Septmber
2005, Issue 182
Signal
Generation Solution
Build
an Inexpensive RF Signal Generator
ARCHITECTURE
My
goal for this project was to design a signal generator
that produces sine waves from 10 to 600 MHz at a constant
output power level of 5 dBm. Let’s take a look at how
I did it.
The
assumed load is 50 W, which
is typical for RF systems. Talking about signal levels
in terms of decibels relative to 1 mW (dBm) is common
when you’re dealing with RF systems. Equation 3 is for
converting from power in watts to power in dBm.
[3]
A
table showing the relationship between power in dBm,
RMS voltage, and power in milliwatts is posted on the
FTP site. The table assumes a 50-W
system.
The
5 dBm design specification for this generator is equivalent
to a 0.4-V RMS sine wave. This will produce 3.2 mW when
driving a 50-W load.
Figure
3 shows the overall architecture for the signal generator.
It consists of two main modules. The RF module produces
the 10- to 600-MHz 5-dBm signal. The controller module
directs the RF module’s actions.
|
(Click
here to enlarge)
|
Figure
3—A voltage from the microprocessor controls the
RF signal frequency. The actual RF and LO frequencies
are measured by the microprocessor. The microprocessor
calculates the actual IF frequency and displays
it. A voltage from the microprocessor controls the
amplifier gain to maintain the desired output level. |
The
signal flow in the RF module is a straightforward mixing
process. The local oscillator is fixed at approximately
1.5 GHz. The RF oscillator varies from 1.5 to 2.1 GHz
depending on the output frequency you want from the
generator. A voltage from the microprocessor sent via
a D/A converter controls the RF oscillator’s frequency.
I
chose the high operating frequencies for the LO and
RF oscillators in order to keep unwanted spurs from
appearing in the desired IF output range of 10 to 600
MHz. Next, the outputs of the RF and LO oscillators
were frequency divided down so the microprocessor could
measure them. These divided-down signals were 23- to
32-MHz digital signals; they were routed to the microprocessor
through time windowing control logic.
Before
the RF and LO oscillator outputs are mixed, the signals
are low-pass filtered to attenuate any harmonics present
at the oscillator outputs. The RF and LO signals are
then mixed by the mixer and an assortment of signals
appear at the IF output. This IF output is then low-pass
filtered to eliminate most of the unwanted spurs. Following
this, a variable-gain amplifier whose gain is controlled
by the microprocessor amplifies the RF signal. This
analog gain control signal is supplied via a D/A converter
driven by the microprocessor. An LCD shows the generator’s
output frequency. The up and down push buttons enable
you to select the desired generator frequency.
