Coupling and Termination Module Diagrams

Mini Modular Instruments > Coupling and Termination > Diagram

A DC block connected between a source and a load is shown in Fig.1, where R_T 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 R_TC, and it has the unit of ms if R_T is in Ohms and C is in µf, or ns if R_T is in kΩ and C is in pf. In most practical cases, the stray capacitance across the load can be neglected. The case where R_S=R_L=50 Ω is of special interest, where R_T 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 R_S 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.

Table I. Transmission of a rectangular pulse train through a high-pass filter with time constant t=R_TC, f_3 dB=1/2πR_TC. R_T=100 Ω.

* For a thorough treatment of this subject, please see Millman and Taub, Pulse, Digital, and Switching Waveforms. Larger coupling capacitor values are available, up to 15 µf, in the 95000-series modules.

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