RF Isolator Uses Differential Amplifiers

(IC version is
recommended over this older design)

An RF isolator is a seemingly magic device that allows signals to
pass in only one direction. Signals applied to the input port are sent to the
test port and signals coming into the test port can only go to the output
port.  This one-way property is usually accomplished with special
non-linear ferrite/magnet structures operating at very high frequencies. Fig. 1
shows an active RF isolator capable of handling signals approaching 16 dBm and
frequencies from well below 1 MHz to above 200 MHz. The circuit really emulates
an isolator in that the actual signal energy is not passed from port to port and
the signal levels must be fairly low. The circuit is well suited for testing the
SWR of a variety of devices connected to the test port.

For a typical setup the test generator is connected to the input port, the
antenna or device to be tested is connected to the test port, and a signal level
monitor is connected to the output port. When the  test port is terminated
with 50 ohms, no energy is reflected and no signal will appear at the output
port. If the load at the test port is not 50 ohms then some of the test signal
will reflect and appear at the output port. For example, if the test port is
completely open, the test signal reflects completely and the output will equal
the input. If the test port is shorted, the test signal will reflect inverted
and the output will be an inverted version of the input. Other loads will
reflect a portion of the test signal - see SWR.

To measure the return loss of a device or antenna, first open or short the
test port and observe the signal level at the output port. Then connect the
device to be tested and observe the drop in signal level at the test port. The
amount of drop is the return loss. This test may be done in the presence of
other signals if a tuned meter or spectrum analyzer is used to monitor the test
signal. Adjust the test signal frequency until it is sufficiently far from the
other signals to easily measure and verify that the other signals present are
not too large for the circuit to handle (below 16 dBm).

The transistors
will run hot so use a low-capacitance heat sink. Increase the 10 ohm resistors
in the emitter circuits to reduce the transistor heating but the large signal
handling ability will be reduced. The isolator is a 50 ohm device due to the
resistors near 50 ohms. (The 56 ohms are a bit higher since they are shunted by
some circuit resistance.) Other impedance like 75 ohms may be achieved by
changing these resistors to, say, 75 and 82 ohms. Ordinary small-signal
transistors like the 2N3904 operating at much lower current make a useful device
for testing antennas and low power devices at lower frequencies - perhaps to 50
MHz. Try 2, 82 ohms instead of the 2, 10 ohms in the emitters. Remember that the
signal handling capability will be quite low so use signals at or below 0 dBm (1
mW).

Some points for the experimenter to ponder:

This circuit has a very wide bandwidth (in terms of octaves) and it will
accommodate complex waveforms so it can be used to test more than just
antennas!

The ports are isolated so the input can be quite poorly matched - even a
coil of wire picking up a signal from a grid-dip meter will work.

The ports may have different characteristic impedance simply by changing
the obvious resistors. (56 and 51 ohms, presently). For example, a 100 ohm
generator could test a 75 ohm antenna and present the reflections to a 50 ohm
spectrum analyzer.

An RF amplifier and diode detector could be added to the output to make a
simple dip meter. Simply tune the input frequency until the meter dips,
indicating little reflection from the device under test.

Also read about an IC version of the isolator: Low Frequency
Circulator/Isolator Uses no Ferrite or Magnets (pdf file)

Copyright 2000 by Charles Wenzel