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.