PDN Probes
12
DC Blockers: When and Why to Use Them
A coaxial DC Block (or Blocker) is used when DC isolation is required to protect sensitive
test equipment or RF circuitry. Since the probe impedance of 50 ohms would load low
impedance sources, it is often necessary to block any DC current. Either the J2130A DC bias
Injector or the P2130A DC Blocker can be used for this purpose. The P2130A has male and
female SMA connectors that allow it to be easily connected to the 1-Port and 2-Port probes.
An SMA to SMA cable, or an SMA to BNC cable can then be used to connect the probe to
the instrument.
Figure 6: The P2130A DC blocker, available separately.
The P2130A DC Blocker lets through frequencies between 500 Hz and 8 GHz while
simultaneously providing low insertion loss and excellent VSWR. Its maximum differential
(input – output) voltage is 50V.
Blocks can still damage RF equipment if voltages are exceeded at the instrument. The
blocking cap is VERY large to get to low frequency (4uF) so it can transfer a lot of energy.
That is one reason for the attenuating probes rather than the blocks.
NISM Stability Measurement
(VRTS3 Board)
NISM is a powerful technique for measuring the stability of control loops. The NISM
software, embedded in commonly used VNAs, converts output impedance to phase margin.
This is extremely useful considering that many of today’s power ICs do not have their
control loops exposed, and in fact, in those cases, NISM is the only way to make an accurate
stability measurement.
In this case, the output impedance of a Voltage Reference is measured. The VNA, in this
