Intermodulation Distortion in Programmable Attenuators

In recent years, however, wireless communication systems
employing complex digital modulation schemes, increased channel
capacity, high transmit power and extremely low receiver
sensitivity have put into question the linearity of passive components.
Even very low level multi-tone intermodulation products
generated by attenuators can seriously degrade the efficiency of
a system/ instrument if these products fall within the user passband.
For two closely spaced tones at frequencies f1 and f2, the
third order IM products at 2f1 - f2 and 2f2 - f1 , are the most
harmful distortion products. They are harmful because they are
located close to f1 and f2 and virtually impossible to filter out. In
today's base stations the multicarrier power amplifier (MCPA) is
replacing banks of single-channel amplifiers and their corresponding
power combining network. MCPAs have the capability
of carrying a number of modulation schemes simultaneously and
can also employ schemes such as dynamic-channel-allocation
(DCA) to use the allocated frequency spectrum more efficiently.
The in-band intermodulation distortion (IMD) performance of
these amplifiers is extremely critical and needs to be measured
using low distortion programmable multi-tone generators whose
IMD performance must be quite superior. This is discussed in the
two case studies cited here.