diode forward voltage
here to go to our main microwave measurements page
here to go to our main curve tracer page
here to go to our page on double-balanced mixers
here to learn about Walter Schottky
Thanks to Daniel T. for correcting
some minor errors on this page (we previously said "460 mV"
when we meant to say "260 mV"!)
Suppose you want to reverse
engineer an old Watkins Johnson double balanced mixer, perhaps
a WJM4A. You might want to know the forward voltage of the
diodes that are inside. You can almost always access the diode characteristics
through the IF port of a double-balanced mixer, without cutting
off the lid. The forward voltage for a Schottky diode is defined
as the voltage at which the device draws 1 ma Low forward voltage
diodes (less than 0.3 volts) are referred to as low barrier diodes,
diodes with VF between 300 mV and 450 mV are medium barrier, and
diodes with VF greater than 450 mV are called high barrier diodes.
There is no better way to measure the forward voltage of a Schottky
diode than using a curve tracer.
Before we continue, when you
measure the diode characteristics of a quad diode through the IF
port of a mixer, chances are you are looking at two diodes in parallel.
So you must measure VF at 2 ma, not 1 ma. Here's a typical double-balanced
mixer schematic, so you can see what we mean. OOPS, we forgot to
show that there is actually a DC ground in the center of
the LO balun, which is needed for this
measurement! (we'll fix the figure one of these days...)
By the way, a great source for
Schottky ring diodes is... (insert your company's name here for
In this example
we will assume you own a Tektronix 370B curve tracer, the best there
is. However, the procedure can be adapted to any model curve tracer
if you will put on your thinking cap for a few minutes. Always
start by disabling the output of the curve tracer to your DUT, and
setting the collector voltage to zero (it's a big knob, you can't
miss it). Turn the collector supply knob all the way counterclockwise
to set it to zero.
Connect the mixer
IF port to the curve tracer (we'll abbreviate it CT) this way:
Center of IF output coax=collector
No connection=base of CT
Mixer package ground=emitter of CT
Cycle the 370B on/off
to clear out whatever settings that last person used. Although the
370B defaults to very benign conditions, you should make a habit
out of checking a few things we will describe here; if you have
an older CT, you will need to manually set more of the control knobs.
You should check to see that the polarity of the collector supply
is positive (set to NPN on old curve tracers) and it is in swept
mode as opposed to DC or AC modes. You won't be using the step generator,
so you don't have to even look at any of its controls. The polarity
of the collector supply doesn't matter for this measurement, you
will be looking a diode IV curve in either polarity.
Now set the knobs
on the CT this way (order is not important, but there are five knobs
you need to adjust):
- Set vertical/division to
- Set horizontal/division
to 50 mV (the lowest setting).
- Set collector supply to
- Set peak power to to the
lowest setting, .08 W (should be there already).
- Set maximum collector voltage
to 16 volts (the lowest setting, it should be there already).
Now you are ready to begin the
- Enable the outputs that
are connected to your DUT (you could be plugged in to either
the left side or the right side of the CT).
- Increase the "collector
supply" until you see the VF curve. Stop when you reach
the top of the scale (two milliamps). Your display should look
The forward voltage
is 260 mV in the above curve (voltage at 2 ma, at the top of the
You can reverse
the leads and measure the other two diodes in the quad to compare
how balanced they are to the first pair. We measured almost exactly
260 mV also, they are nicely matched.
An optional way
to look at them together is to set the polarity to "AC"
which sets the origin to the middle of the display. Now when you
crank up the voltage you can see both diodes. Here we change the
current per division to 500 uA so we could measure both VFs at 2
ma Again, we read 260 mV for both diodes (here you need to look
four divisions from the center line to read 2 ma).
If this still isn't
clear, please send
us your questions!