Evaluating the ORCA ORSO42G5 with the
Lattice Semiconductor
High-Speed SERDES Board
11
Q: What is inside a bias tee module and how does it work?
The bias tee module is a passive device, consisting of a capacitor and inductor, as shown below.
The capacitor blocks DC between the right and left signal ports. The inductor provides DC coupling/AC isolation
between the upper port and the left port. The inductor is the most critical element. In practice this element consists
of several inductor and resistor components very carefully assembled to provide high impedance over multiple
decades of frequency. This is the key to providing low signal line reflection and low signal distortion.
Not all bias tees are the same. There are a variety of bias tees designed for specific frequency ranges and DC cur-
rent levels, from several different vendors. Lattice uses a Picosecond Pulse Lab bias tee. More detailed character-
ization and application documents are available from this vendor. See the References section at the end of this
document.
Q: Can the SERDES output be observed without a bias tee module?
Yes, in two different ways as shown below.
1. For AC-coupled SERDES interface applications, external shunt resistors, tied to V
DDOB,
are suggested at
the Tx output. In this case the outputs may be connected to a high speed oscilloscope without using bias
tee elements. Simple coupling capacitors can be used as shown below.
In this interconnection the 120
Ω
external resistors provide the necessary DC bias to the Tx CML buffer outputs.
AC-Biased
Port
DC-Biased
Port
AC-Unbiased
Port
Bias Tee
V
DDOB
120
120
50
High Speed
Oscilloscope
with 50
Ω
Inputs
Evaluation Board
SERDES
TX Output
50
Note: The symbol above indicates a region where shielded (SMA) cable should be used.
