HMC349AMS8G Data Sheet
Rev. C | Page 8 of 10
THEORY OF OPERATION
The HMC349AMS8G requires a positive supply voltage applied
to the VDD pin. Bypassing capacitors are recommended on the
supply lines to minimize RF coupling.
The HMC349AMS8G is internally matched to 50 Ω at the RF
common port (RFC) and the RF throw ports (RF1 and RF2);
therefore, no external matching components are required. All
of the RF ports are dc-coupled and dc blocking capacitors are
required at the RF ports. The design is bidirectional; the RF
input signal can be applied to the RFC port while the RF throw
port (RF1 or RF2) is output, or vice versa.
The HMC349AMS8G incorporates a driver to perform logic
functions internally and to provide the user with the advantage
of a simplified control interface. The driver features two digital
control input pins, CTRL and EN.
When the EN pin is logic low, the RF1 to RFC path is in an
insertion loss state, and the RF2 to RFC path is in an isolation
state, or vice versa, depending on the logic level applied to the
CTRL pin. The insertion loss path (for example, RF1 to RFC)
conducts the RF signal equally well in both directions between
the throw port (for example, RF1) and the common port (RFC).
The isolation path (for example, RF2 to RFC) provides high loss
between the insertion loss path and the throw port (for example,
RF2) terminated to an internal 50 Ω resistor.
When the EN pin is logic high, both the RF1 to RFC path and
the RF2 to RFC path are in an isolation state, regardless of the
logic state of CTRL. The RF1 and RF2 ports are terminated to
internal 50 Ω resistors, and RFC becomes open reflective.
The ideal power-up sequence is as follows:
1. Connect GND.
2. Power up VDD.
3. Power up the digital control inputs. The relative order of
the logic control inputs is not important. However, powering
the digital control inputs before the VDD supply can
inadvertently forward bias and damage the internal ESD
protection structures.
4. Apply an RF input signal. The design is bidirectional; the
RF input signal can be applied to the RFC port while the
RF throw ports are outputs, or vice versa. All of the RF
ports are dc-coupled to VDD through internal resistors; there-
fore, dc blocking capacitors are required at the RF ports.
Table 4. Control Voltage Truth Table
Digital Control Input RF Paths
EN CTRL RF1 to RFC RF2 to RFC
Low Low Isolation (off ) Insertion loss (on)
Low High Insertion loss (on) Isolation (off )
High Low Isolation (off ) Isolation (off)
High High Isolation (off ) Isolation (off)
