Why measure it if you can’t see it?

　　Most people believe that the differential gain (DG) and differential phase (DP) parameters in video cannot be observed by the naked eye. This is because the amplitude is too small and the screen brightness changes often mask the effects of errors, so DG and DP are generally not observable. In this case, why do we need to measure these invisible parameters?

　　DG and DP testing is used to detect very small errors before they affect the visual effect. This ensures that the video signal maintains high image quality after passing through hundreds of amplifiers from the signal source to the final destination. For amplifiers, analog/digital converters (ADCs), and digital/analog converters (DACs), their DG and DP performance can be evaluated using some simple methods and the performance can be evaluated at a few test points or close to the power rails. Once again, very small errors are detected here to ensure signal integrity at each level. In order to better understand the impact of DG and DP errors, we first examine the impact of DG and DP errors on amplifiers, ADCs, and DACs.

　　In simple terms, we can think of the effects of DG or DP video errors as changes in skin color when moving from a brighter environment to a darker environment. For TV systems that use subcarriers, such as NTSC (North America and Japan), changes in DG directly affect saturation or the brightness of the color, just like the chroma control of a TV. DP errors change the hue (making the image more green or purple), just like the color control of a TV.

　　Furthermore, in subcarrier TV systems like PAL (Europe and China), DG will directly affect the DP result in second-order saturation. Finally, for high-definition (HD) and component systems, differences in DG and channel gain will result in colorimetric changes. Although the US NTSC system is not broadcast over the air, industrial and security video surveillance systems are still dominated by traditional technology.

　　Why are DG and DP specifications needed?

　　We can look at the production process of TV programs. After switching, multiplexed camera signals are transmitted, recorded, played and edited through special equipment to finally produce programs. Channel programs may be transmitted over long distances through microwaves, optical fibers or satellite systems, and finally converted into over-the-air broadcast TV signals. Cable systems, DVDs or satellite systems can deliver TV programs to every home so that we can watch them at home. During the entire processing process, the video signal may pass through hundreds of amplifiers, each of which will produce a small amount of DG and DP in the video signal. In order to maintain the integrity of the video signal, engineers must design highly sensitive detection signals.

　　All amplifiers have some nonlinear amplitude response, and using negative feedback can help reduce nonlinearity. DG and DP are measurements that really emphasize linearity and take frequency response into account. NTSC and PAL television systems carry color information in a subcarrier (3.58MHz and 4.43MHz, respectively). Differential gain is defined as the change in amplitude of the high-frequency subcarrier for a change in the low-frequency video level or brightness. In the NTSC video waveform (shown in Figure 1), the 3.58MHz subcarrier is superimposed on the lower frequency brightness signal and has five brightness levels. For clarity, the subcarrier is represented by a large amplitude sine wave. In fact, there are more than two hundred subcarrier cycles in one line.

　　

　　Figure 1: DG and DP of video. (white, black, subcarrier for each step, reference color pulse, parallel sync, one parallel line)

　　Differential phase is defined as the phase change of the high-frequency subcarrier caused by changes in the level or brightness of the low-frequency video signal in NTSC and PAL signals. The hue or color display is governed by the phase relationship between the video signal sync pulse and the subcarrier that appears in the active video image segment. In order to correctly display color information, accurate control of the phase is necessary.

　　Amplifiers, ADCs, and DACs have an optimal operating point where the amplifier has the best linearity and meets the highest specification or standard. The optimal operating point is usually located in the center of the power rails (Figure 2), but of course, the IC designer can place it elsewhere as needed. The amplifier has the best feedback control and the best linearity at the optimal operating point. This means that as the signal deviates toward the power rails, the linearity deteriorates.

　　

　　Figure 2: DG and DP of an amplifier close to the power rails. (Positive rail, negative rail, undistorted sine wave, sweet spot, in the middle of the rails, peak signal compression near the rails, hard clipping at the rails)

　　By superimposing a high-frequency sine wave on a low-frequency signal, the entire operating range of the amplifier can be tested. For example, the MAX4389 amplifier can provide DG as low as 0.015% (typical) and DP as low as 0.015° (typical) in a subcarrier TV system. However, this DG indicator is also applicable to wider-band TV signals and non-video applications. If we need a 10MHz signal, we can apply a 7MHz sine wave to the MAX4389 and change the DC bias to test and evaluate the amplifier. If you need to operate at a bandwidth of 30MHz, you need to apply a 22MHz sine wave.

　　Typically, the best response is obtained by selecting a high-frequency sinusoidal signal with a frequency between 2/3 and 3/4 of the system bandwidth, and biasing the sinusoidal signal at the midpoint of the DC supply voltage (Figure 2). When the DC voltage changes, the sinusoidal signal will shift to one side of the power supply, and the sine wave amplitude will change. Usually, when approaching the power supply rail, the high-frequency response will be reduced and the operating current of the transistor will be reduced. Under extreme conditions, the amplifier will exceed the current range, stop working, or clamp. ADCs and DACs will also encounter similar problems.

　　As chips become more complex, they are no longer simple amplifiers, and larger DG and DP errors may occur in the circuit. Such highly complex circuits may include multiplexers, six-pole Sallen-Key filters (with three or more amplifiers), video buffer amplifiers, etc. The MAX7428 is such a circuit, with a typical DG error of 0.2% and a DP error of only 0.2°.

　　Amplifier testing starts with DG testing, while video signals must add DP to increase test sensitivity and maintain signal integrity in the case of multiple stages in series. DG testing is also used in other products such as ADC and DAC. For applications that require a wider bandwidth, the bandwidth requirement can be met by changing the DG test signal. DG measurement is a common test method used to evaluate the linearity of different components over the entire power supply voltage range. Measuring the invisible DG is very useful. It acts like a microscope to more closely examine the signal integrity to ensure that good signal quality is maintained after passing through a long series of analog circuits.