Septmber
2005, Issue 182
Signal
Generation Solution
Build
an Inexpensive RF Signal Generator
REAL
MIXERS ARE MESSY
A
classical mixer has two input ports (RF and LO) and
one output port (IF). Ideally, the sine wave FLO at
the LO port modulates the signal FRF at the RF port,
and the output port IF contains FIF = FLO ± FRF. In
the commonly available double-balanced mixer (DBM) used
in this project, this result is true; however, the output
also contains undesired spurious outputs (called spurs)
that can cause distortion if they aren’t handled carefully.
Why
are so many spurs produced? For one thing, the mixing
action in a DBM is achieved by turning various diodes
on and off to produce modulation. This process is called
biphase modulation. The mixer’s output can be mathematically
modeled as follows:
[1]
With
a DBM you’re basically modulating the RF signal with
a series of sine waves that are odd harmonics of the
LO, each producing signals at the mixer’s output.
Let’s
look at another source of unwanted mixer output components.
Real world RF mixers have signal leakage between all
the three ports. This causes signals to appear at the
output IF port located at the RF and LO frequencies.
According to classical communication theory, this isn’t
supposed to happen in DBMs. This leakage is unavoidable
for RF signals.
The
final source of spurs is generated because the conducting
diodes are nonlinear. Although the nonlinearity is required
to produce the wanted sum and difference frequencies,
the higher order nonlinearities produce unwanted spurs.
The
net result is that the mixer produces output at various
frequencies:
[2]
where
m and n are integers. Figure 1 shows how catastrophic
this can be. It shows the output of a spectrum analyzer
that was attached to the IF port on a DBM mixer. The
LO and RF signals were provided by two commercially
available signal generators. FLO equals 500 MHz. FRF is 700 MHz. As you can see, the output spectrum
is loaded with spurs that would cause problems in a
wideband system.
|
(Click
here to enlarge)
|
Figure
1—Take a look at an example of the frequency spectrum
at the IF port of a real world DBM mixer. The LO
signal and the RF signal were provided by two commercially
available signal generators (FLO = 500 MHz and FRF
= 700 MHz). The output spectrum is loaded with spurs
that would cause problems in a wideband system. |
Hittite’s
spur calculator is an excellent tool for predicting
a DBM’s output (www.hittite.com). If you specify the
frequencies and the levels of the RF and LO ports, the
tool will show you the frequencies at which all of the
spurs will occur and what their power levels will be.
Figure
2 shows an example of the output from the Hittite tool.
In this example, the LO is 1.5 GHz and the RF signal
varies from 1.5 to 2 GHz. If, for example, you want
to know which mixer IF outputs will occur if the RF
signal is 1.7 GHz, you go to 1.7 GHz on the RF frequency
axis and move up the graph vertically. Every line you
cross is another output that will be at the IF port.
The level of each output signal is given in the boxes
below the graph.
|
(Click
here to enlarge)
|
Figure
2—The LO signal is 1.5 GHz. The RF signal varies
from 1.5 to 2 GHz. The graph shows the different
IF port outputs that will occur as the RF frequency
is varied. The level of each output signal is given
in the boxes below the graph. |
You
can use this tool to come up with a frequency plan for
your design. It will help you choose things like the
frequencies and drive levels required for the LO and
RF oscillators in order to get the IF results you want.
It will also help you plan which filtering is required
around the mixer.
