1993_Motorola_Linear_Interface_ICs_Vol_2.pdf

In addition, there are many PC and Macintosh-based CAD programs available today. In general, these
programs work well for digital and low frequency analog circuits, but are of very limited value in RF
applications. SPICE models are not currently available for the communications circuits. Several circuits do
show S-Parameter data or admittance plane information which may be used to optimize input or output
matching for gain or noise. The most useful method of utilizing the applications circuits at different
frequencies is simple linear scaling of the tuning and reactive elements. This method is generally
applicable over a 2:1 frequency range lower than the documented application.
Many communications applications include some digital signaling, data conversion, or microcontroller
interface. The RF designer must take great caution to avoid interference with the low level analog circuits
in these mixed-mode systems. The receivers are particularly susceptible to noise as they respond to
signals of only a few microvolts. Make sure the clock frequency is not a sub-multiple of the receiver input
or IF frequencies. Keep the DC supply lines for the digital and analog circuitry separate. Avoid ground
paths carrying common digital and analog currents. As much common sense as analytical skill goes
into a successful RF design. A good consultant may well save many times their fee in material, lost time,
and rework.
TEST EQUIPMENT
Establishing a new RF communications lab can be a very costly investment. The normal DVMs and
regulated power supplies are generally acceptable if they do not generate spurious RF, and are not
sensitive to RF voltages. You should pick an oscilloscope with a frequency response three or more times
higher than your operating frequency. In addition, a low capacitance probe - FET probe - would be
useful. Remember, while conventional probes have very high input resistance, their capacitive reactance
decreases with frequency and becomes a limiting factor above 30 MHz. For most transmitter work, a basic
spectrum analyzer is a must. It will help confirm power output, spurious output levels, stability, and
modulation characteristics.
Rental and used equipment are often a good source of test equipment. Recently, Communications System
Analyzers have become available at very moderate prices. The Motorola R2600, for example, combines 16
different instruments into one portable package. The signal generator, receiver, counter, oscilloscope and a
"best-in-class" modulation meter make this package a very attractive design and production test tool.
Further information, including a demonstration, are available from your local Motorola Communications
and Electronics sales office.
MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA
8-183