DC Block, SMA M/F

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.

(0° C ≤ TA ≤ 35° C)*

DC Blocks, Series-Connected Capacitor:

RF Chokes, Series-Connected Inductor:

Series-Connected Schottky or PN Junction Diode:

Series-Connected Resistors:

Shunt-Connected Capacitors:

Shunt-Connected Resistive Terminations:

Shunt-Connected Diodes:

Attenuators:

As of February 2011, PRL has discontinued the ready-to-use attenuator series, due to low demand. We will honor and support all outstanding quotations and orders, but we will not be offering new items for sale in quantities of less than 100. For volume orders, please contact sales@pulseresearchlab.com . To build your own attenuator, please see our Signal Conditioning Kits and our Custom Attenuators application note.