Vertical Amplifier Filter Responses
The filters needed for ao urate VITS distortion measurements are built into [lit vertical amplifier of the new hp-TV waveform osi illosi ope.
The i"i at response is used for general observation of the waveforms, foi che< ting the frequent-V response of the channel In means ol the muitiburst signal, and for checking phase and transient responses by means of the sine-squared pulse and bar.
With tlie responsk control in the
* See P. 7 herein far a mora complete discussion of VITS.
flatposition, the vertical amplifier lias a frequency response which is flat within it small frai tion of one dB (see spei i lu a lions) up to a frequency ol 1.5 Mil/, The response then decreases mouoton-icallv. and is down 21) dB at 20 Mil/ (Fig. 3). The function of this last roll-oil, whit h is f:ister titan a Gaussian loll off, is to eliminate high-frequency noise, which can l>c troublesome in TV waveform observations.
The phase characteristic ol ilie fiat response is linear, enabling (lie oscilloscope to produce extremely symmetrical displays of the three sine-squared pulses (the T/2 pulse contains significant frequency components up to 16 MHz). The symmetry of the sine-squared pulse is a measure of the freedom of .i channel from phase distortion. so it is important that no assym-meti y is introduced by the osi i!Ios< ope.
In achieving its excellent amplitude and phase characteristics, the FLAT filler response causes small preshoots and overshoots. However, these are significant only on the T/2 pulse, where they are typically 3%* and equal to each other within )%■*
The low pass response is used for filtering out the multihursi frequencies in order to < he< k the baseline of the muitiburst waveform for dc components which indicate amplitude non-linearities. It is also used for eliminating the 3.58 MH/ color subcarrier bursts from the modulated stairstep signal in order to i heck for low-frequency differential gain.
The low pass position of the respond control giv's the vertii .d amplifier a frequency response which is down more than 30 dB near 0.5 Ml!/ and more than 10 dB at all frequencies
* Similar tolerances apply to the step response. Rise time it less than 50 ns.
greater than 1.5 Will, so that the displays of the muliibursL baseline and unmodulated stairsteps are free of residual sine waves. The response is flat within 2 dB to 150 kHz so that the muitiburst baseline, the unmodulated stairsteps and the synt pulses are dis played without appreciable rounding.
A typical muitiburst waveform consists of a white flag followed by about 6.4 /is of 0.5 MHi sine wave and about 4.7 Fs of 1.5. 2.0, 3.0, 3.6, and 4.2 MH/ sine waves. The waveform is generated simply by gating the outputs of six oscillators in sequence, without regard to relative phase. Consequently, when the muitiburst generator switches from one burst frequency to the next, an abrupt change of voltage may occur in the output waveform. This makes the step response of the oscilloscope's low pass filter critical, for if ihe transients induced in ihe osiilloscope do not die out quickly, ihe baseline of the multi-burst waveform will be obscured, hi the new -hp- instrument, the 0to-100% rise time of the low pass response is about 2 jus, and overshoot is less than 2%. This means that, if an abrupt voltage change does occur in the muitiburst waveform when the low pass filler is being used, all transients are reduced to less than 2% of their initial amplitudes within 2 ^s, leaving 1.1 or 2.7 jis of undisturbed baseline for measurement purjioses.
The IRK response of the vertical amplifier is used for attenuating the color signal and an\ noise that may be present in order to make amplitude mcas-
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Fiy. 3. Basic FLAT frequency response of oscilloscope is down less than 0.1 dB helou 1.5 Ml!;, less than 1 dB at S MH;. 3 dB at 10.5 Mil:, then has eery fast roltoff and is down 20 dB nl 20 MHi. This eliminates high-frequency noise if present on incoming line. Lower diagram shun s ±0.05 dB flatness below 1 MHz.
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Fig, 4. Bloel; Diagram of Vertical Amplifier in -hp- Model 191A Television Waveform Oscilloscope. Passive filters of special design give accurately-shaped responses with high stability. Differential-to-single-ended input amplifier permits use ol more a!able single-ended fillers, has high common mode rejection over wide frequency range.
Fig, 4. Bloel; Diagram of Vertical Amplifier in -hp- Model 191A Television Waveform Oscilloscope. Passive filters of special design give accurately-shaped responses with high stability. Differential-to-single-ended input amplifier permits use ol more a!able single-ended fillers, has high common mode rejection over wide frequency range.
urements on ihe monochrome signal. Ihf IRK response gives the verticil 1 amplifier a standard roUolI .is dest ribed in IEEE Standards,1 It is down 20dBai the color subcarriet frequency ol 3.5S MHz.
l ite CMRoMiN.vNCt response is u\e<l to remove the stairsteps from the modulated stairstep signal in order to chec k a channel for high-frequency differential gain. The filter puts the iii bursts of 3.58 MHz on I lie same line to permit easy comparison of their amplitudes. With the rksponsi control in the chrominance position, the verti< a! amplifier has a bandpass response with t enter frequency of ?>.r>R MM/, the < olor subcarrier frequency.
The differential gain response is the same as theghrominanc i response, except that an additional voltage gain of 5 (14 dB) is provided for making differential gain measurements with respect to 100%.
VERTICAL AMPLIFIER Fig. I is a block diagram of the vertical amplifier. To permit 1% distortion measurements, the four frequency responses of this amplifier had to be carefully shaped and held within tight tolerances. This kind of requirement is unusual in ost illoscope design; usually only the 3 dB bandwidth is specified, and the shape of the rolloff is not. In order to achieve the four ire-
I-e fRt fiHi' meets tfie requirements of the IEEE Stand■ on Teleins.on Measurement of Luminance Srgrial Levels (ASA C16.3! - !95Sj ficejt tint tire toltisricei 3'e tifhtir than tnose these ttandardi quency response t harat (eristics with accuracy and stability the vertical amplifier is designed to have :i wide hand width, and passive filters are used to shape the response. This eliminates any possibility that changes iti transistor characteristics with temperature or age. or differences between it.uisis-tors, will change the responses. Each amplifier block has adequate feedbai k, even at the high frequencies, so that expected component and tenqjerature variations will not alien eithei the basic response or the overall response.
The input amplifier is a differential to-single-ended converter, wlu<h allows the oscilloscope to accept the- normal two-wire TV cables, but permits the use of single-ended filters, whirh are mm h less sensitive to component variations and stray capacitance. ! lie interstage amplifiei is alsosini]ilified by this arrangement.
Common mode rejection of a differential amplifier is the amount by which signals common to both inputs are reduced in the output. In a TV os-< illoscope, high common mode ieje(-tion results in a display which is free of any noise and interference common to both wires of i wo-wire TV cables. In the new -hp- oscilloscope, the t onimort mode rejection < i| the iuptiI amp!iiiel is greater than Iti dB up to 2 MH/, with a d ilB-|>er-o( tave rolloff up to 20 Mil/. In actual produt tion instruments the amplifiei-filter < ombi nations haye overall common mode rejections
- Fiji. 5, HO !-!'■>! A version oj new TV Waveform Oscilloscope hnx front and rear tclephoiit -style input jacks, no WRUti su eep mode.
heLtet than 30 dB up to 20 Mil/.
lii jiermit video signals to he observed without disturbing the waveforms, the input connet lions to the vertical amplifier ate made by means of 75 ohm and 12-1 ohm 'loop-through' systems. Video signals in ojieratitig centers and control rooms are tarried either by a i oaxial < able with a < harac teristic im pedancc of 75 ohms oi by a balanced two-wire cable with a cliarat teristic impedance of 12-1 ohms. The input circuits of the "s< illosiope are designed to look like short sections of 75 ohm and 124 ohm lines, so dial the instrument t an be inserted into cither t\ pe of t able without mismatch.
When distortion measurements auto he made, the display of ilie video signal is adjusted for a standard maximum amplitude of I lo IRE units. !n the polarity most often used, the syut pulses have a magnitude of — 10 IRE, the blanking level is 0 IRE, black is + 7.5 IRE. and white is -f 100 IRE, In standard TV practice, 110 IRE units is equal to ] volt. The new oscilloscope has a swilch-selet ted gain setting which will display 101) IRE units for an input signal amplitude of 0,71 I volt peak-to-peak. V variable gain control permits proper adjustment of the display foi other input signal amplitudes. The gain tan be switched to the fixed gain posit ion without (lis i uibiiig the setting of the variable gain control, so that the display < an be > a libra ted quickly without readjustment. Also provided is a calibrator which supplies to the vertical amplifier a 30 kHz signal with a peak-to-peak amplitude of 0.714 volt.
Because of the presence of pii i ure information in the composite video signal. the average value ol the signal i hanges continually. To make the display stable, the oscilloscope lias a dc restorer which, when the front-panel control is in the ON position, references ilie display to the blanking level instead ol to the average value of the signal. lb deteimine the blanking voltage, the restorer circuit samples the 'bat k porch; which is about 5 microseconds ol blanking level following each horizontal synt pulse. The presence or absence of the color synchronizing burst on the back porch does not affect the restorer.
I he dc restorer is a soft < lamp; that is, it has long sampling time constant, so that it takes many samples to establish the final reference voltage. This means that if fiO H/ lium is present on the video signal it will appear on the display .mi! he detected. \ hard clamp would remove the i 1i hum.
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