectly linear with respect luminance. Comlinear never measured such device from manufacturer. When represented graphically, Figure represented straight line, appear more like quadratic even cubic function.
trick which improve situation expense burning more circuit power) place "pulldown" resistor output. This increases collector current relieves burden feeble PNP. Notice effect this pulldown resistor Increasing output stage collector current flattens curves. possible reduce peak-to-peak force zero endpoints. Figure shows CLC430 driving video loads (total 75ohms). After adding pulldown resistor, went from 0.04%, 0.01%,
Figure Let's take closer look endpoint measurement. could change intercepts shape this "parabola," could come with just about number wished (see Figure could change bias current typical complementary emitter follower output stage (Figure this accomplished. Even complementary bipolar processes leave something desired matching. better than lateral PNP, still worse than NPN. Beta, thing, Early voltage, another, good NPN. limitation output stage. Many Comlinear amplifiers specify maximum output voltage current capabilities based PNP. Often device significantly better than this especially when output sourcing current from NPN. simply cannot sink equal magnitude current without compromising other device performance specifications. Some devices, like CLC406, specify output voltage ranges, e.g. +3.1, -2.7 (see Figure Figure Another test method used attempt measure individual op-amps employs vectorscope, spite limited resolution. assume that five amplifiers each have +0.2 could theoretically estimate accumulated error this chain. have already seen that amps selected cancel errors, this method highly suspect (see Figure Careful reading data sheets from other manufacturers will reveal anomalous devices' test conditions frequent dodge specify even 1000 load. Most video designers need know into load. frequently best gain two. "measured" gain three, another gain where DG/DP best. Always look amplitude test signal. rarely specified. proper luminance stated test conditions, chrominance will vice versa). should come surprise that amplifier will have better when delivering 100mV 3.58MHz into manufacturer's specifications appear meet required test conditions carrier frequency, luminance levels, load gain, there still question whether RMS, peak-to-peak endpoint delta error measured. This rarely, ever, stated. Finally, wish maximum guarantees National guarantees others offer only typical (see Figure
Figure
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References More Random Walks Through Science, Robert Weber, editor, Institute Physics, Bristol, England. Television Measurements, NTSC Systems, Margaret Craig. Video Technology Tutorial, 1992, Dennis Brunnenmeyer, Cedar Ridge Systems, Cedar Ridge. Figure Summary Please read data sheets carefully. "measured" expect. Consider measurement technique, test conditions guaranteed specifications. Does data sheet really specify what need know? Operational Amplifiers, Theory Practice, Roberge. Solutions with Speed 1993-1994, Comlinear Corporation. Raster Graphics Handbook, Second Edition, Conrac Division, Conrac Corporation.
Figure
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Measuring Improving Differential Gain Differential Phase Video Ar

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