Septmber
2005, Issue 182
Signal
Generation Solution
Build
an Inexpensive RF Signal Generator
Tired
of going to a local university lab to use a signal
generator, Neal designed his own. In this article
he explains how he built the PIC16F877A-based controller
and RF module.
by
Neal Martini
I
worked at home for a year and a half on a project that
required a frequency source that generated sine wave
signals in the 100- to 375-MHz frequency range. I had
access at a local university to high-quality Hewlett-Packard
signal generators, but going back and forth to the lab
was a bit inconvenient. My first thought was to look
on the Internet for an affordable signal generator to
suit my needs. Needless to say, only expensive generators
were available.
With
many years of professional and hobby experience in lower
frequency digital and analog systems, I decided to venture
out and build my own RF signal generator. I realized
that things were a little trickier at higher frequencies,
but I was confident it would be fairly straightforward.
I had some experience with low-frequency narrowband
mixing circuits, so I thought I had a pretty good handle
on frequency translation using mixers. So, I proceeded
with reckless abandon. Boy, did I ever underestimate
the task I was about to undertake!
The
more I dug in, the more I realized that almost everything
is different when working at RF. The terminology (e.g.,
VSWR, compression points, frequency pulling/pushing,
intermodulation distortion, and dBm) was foreign to
me. Much more attention was given to design details
like complex impedance matching and signal path leakage.
The way you breadboard, lay out the PCB, and test were—you
guessed it—all different. Even the parts suppliers were
companies that I wasn’t familiar with. And finally,
and most importantly, the difference between theory
and reality was dramatic.
Nevertheless,
it was a great learning experience. If you want to expand
your electronic horizons a little, this is a good project
to try. Before I describe my design, I’ll cover some
of the things I learned that might help you understand
some of the trade-offs I made.
