# Coupling and Termination Module Diagram A DC block connected between a source and a load is shown in Fig.1, where RT is the total resistance of the circuit, consisting mainly of the sum of the source and load resistance, and C is the coupling capacitor. The time constant τ of the circuit is RTC, and it has the unit of ms if RT is in Ohms and C is in µf, or ns if RT is in kΩ and C is in pf. In most practical cases, the stray capacitance across the load can be neglected. The case where RS=RL=50 Ω is of special interest, where RT is 100 Ω, and this value is used in Table I below.

When a voltage step with amplitude E is applied to the input at t0, the output immediately rises to E/2. At t0+,the output starts to decay towards zero with a time constant τ. After 4τ, the output will have discharged 98% of E/2 and is nearly at ground potential.

For coupling pulse signals, it is clear that one needs a capacitor large enough so that the output signal remains essentially rectangular in shape as the pulse duration increases. Table I lists the coupling capacitor values verses % pulse level tilt* for different values of pulse width. Instead of the exponential decay, a linear decay approximation of the output is used.

It should be noted that the value of RS is generally not 50 Ω when the drive circuit is an ECL device, because the output resistance of an emitter follower is typically 5 Ω. Therefore, the values shown in Table I need to be modified when AC coupling a signal from an ECL emitter follower. A simple and quick approximation is to either divide all the PW values by two or multiply the % tilt values by two.

C(µf) f3 dB τ=RTC PW (1% tilt) PW (2% tilt) PW (5% tilt) PW (10% tilt)
0.01 159 KHz 1 µs 10 ns 20 ns 50 ns 100 ns
0.1 15.9 KHz 10 µs 100 ns 200 ns 500 ns 1 µs
1.0 1.59 KHz 100 µs 1 µs  2 µs  5 µs  10 µs
10 159 Hz 1 µs  10 µs  20 µs  50 µs  100 µs
100 15.9 Hz 10 µs  100 µs  200 µs  500 µs  1 ms

Table I. Transmission of a rectangular pulse train through a high-pass filter with time constant t=RTC, f3 dB=1/2πRTC. RT=100 Ω.

* For a thorough treatment of this subject, please see Millman and Taub, Pulse, Digital, and Switching Waveforms.