The PIN Modulator is a high-speed, current-controlled
absorption type attenuator. A simplified illustration
of the hp Model 8730A''B series PIN Modulators is
shown in Figure 1. Each PIN unit includes a low-pass
filter, two high-pass filters, a number of PIN diodes,
and a 50-ohm strip transmission line (ridged waveguide
in the higher frequency units).
The PIN diode is a silicon junction diode whose Pand
N trace regions are separated by a layer of intrinsic
(I) semiconductor (silicon). Thus the name PIN diode.
6730A-A-3
g Figure 1. Simplified Block Diagram of PIN Modulator
At frequencies below about 100 Me the PIN diode rectifies
as a junction diode. However, at frequencies
above 100 Me, rectification ceases due to stored charge
in the intrinsic (I) layer and the diode acts like a
resistance by conducting current in both directions.
This equivalent resistance is inversely proportional to
the amount of charge in the I layer. An increase in
forward bias current (current at a negative voltage) increases
the stored charge and decreases the equivalent
resistance of the PIN diodes. When reverse bias is
applied, reverse current flows until stored charge is
depleted at which time equivalent resistance becomes
a maximum, in the order of thousands of ohms.
To understand how a PIN Modulator works, consider
the following: the PIN diodes are mounted as shunt
elements between the RF transmission path and ground.
The transmission path has a characteristic impedance
of 50 ohms. When the PIN diodes are forward-biased
the equivalent diode resistance is about 30 ohms and
most of the RF energy is absorbed by the diodes instead
of propagating down the 50-ohm transmission path.
However, when the diodes are reverse- or back-biased
the equivalent diode resistance is in the order of thousands
of ohms and the microwave currents will flow
down the transmission path because diode resistance
compared to the 50-ohm path impedance is negligible.