Specifications

5-18
5.4 Diode measurement
The junction capacitance of a switching diode determines its switching speed and is dependent on
the reverse DC voltage applied to it. An internal bias source of the measurement instrument is used
to reverse-bias the diode. The junction capacitance is measured at the same time. Figure 5-26 shows
the measurement setup.
For variable capacitance diodes (varactor diode) that use capacitance-bias characteristics, it
is important to measure capacitance accurately while applying an accurate DC bias voltage.
Figure 5-27 shows an example of measuring the C-V characteristics of a varactor diode. Use a low
test signal level (typically 20 mV rms) to precisely trace the relationship of the capacitance to the DC
bias voltage.
The varactors for high frequency applications require Q factor or ESR measurement along with
capacitance at a frequency above 100 MHz. The RF I-V measurement instrument is adequate for this
measurement. It is possible to measure Q or ESR with the same setup as for the C-V measurement
by merely selecting the desired parameter.
Figure 5-26. Reverse biased diode measurement setup
Figure 5-27. Varactor C-V characteristics