Transmitting video over CAT5 cable

With the world dominance of personal computer systems, demand for long distance component video transmission is increasing at an unprecedented pace. The applications areas of greatest demand are KVM systems, server farms, information message boards and educational classrooms. This application note presents the most current design methods for transmitting high bandwidth SXGA video signal over long distances of Cat 5 cable (300m or more). The enormous cost benefits of Cat 5 cable will also be discussed; for instance, the average cost of a 100m of Cat 5 cable is $20 while the average cost of a 100m of Coax Cable could easily exceed $240. Furthermore, wiring is reduced from a bulky hard to manage bundle of 3 cables to 1 easily pulled cable. Additionally, Cat 5 cable has a 4th twisted pair available, which can be used for KVM signal, audio, timing or control signal transmission. This applications note provides in-depth information on some of the most important physical support technologies and constraints; CAT 5 cable characteristics, SXGA video standards and video amplifier/line drivers and receiver bandwidth and slew rate requirements. The trade-offs of differential line driver and receiver topologies are discussed in detail. We also presents termination techniques and video equalization strategies. Finally, the Hermes video demo system is described in detail.

SXGA video standard

Table 1 presents key parameters of 76Hz SXGA video signal.

The signal bandwidth comes from the following equation,

BWS = 1/2 [(K*AR*(VLT)²*FR)*(KH / KV)] = 51.9MHz

Where BWS = Signal bandwidth K = Kell factor, Visual information is lost due to the probability that some of the video information will be displayed during the retrace rather than the active portion of the scan line. Assuming 30 percent of the visual information is loss, we have K = 0.7.

AR = Aspect ratio (the display width divided by display height) =1.33 VLT = Total number of vertical pixels = 1067 FR = Frame rate or refresh rate = 76 KH = Ratio of total horizontal pixels to active pixels = 1720/1280=1.34 KV = Ratio of total vertical lines to active lines = 1.04

Amplifier bandwidth and slew rate requirements

To maintain video signal integrity, we need to maintain 0.1dB bandwidth to the signal bandwidth (BWS). When selecting amplifiers, special attention should be given to its frequency response characteristics; for instance, for a signal pole amplifier the 3dB bandwidth required to handle 51.9MHz is 6.5*51.9MHz = 337MHz. For multiple pole amplifiers (most modern high speed amplifiers are multiple pole amplifiers), the 3dB bandwidth should be set to 3 times the signal bandwidth, which for the previous example would be 155.7 MHz. The slew-rate can be calculated from the signal amplitude and pixel rate. So to maintain video signal integrity with a pixel rate of 139.5MHz while allowing the signal to complete its transition during ¼ of a clock period use the following equation.

Slew Rate = 1/(¼*Pixel Time) = 1/( ¼*(1/139.5MHz)) = 558V/μS

VESA DMT standards also define 60 Hz refresh rates and 80 Hz refresh rates but the most common usage is the 76 Hz refresh rate.

CAT 5 cable characteristics

Figure 1 shows the cross section of Standard Cat 5 cable consists of 4 twisted pairs of AWG 24 cable, which has a characteristic impedance of 100 ohm. The DC resistance is 10 ohm/100m with a capacitance 4.6nF/100m. One important characteristic of SXGA video transmission is high frequency cable attenuation, which increases exponentially over frequency and distance. Figure 2 shows the effects of signal frequency and cable length on the signal attenuation. The relationship between cable attenuation, signal frequency and cable length is

L is cable distance in 100s of meters and F is the signal frequency.

Differential line driver topologies

Figure 3 illustrates a standard differential input and output line driver system built with discrete operational amplifiers. The differential output driver doubles the output voltage swing " while the resistors R_F and R_G determine the circuit voltage gain with the following equation:

V_out/V_in = 1+2*R_F/R_G.

High noise rejection such as 60Hz power line interference is accomplished by amplifying only the differential input voltage signals and not amplifying the common mode input voltage.

The only real disadvantage of this circuit is the required differential input signal sources.

Typically, signals originate in single ended rather than differential form. Converting a single ended signal to differential mode prior to line transmission reaps the benefit of high common mode noise reduction. The circuit in figure 4 provides a very simple way to generate a differential output signal from a single ended input signal using two operational amplifiers; the upper amplifier is non-inverting while the bottom is inverting. Note the amplifiers have different feedback ratios (close loop gain) which results in different bandwidths for voltage feedback amplifiers. The difference in bandwidth causes higher frequency signal mismatch and can lead to higher distortion. For current feedback amplifiers, the bandwidth stays relatively constant at different gain settings. Since the bandwidth is primarily a function of the value of the feedback resistors, one should keep the feedback resistor the same for current feedback amplifiers.

Figure 5 illustrates the complete block diagram of the EL5177, a 550MHz single/differential input to differential output amplifier which can be used as singled ended to differential converter. This device is internally compensated for a closed loop gain of +1 stable, the gain is set by R_f and R_g. V_ODM is the output in differential mode and V_OCM is the common mode output voltage.

V_ODM = (V_in+ - _Vin-) * (1 + (2R_f/R_g))

V_OCM = Vref.

The voltage applied at REF pin sets the output common mode voltage.

Differential line receiver topologies

Figure 6 shows a differential to single ended converter implemented with high speed amplifiers. This circuit receives a differential voltage, reduces the common mode input gain to zero and terminates in a single ended output. The advantage is both a very high input impedance and very high common mode rejection achieved with simplicity. Bandwidth mismatch of the two amplifiers introduces the possibility of high frequency distortion. Further more, high output swing is required to achieve good common mode rejection. The differential gain is determined by R1 and R2 resistors with the relationship

Gain = 1+R1/R2

Figure 7 shows the complete block diagram of the EL5175, a 550MHz differential input to single ended output amplifier which can be used as a differential to single ended converter. This device is internally compensated for closed loop gain of +1 stable and the gain is set by R_f and R_g. The output voltage is equal to the difference of the inputs plus V_ref and then multiplied by the gain.

V_ODM = (V_in+ - V_in- + V_ref) * (1 + (R_f/R_g))

Author biography:

Mike Wong is the director of application engineering for Intersil's Elantec Product group where he has worked for over 10 years. He specializes in high performance analog circuit and power management applications. He has previously worked at ASTEC. He has a BSEE from University of California at Davis.

Part 2: Termination and Equalization Techniques Part 3: Details on the Hermes Demonstration Board