Radio Frequency Integrated Circuit Design - Webs

Introduction to Communications Circuits
can drive a measurement device efficiently. The freedom to choose arbitrary
impedance levels provides advantages in that circuits can drive or be driven by
an impedance that best suits them. On the other hand, if circuits are connected
using transmission lines, then these circuits are usually designed to have an
input and output impedance that match the characteristic impedance of the
transmission line.
1.2.2 Units for Microwave and Low-Frequency Analog Design
Signal, noise, and distortion levels are also described differently in low frequency
analog versus microwave design. In microwave circuits, power is usually used
to describe signals, noise, or distortion with the typical unit of measure being
decibels above 1 milliwatt (dBm). However, in analog circuits, since infinite or
zero impedance is allowed, power levels are meaningless, so voltages and current
are usually chosen to describe the signal levels. Voltage and current are expressed
as peak, peak-to-peak, or root-mean-square (rms). Power in dBm, PdBm, can be
related to the power in watts, Pwatt, as shown in (1.1) and Table 1.1, where
voltages are assumed to be across 50�.
PdBm = 10 log10� Pwatt
(1.1)
1mW�
Assuming a sinusoidal voltage waveform, Pwatt is given by
Pwatt = v 2 rms
R
3
(1.2)
where R is the resistance the voltage is developed across. Note also that vrms
can be related to the peak voltage v pp by
Table 1.1
Power Relationships
v pp v rms P watt (50�) P dBm (50�)
1 nV 0.3536 nV 2.5 × 10 −21
1 � V 0.3536 � V 2.5 × 10
−176
−15
−116
1 mV 353.6 � V 2.5 nW −56
10 mV 3.536 mV 250 nW −36
100 mV 35.36 mV 25 � W −16
632.4 mV 223.6 mV 1 mW 0
1V 353.6 mV 2.5 mW +4
10V 3.536V 250 mW +24