Build a breadboard
pulsed RF source
here to go to our main page on pulsed RF sources
here to learn about measuring pulsed RF on a spectrum analyzer
Build the breadboard drain-pulsed
amplifier described here and you will have a useful piece of test
equipment for your microwave laboratory, for very little cash, you
cheapskate! It provides up to 20 dBm peak power, and operates from
2 to 20 GHz, when driven by a laboratory sweep oscillator and a
Why would you build your own
modulator when a basic sweeper provides this function? Our modulator
is much more versatile than the cheesy modulation function that
is built into a $50K synthesized sweeper, and it will only cost
you a couple of bucks! We'll give you faster rise/fall times, and
shorter pulse widths than Agilent does!
Here's a clickable outline of
of caution on the terminating resistor
Testing the breadboard
NASA takes over
the project (some great hardware photos!)
measurements with detector
Much of the material on this
page was contributed by Justin, Wayne, Romeo, and Greg of NASA,
who took an idea they got from Microwaves101 and made it into real
hardware. Thanks guys, especially for all of the different data!
The heart of the Microwaves101
breadboard pulse modulator is an amplifier that you turn on and
off by pulsing its drain bias voltage. There are tons of amplifiers
out there that you could use to create a breadboard modulator. We
have two suggestions (from Hittite), based on availability (free!),
wide bandwidth, and pretty good power.
The HMC462 and the HMC463 are
marketed as low noise amplifiers. Click on the links below and we'll
take you to their data sheets. Both are surface-mount products,
so you don't need to worry if you don't have a wire bonder at your
disposal. If you ask them nicely, Hittite will supply you with some
free samples of either amplifier mounted on a connectorized evaluation
board, all built up. Hittite has already done the heavy lifting
for you. However, you have to
fill out an annoying request sheet before you get any free parts
from these MMIC misers. Tell them the Unknown Editor sent you!
Here's pictures of the two choices,
mounted on Hittite-supplied evaluation boards. The pictures are
not to scale, the amplifier packages are the same size. Note the
nice heat sinks on the backsides, and well-wetted RF connector solder
joints. Hittite thought of everything... except they assembled the
HMC463 with one of the tantalum capacitors backwards (it has been
fixed in the photo). Obviously they don't test this stuff before
they ship it!
Here's the tradeoff: the HMC462
is self biased, so you only need a single +5 volt supply. The HMC463
has three bias voltages: VDD ( nominally 5 volts), VGG1 (nominally
-0.6 volts but should be adjusted to set the proper drain current),
and VGG2, which is the cascode gate connection which allows some
gain control (but it can be grounded for this project). The HMC463
might be slightly more complicated to operate, but it delivers more
power than the HMC462. According to Hittite's data sheets, the HMC462
delivers 15 dBm at 2 GHz, and 12 dBm at 20 GHz, while the HMC463
delivers 20 dBm at 2 GHz and 13 dBm at 20 GHz. Self-biased designs
always put out slightly less power then their normally-biased counterparts,
because the source resistor that biases VGS to a negative voltage
eats up some of the available output voltage swing. You decide which
one you can live with, but always remember "the best idea is
the simplest one that works". That's an Einstein quote in case
you didn't recognize it.
Both the HMC462 and HMC463 draw
less than 70 ma at +5 volts on the drain bias. The HMC463 should
not have its drain voltage (+5 V) applied without its gate
voltage (~-0.6 V), but with low power amps (such as LNAs) you can
get away with short periods of time where this is violated. We've
operated it for hours with VGG1=0 volts and never observed reliability
problem, but don't expect its RF behavior to be OK without proper
biasing. In any case, you won't need any elaborate power supply
sequencing circuits to "baby" the device.
Note 1: in this modulator application
you don't need to pulse the VGG1 signal, just leave it on while
you pulse the drain (VDD).
Below is a schematic
of the Hittite evaluation board for the HMC463LP.
The schematic for the self-biased HMC642 is very similar, but simpler
(there are no VGG1 and VGG2 connections.) The drain bias network
is the same, with three bypass capacitors (100 pF, 1000 pF and 4.7
the breadboard RF modulator
Basically, you will be following
our directions for using a MOSFET driver for pulsing the drain of
a MMIC amplifier, which we have detailed
on a separate page.
You will have to make some modifications
to Hittite's evaluation board to operate the drain modulator. They
show a 4.7 uF cap on the drain line (C3) . You will move the capacitor
on the supply side of the MOSFET modulator; the Hittite amp will
still "see" the capacitor when the modulator is on. The
cap is there for stability. Trust us, it will be stable with this
mod, and it will help act as charge storage to keep the drain voltage
constant while the pulse settles.
The schematic below
shows how to modify the evaluation board (once again, the HMC463
is shown, the modified HMC462 schematic would be the same but without
the VGG1 and VGG2 connections). The MOSFET driver must be mounted
no more than an inch or two away from the amp. Farther away and
you will slow down the DC pulse. You can either dead-bug
the TSC427 MOSFET driver right onto the evaluation board (there's
room), or make a second PC board to hold the MOSFET driver, (like
Justin did below). One thing you will have to modify on the
Hittite board: the 4.7 uF capacitor on the drain line must be moved
"upstream", ahead of the MOSFET driver as shown, otherwise
it will slow down the modulator. If you have long power supply leads
(more than a few inches), you probably want to mount even more charge
storage on the MOSFET driver supply (as many 4.7 uF caps as you
Here we have violated
a "rule" of MOSFET drivers, by ganging the dual driver
in parallel (oh no Mr. Bill,
one of them might hog the current!!!) This improves the switching
speed, but if you are planning on sending the Microwaves101 Pulse
Modulator to Mars, you might not want to do that. In any case where
you only use one of the two available drivers, be sure to tie the
input high or low on the unused driver to keep it out of trouble.
Keep the power supply
leads short for high-speed performance. You can experiment with
some axial capacitors across the power supply output terminals to
act as charge storage when the modulator is operated at various
pulse widths and duty cycles.
It's good practice
to bring the input pulse to the MOSFET driver through a coax line.
You can probably install a BNC or MCX connector by drilling a hole
on the eval board. Again, shorter connections are better for speed.
word of caution on the terminating resistor
A resistor is required
for terminating the SW input signal, with a value from 50 to perhaps
500 ohms. It serves two purposes: it ensures that the pulse modulator
stays "off" when no switching signal is present (trust
us, you will thank us one day for this), and it helps provide an
impedance-match to your high-speed pulse generator. The value of
terminating resistor that you use depends on what type of pulse
generator you are going to drive it with. In the NASA example below,
an old HP 8112A 50 MHz function generator was used, and it "expects"
to see fifty ohms. Note that if someone wants to turn on the modulator
continuously (CW), that 50 ohm resistor is going to have to dissipate
a half-watt at 5 volts. In order not to fry it, we recommend you
consider two things: use a 2010 sized resistor (200 mils by 100
mils) for dissipation up to 1/2 watt . Alternatively (if only a
smaller resistor will fit), when you want to operate CW, cut the
power dissipation by reducing the input voltage to 3 volts instead
of 5 volts (the TSC427 driver will turn on at 2.4 volts).
breadboard RF modulator
When you first try out the modulator,
try it "ON" (not pulsed) to be sure the amp is working
(measure the gain of the amp to verify).
Note 3: when you pulse it, the
drain current will decrease roughly by the duty factor of the drain
pulse. At 10% duty, you should only have 7 ma DC current.
You can get an idea of the RF
rise time by looking at the VD waveform. If it has 5 nanosecond
rise/fall times, the RF will too. If the rise time is too slow (like
50 ns), put on some more charge storage or shorten the power supply
leads: maybe 20 uF will be better than 4.7 uF.
Note 4: remember to check out
the DC input waveform with the MOSFET driver turned ON, otherwise
it will load the pulse generator down and look ugly!
The final proof that you have
a well modulated signal is to put a detector on the output and display
the pulse envelope on an oscilloscope. Be sure not to exceed your
detector's maximum input power rating.
Note 5: you may have to buy a
"high speed" detector (like one from Krytar)
so that the detector is fast enough to measure the pulse characteristics
accurately. Even better, get a high-speed sampling o-scope and look
at the modulated waveform in real time!
takes over the project...
Again, we can't thank you space
cowboys enough for submitting this stuff to Microwaves101!
Below are two pictures of the
Hittite HMC462 amplifier and the new "daughter
board" that Justin, Wayne, Romeo and Greg fabricated to
add the MOSFET driver to the amplifier. The first photo shows a
fit check prior to assembly, the second picture shows all the components
mounted. Looks like an MCX connector brings in the pulse command,
and there's the 4.7 uF cap that is used for charge storage. Yup,
the polarity seems correct! In this case they took off the 1000
pF ceramic cap from the drain connection of the amp, but didn't
add it back on the supply side of the MOSFET driver. We recommend
that they put it in parallel with the 4.7 uF tantalum cap, it can
only help sharpen the pulses (but perhaps an insignificant amount).
By the way, that's the HMC462
amplifier being used here, not the HMC463. Thanks to Gerry and Justin
for clearing that up!
Here's the chassis that the boys
from NASA mounted the part in, which contains its own 5 volt DC
power supply (looks like it was recycled from a seven-year-old project,
such reuse is always admirable). Nice job keeping the power supply
leads short, and the coax line going to the pulse input is another
good thing you might want to copy if you try this yourself! Semi-rigid
cables bring the RF in and out to bulkhead SMA jacks on the front
panel. A truly fine piece of breadboard equipment is born!
Here are some thumbnail pictures
of the box, which has been nicely labeled for posterity. Click on
them and you'll see larger views.
The modulator is driven by an
HP 8112A 50 MHz pulse generator, which is set up for fifty ohms
(hence the modulator's terminating resistor is fifty ohms). Here's
a scope shot of the input pulse, the modulated drain voltage, and
detected output, with the RF off. The "glitches"
on the detected output can be cleaned off using a high-pass filter
(which is on order).
measurements using a detector
See our page on oscilloscope
measurements if you are interested in this topic.
Channel 1 (yellow) = square wave
from function generator to pulse input
Channel 2 (pink) = Pulsed RF Output (no detector)
Channel 3 (blue) = Output of the driver
As you can see, there is one
"spike" that occurs when the amp is turned on and another
when it's turned off. The detector didn't pick up the turn-on spike
because it's positive (or negative, whatever). The spikes are high-amplitude
but relatively low frequency. According to the spectrum analyzer,
it seems as though their frequency components are mostly contained
below 250 MHz, and almost completely gone at 600 MHz. Therefore,
I don't think they'll cause any huge problems. A coaxial hi-pass
filter on the output would to clean this up.
Here's a scope shot of the detected
output with the RF on. Notice the detector produces a negative voltage,
this is due to the way the detector diode is oriented.
Here's two shots taken on a sampling
oscilloscope, where you can see the RF waveform. There isn't a lot
of lab equipment that is cooler than a high-speed sampling o-scope!
In the first shot you can see the switching delay of about 20 Ns,
and a rise time of perhaps five or ten nanoseconds, depending on
where you consider it "on".
The second shot is zoomed in,
you can roughly determine the RF frequency by observing that in
1 division (1.25 Ns) it goes through 3 cycles:
3 cycles/1.25 Ns=2.4 GHz
Good thing this wasn't a "classified"
Here's a spectrum analyzer plot
showing the pulsed spectrum at the output, during "normal"
pulsed operation. This shot was staged to nearly replicate our example
in the pulsed RF section
of our discussion on spectrum analyzer
measurements. The only difference is the amplitude. Here are
the timing specs:
PW = 1s
PRI = 20s
(pulse repetition interval)
Notice that 1/PW is the distance
between the spectral lines (1 MHz), and 1/PRI is the distance between
the minimum of the smaller lobes (1 MHz).
Taking -20xlog (duty cycle) yields
a pulse desensitization factor of 26 dB (the difference in magnitude
of the peak output power and the main lobe power). The power at
the input is a -10 dBm RF signal and the pulse mainlobe power can
be seen on the plot at -23 dBm, right where it should be. Using
the desensitization factor of 26 dB:
Peak pulse power = -23 dBm+
26 dB= 3 dBm
This is correct because the amplifier
added 13 dB to the -10 dBm input signal. It's nice when algebra
The plot below is the measured
on/off isolation of the modulator measured
on a network analyzer. The isolation is a function of frequency
(isn't everything?), providing more than 50 dB isolation at 2 GHz
and more than 35 dB at 20 GHz. Trust us, this is better than you
could do with a cheap MMIC switch! If you need better on/off isolation,
you will have to spend more money, and perhaps build an expensive
hybrid with PIN diodes and a fancy driver circuit. Shielding on
the board will become a big headache as well.
That's all for now, but we'll
keep adding to the page as Justin sends us more data...
Update March 2006
Justin sent us some more info
on the modulator project, it just keeps getting better and better.
One of these days the Lazy Unknown Editor will distill this page
down a little, but right now, it's Miller Time!
"Finally, I made a few changes
to the Pulsed RF Unit:
1. Replaced the 50 Ohm square wave input resistor with two piggybacked
100 Ohm resistors. The new resistors are each 1210 surface mount
size so together they can take up to 1/2 Watt. Now it's OK if somebody
accidentally uses +5 VDC on that input. I should probably put a
5.6V zener on there to further protect things, but there's not much
space left now.
2. Put the 1000 pF capacitor in parallel with the 4.7 uF capacitor
like you suggested.
3. Installed a coaxial in-line highpass filter. It's an RLC electronics
F-100-1500-2-R, 2-pole with cutoff at 1.5 GHz. It cost $320 plus
a not-insignificant NRE, so the project turned out to be over budget
in the end (this is NASA, after all). Now if somebody else wants
to duplicate this work they can be sure to keep RLC honest if they
try to re-quote the NRE charge.
I've attached updated plots that
should match plots I already sent you. You can see now that the
glitches are completely gone thanks to the highpass filter. I even
turned the volts/division knob all the way down to 2mV/div and there
is absolutely nothing there when the RF is absent. The high-speed
O-scope plots look cleaner now that the RF isn't offset by the glitch.
The risetime didn't really change
much with that 1000 pF capacitor. I'm showing it to be about 8.6
Ns The falltime is really fast -- about 2.4ns! That was hidden before
due to that glitch.
I sent 2 new pictures as well.
You can see that we had to move that 4-pin molex connector over
to make room for the 1000 pF cap.
The network analyzer and spectrum
analyzer plots you have up should still be valid. Hopefully we can
call this page complete now!"
You can see the high-pass filter in this picture:
Here's the DC and RF pulses,
with NO RF:
The next two plots show the rise
and fall times, for a 60 Ns input pulse. Rise time is measured at
Fall time is measured at 2.4
"I noticed that the pulsed
RF spectrum didn't look quite right on my measurements so I plugged
back into the high-speed oscilloscope and learned something useful:
the pulsed RF output is about 15 - 20 Ns longer than the 5V input
pulse. Regardless of the input pulse width, there's a constant "stretching
factor" of about 18 Ns or so. I should have noticed this earlier
-- you can even see it on the plots I've sent you with the driver
voltage shown. For some reason, that driver is a little slow to
turn off. I've attached one more plot to show the phenomenon in
the pulsed RF output. You'll see that my input pulse is 60 Ns wide
but the RF output lasts for about 78 Ns Looks like I need to make
a new label for my box!
As a side note, the spectrum
plot you have is a little off since I really had a PW=1.018 us.
I won't tell if you don't. "
"The perfectionist in me
has won the battle over the lazy man. Here is a new spectrum plot
with a true 1us pulse width (I used a 982 Ns input pulse). Now those
nulls fall in the correct places."