Septmber
2005, Issue 182
Signal
Generation Solution
Build
an Inexpensive RF Signal Generator
PROTOTYPING
& TESTING
When
there are 2-GHz signals running around in a circuit,
you have no choice but to prototype with a PCB. I tried
hand soldering and wire wrapping, but the results were
chaotic.
You
can get away with a two-layer PCB design as I did for
this project, but four layers are better. You have to
use surface-mount devices to achieve quality results.
At
RF frequencies, components with axial leads add too
much inductance and capacitance to be practical. I refused
to go any smaller than 0805-size SMT devices because
that was the smallest I could handle with tweezers and
magnifying goggles. It seemed to work fine.
I
resorted to water-soluble solder paste and a toaster
oven for the reflow. I was amazed at how well this simple
approach worked. I controlled the toaster oven’s temperature
and timing manually.
Laying
out a PCB at these frequencies can be tricky. Given
the trace thickness used by your PCB supplier, you need
to use trace widths that provide a 50-W
system impedance. I needed approximately 100-mil trace
widths for my PCB.
In
addition to controlling trace widths, try to keep the
main signal path as close to a straight line as possible
to minimize any spurious modulations. If you need to
make a right angle, it’s recommended that you do it
with two 45° steps. Even the size and length of a via
is a factor at these RF frequencies, but I chose to
ignore this precaution without apparent consequences.
It’s
also necessary to put an RF circuit in an RF tight enclosure
in order to keep external signal sources out and the
generated signals in. RF tight means no open holes in
the box to pass wires through (they would leak RF like
crazy). It also means shielded connectors in and out
for signal lines and pass-through capacitors for filtering
DC supply lines. I obtained pass-through capacitors
from a surplus house for $1.50 each. It’s worth searching
for these because they can be pretty costly.
Incidentally,
if you study the information available online, you’ll
find that standard BNC connectors work well at 2 GHz.
The BNC connectors and cables are cheaper and more readily
available than the higher frequency SMA counterparts.
A
final note on testing and debugging an RF circuit. It’s
best if you hard-wire shielded cable to test points
rather than use pigtail leads. The parasitics and extraneous
coupling that exist with unshielded lengths of wire
will produce confusing results.
Enough
about all the gotchas! It’s time to move on to the RF
signal generator’s design.
