An LMX1601 Phase locked loop, a
discreet FET VCO, and an AVR micro controller combine to make a
stable, easy to use monophonic FM transmitter that includes a an audio
switch that turns the transmitter on only when its being used.
The common characteristic of all of the previous low power FM
transmitters I've built over the decades, is that their operating
frequency is determined by an LC resonant circuit. Some of them had
excellent stability, some of them didn't, but I had always wanted to
make one that is crystal controlled. Various schemes had been
considered from time-to-time, including the direct approach of
modulating the load capacitance of a a crystal oscillator, a whimsical
phase modulation scheme involving a phase shifter, some balanced
modulators, and limiting amplifiers, and at times, the
down-to-earth and sober approach of modulating a VCO within a phase
locked loop (PLL). While browsing Digikey's online catalog, I found the
LMX1601 frequency synthesizer chip and thought: "Just maybe, the
approach is finally within my grasp."
The LMX1601, which apparently was designed for use in cell phones,
includes everything need to make two phase locked loops except for the
VCOs. More importantly, one of the PLLs, specifically the "AUX"
specified to work in the FM broadcast band. The LMX1600 and the LMX1602
were also considered, but the LMX1601 was selected because it has a
"500 MHz option",
meaning that it can work down to about 50 MHz.
The painful part was dealing with the
packaging of the chip. The lead centers of this 16 pin package
0.65 mm apart. Dealing with the fine printed circuit board features
that would be needed to mount this chip was the most daunting aspect of
using the chip. I had only experimented with the laser printer toner
etch resist method of making PC boards a few months earlier, with some
success. As it turned out, the laser printer toner etch resist method
could be pushed to make a board that supports this chip. But just
Fig 1. Click on the image to see
the larger size version.
This board uses layout accomodates
both the DIP,
on the same board as the
which is specified
millimeters. There were some compromises. The artwork only includes
the AT90S2313, LMX1601, and the minimum components needed to make
the LMX1601 operate. Artwork was generated using a
In order to function, the LM1601's registers need to be loaded. In
particular, they need to be loaded with the divide ratios for the
reference and signal (VCO in this case) counters, and to set some
control parameters. To load the registers, I used a Atmel AT90S2313. It
could have been any microcontroller with 3 I/O pins, but I had chosen
the AVR processor, and I still have a lot of AT90S2313 chips left over
from another project. To the best of my knowledge, the ATtiny2313 can
substituted without changing the source code. The 20 pin DIP
appears monstrously large compared to the diminutive LMX1601. The size
difference is an indication of the passing of the three of four
decades that elapsed between the introduction of these two packages.
The LMX1601 has a reference divider, which I used to divide the 4 MHz
clock from the micro controller's crystal oscillator, down to 12.5 kHz
The feedback divider, referred to as "16 Bit AUX N Counter" in the data
sheet, is proceeded by a divide by 8 prescaler. These factors determine
the channel spacing for the phase locked loop. 8 X 12.5 kHz = 100 kHz.
The AUX N register contains the divide ratio for the feedback divider,
and the phase locked loop tries to make the VCO operate at a frequency
equal to the value written to the AXU N register, times 100 kHz. I say
"tries" because the actual frequency of oscillation depends on the
range of the VCO itself. This means that the center requency of the
resulting frequency modulated oscillator can
be set with 100 kHz resolution, enabling it to be set either on or
between allocated FM broadcast channels throughout most of the world.
If a value of 1000 is written into the AUX N register, the PLL will try
to run at 100 KHz X 1000 = 100 MHz. The math isn't so hard, and I
skipped the hard math -the loop gain and bandwidth calculations- so
this should be an easy project, mathwise.
In the firmware, an 8 bit count is used as the "channel" number.
The highest frequency, which occurs when the channel number is
255, is set to 108 MHz by adding an offset of 825 to the channel
number. I did this because the part of the spectrum just above the FM
broadcast band is used for aviation communications in most, if not all,
parts of the world. When the frequency increment and decrement
buttons are pressed, the channel number increments or decrements,
respectively. The button increment and decrement routines that limit
the channel number to those that correspond to frequencies between 88.0
Mhz and 108.0 MHz.
After the channel number is changed, the microcontroller waits for 30
seconds before writing the channel number to EEPROM. EEPROMs are only
capable of a limited number of cycles, and the 30 second timer prevents
the EEPROM from being written to each time a button is pressed. Going
from 100 MHz to 108 MHz requires pressing the increment button 180
times. That's not really that much work, considering that it would
normally only have to be done a few times in the life of the
The actual frequency of the oscillator depends on the microcontroller
crystal oscillator. I am using this with a couple of radios with
digital tuning, and one of them has a pretty narrow detector, so for
low distortion, I needed to have the transmitter very close to the
frequency the receiver expects it to be. In other words, the
transmitter, and therefore the microcontroller crystal oscillator had
to be accurate. I solved the problem in my case by selecting a 4 MHz
crystal this is close, and then adding a small capacitor in series with
it to drop the frequency by a few more parts per million.
The oscillator I chose for the VCO is the same one used in the Band Switched Test Oscillator
project because I
know from experience, that it can tune through the entire FM band, even
with a 3 volt tuning voltage, and also because it is quiet enough to
use for this sort of thing. I had listened to the basic oscillator
plenty on an FM radio while working on the Band Switched Oscillator
Figure 2. The
voltage-to-frequency transfer function of the 2N5485 VFO.
The tuning range during the test,
with a 3.05 volt power supply, was
84 MHz (tuning voltage = 0 VDC)
to 115 MHz (tuning voltage = 3.05
The average sensitivity of the
VCO is approximately 10.4 MHz per volt.
The oscillator inductor is 8 turns of
#22 enameled wire with 1/8 inch inside diameter initially wound
closely-spaced, then stretched only a little to obtain desired turning
range. When I wound the coil, I first stretched a length of wire a
little bit with a pair of pliars - that made it so that it would keep
its shape after winding. Then, I wound the wire on the smooth part of a
1/8 inch drill bit, slid the coil off, trimmed the leads the proper
length, and tinned the leads.
The VCO was built dead-bug style on a small piece of copper clad board
that was soldered onto the printed circuit board with thin copper
strips. Noise, and in particular hum from the power supply and spikes
from the microcontroller were a problem. The 3 volt power supply, made
from a TL431 shunt regulator with an external series pass
transistor, gives some power supply ripple rejection, but I
added a 7805 regulator ahead of it to give an additional 60 db or
ripple rejection. This was enough so that there was very little
50 Hz, or 60 Hz -I worked on this on two different continents - to be
heard in the signal. The 7805 being ahead of the TL431 circuit also
handles a wide range of input voltages, which along with the varistor,
gave me some peace of mind.
With the approximately 100 db of power supply ripple voltage rejection,
and the power supply hum all but inaudible, a musical tinkling sound
was heard through the speakers. This was noise from the microcontroller
getting into the VCO circuit through the power supply. This was
stopped by adding a 330 uf decoupling capacitor to the VCO.
Figure 3. The two transistor LED
driver circuit drives a red/blue common cathode LED. If only one LED is
used, both drivers may be eliminated and an LED in series with a
resistor can be connected directly to pin 9 of the microcontroller,
provided that an LED with sufficiently low forward voltage
requirements is used. Extensive use of Berg style connectors was used
that the circuit board can be completely detached from the enclosure,
facilitating repairs and modifications.
Referring to the schematic in figure 3, the circuit in the upper left
corner is the audio detection circuit, which operates in a maner
similar to that of a voice
operated switch, or VOX. Pins 12 and 13 of the AT90S2313 are the
inputs to an analog comparitor. The 50k, 1k, and 100k resistors
connected to these pins provide bias to that forms the threshold for
the detection of the audio signals coupled through the 10 uf capacitor.
The 47 uf capacitor keeps voltage on the comparitor input on pin
12 of the comparitor from changing with the incoming audio signal.
Whenever the comparitor's output changes state, it causes an interrupt.
The interrupt resets a 5 minute timer and blinks the LED from blue to
red for a short period of time. The two 4700 pf capacitors from pins 12
and 13 are chip capacitors that have their other ends soldered directly
to the ground plane, to keep the FM oscillator signal from tripping the
comparitor when 5 minute timer times out and the oscillator shuts off.
Even with these RF bypass capacitors in place, sometimes, the
comparitor would trip when the oscillator shut off, thereby turning the
oscillator back on again. Eventually, I added code to the subroutine
that turns off the oscillator, so that interrupts would be ignored for
about a second after the oscillator was turned off. This has finally
solved the problem.
AT90S2313 Pins 17. 18. and 19 are drive the three line Microwire
interface to the LMX1601. Pins 13 and 14 have internal pullups and are
connected to the two pushbuttons, the other end of which are grounded.
The output of the crystal oscillator on the AT90S2313 drives the
oscillator input of the LMX1601. The accuracy of the synthesizer's
output is the same, in PPM, as the accuracy of the 4 MHz oscillator.
You could select or trim the capacitors from pins 4 and 5 of the
AT90S2313 if your 4 MHz crystal is close enough, or if you have several
crystals to choose from, you could do as I did, which was to go through
a pile of 4 MHz crystals until I found one that was within a few Hz of
4 Mhz in this circuit.
While on the subject of the microcontroller, I used an AT902313-10,
which is specified to operate wtih a power supply voltage of 4 to 6
volts. I am lucky that this chip works at 3 volts, though I suspect
many do. If you don't what this element of risk in your version, I
suggest that you use either the 4 MHz version of the AT90S2313 or an
ATtiny2313. Both the 10 Mhz and 20 MHz versions of the ATtiny2313 will
operate from the 3 volt power suppply.
LMX1601 pin 7 is the phase comparitor output, which is a current pump.
A .0047 uf and a 22 uf capacitor make up the loop filter, creating a
voltage that is supplied to the left end of the 10k resistor. The right
end of the 10k resistor connects to the varactors in the VCO through a
low pass filter, to control the VCO's oscillation frequency. Also
connected to the right side of the 10k resistor is a 50 k resistor that
is in series with a 20k resistor paralleled with a .01 uf capacitor.
is where AC coupled audio is mixed with the VCO frequency control
voltage. That's how the oscillator's frequency gets modulated.
The combination 50k, 20k, and .01 uf sets both the modulation
sensitivity and the frequency response of the audio signal. I started
with a 75 microsecond time constant in this circuit, which is the
standard pre emphasis in Europe, but the signal sounded too tinny, so I
raised the time constant to one that sounded better, and limited
the amount of boost to about 3:1 with the resistor ratio. What I have
sounds ok for the kind of audio material I listen the condition of my
hearing. You might want to experiment with other time constants and
The audio input is through a stereo jack. The left and right signals
summed in the two 150 Ohm resistors, and the voltage swing at the point
the signals are summed is limited to two diode drops peak-to-peak, or
about 1.2 volts peak-to-peak. That might be a nice thing to do, as it
could keep the FM signal from splattering all over the VCO's range if
the audio input signal is cranked up too high, but I really put the
diodes there to protect the LMX1601's phase comparitor output from
electrostatic discharge. While in the early stages of this development,
I "frapped" the phase comparitor output, and both the decision to
replace the chip and the actual act were both unpleasant, and this
clipper is meant to avoid future accidents.
The VCO is a modified Clapp oscillator, and by making both capacitors
varactors, the oscillator can tune across the entire 88Mhz to 108 MHz
FM broadcast band. The left-most 2N5485 JFET is the oscillator
transistor. Its source resistor is the collector load of the 2N2222.
When the 2N2222 is no biased on, the oscillator does not oscillate.
When it is biased on, the oscillator does oscillate. That way, I can
switch the oscillator on and off without having to switch the 3 volt
power supply on an off.
A second 2N5485, the right-most JFET, buffers the sine wave on the gate
of the oscillator JFET. This signal is much lower in harmonic
distortion than the signal on the drain. The 130 and 50 Ohm
source resistor form a divider that reduces the amplitude of the
oscillator's output to within the range specified on the LMX1601's data
sheet, and this signal across the 50 Ohm resistor is AC coupled to the
LMX1601's Fin Aux input, where is conditioned and applied to the divide
by 8 prescaler and then to the AUX N divider, to complete the phase
The larger signal on the source is capacitively coupled to an RCA
connector, to which a small antenna may be attached, if needed. The two
1N5711 Schottky diodes limit the voltage on the source to between one
Schottky diode drop below ground and one Schottky diode drop
above the 3 volt power supply. It is my expectation that in the
event of an electrostatic discharge into the antenna terminal, most of
the discharge current will go into the ground or the power supply,
rather than into the buffer, or even worse, through the source
resistor, and into the LMX1601.
The piezoelectric transducer serves to make button clicks when the
button contacts close. It also beeps for a moment when power is first
applied and when a new frequency selection is written to the EEPROM.
The piezoelectric transducer and its drive circuit can be eliminated if
you don't want it. If you do use the transducer, and if it sounds too
quiet, you can find the place in the assembly code that adjusts the
tone frequency to get a frequency closer to your transducer's peak.
There is a comment in the code, so it won't be hard to find.
Total current drawn from the circuit while the oscillator is on, is
only 30 to 35 milliamps. This means that it can be operated for a while
from a 9 volt transitor radio battery, which I did occasionally during
The LED, or "Status LED" as it is referred to in the code, has two die
inside. One red, and the other blue. The LED indicates the following
• Dark: No power is applied to the circuit.
• Steady red: Power is applied and oscillator is off.
• Steady blue: Oscillator is on and a audio signal is not detected.
• Blue, occasionally blinking red: Oscillator is on and an audio signal
is being detected.
You can leave the LED off altogether, but the audio signal detection is
very helpful in adjusting the volume of the audio signal. The LEd could
also just be a single color LED, in which case, a singled LED would be
connected such that it is on when the oscillator is on, and blinks to
dark when audio is detected. If you can view QuickTime files, you might
want to see the somewhat cheesy demonstration of audio detection by
clicking on the image below.
MOV1. Click on the picture
above to view
an approximately 360 KB Quicktime
Remember to use your BACK BUTTON to
return to this page. Note: The
used in the Quicktime move is
to purpose of the movie, and the
the music constitutes fair use
The VCO, which was built on a separate piece of copper clad circuit
board was soldered onto the PLL PCB using thin copper strips to assure
a low impedance connection. The PLL
printed circuit board was mounted on a piece of punched phenolic board
with one pad per hole. The phenolic board which includes the power
supply, audio circuit, and the LED and piezoelectric driver circuits,
mounted in the plastic box. You can see from the enlarged version of
Photo 2 there are some air circuits -where components hang in the air
The red connector shown at the top of Photo 2 is the antenna
connector. A length of insulated wire is attached to it. If, and this
is a big "if". the power cable and the audio cable provided a suitable
ground image of the radiating element, an optimum length for the
antenna wire would be 1/4 wavelength, or 75 cm at 100 Mhz.
It might well be that your won't need an antenna at all. If you are
going to use this within a few meters of the receiver, then don't
bother with an antenna. There is no point in your neighbors fully
appreciating your taste in music or audio programming.
The phenolic board was hand wired point-to-point, mostly on the
There are some colored wires on the top of the board that connect the
LED, piezoelectric transducer, and audio circuits to the
microcontroller, and that is because its only a one-sided board with no
Photo 3. Click, if you have a strong
and you can see a larger
version of this
picture of the back of the
The colors of the wire are
merely a function
of which workbench I sat at
them in place.
One thing to remember while building this, is to keep the circuit board
iron grounded to the same point.
Interface (How to use it).
Here are the rules of operation - a way to describe how the
microcontroller controls the transmitter and how the user interacts
with the microcontroller. In the description that follows, when the
oscillator is on, audio signals applied to the audio input will be
It is important to press the frequency increment button, hold it
down, then hold down the frequency decrement button right after POWER IS APPLIED FOR THE FIRST TIME and
BEFORE AN ANTENNA IS CONNECTED
. This sets the transmit frequency
to 100.0 MHz. Before this initialization, the frequency could be
outside the FM broadcast band.
When power is first applied, the oscillator is turned on and the LED
illuminates with blue , and it will remain on for 5 minutes, even is no
audio is applied. If audio of sufficient amplitude to trip the
comparitor is applied, the LED will wink red on audio peaks and the 5
minute time will be reset. Audio applied to the audio input will
be transmitted on the transmit frequency that was last saved in
nonvolatile memory. This is the typical mode of operation.
After 5 minutes without audio of sufficient amplitude to trip the
comparitor, the LED will turn red and the oscillator will turn off.
If audio is applied after the oscillator is shut off, the oscillator
will turn on again and the LED will turn blue, and it will blink red
occasionally to indicate that audio of sufficient amplitude to keep the
5 minute timer from time out is present.
When the frequency increment button is pressed, the transmit
frequency will increase by 100 KHz, unless the transmit frequency is
already 108.0 MHz.
When the frequency decrement button is pressed, the transmit
frequency will decrease by 100 KHz, unless the transmit frequency is
already 88.0 MHz.
When either then frequency increment button of the frequency
decrement button is held down, then the other button is held down, the
frequency will be reset to 100.0 Mhz.
When 30 seconds elapses after a button depression, during which
period another button is not encountered, the then current transmit
frequency will be stored in nonvolatile memory.
Now, with this crystal controlled transmitter, I can use a wireless
headset to listen to internet audio feeds while in a large city, where
the band is completely jamed with both legitimate and the occasional
bootleg radio stations. This low power transmitter can be nestled in a
reasonably quiet slot between the competing behemoth signals, and since
its crystal controlled, I know it will be in the same place, day after
day. Similarly, using the LED indicator, I can set up the transmitter
in my workshop and be confident that I will find it on the
assigned frequency, with the right modulation level, when I switch on
radio on the front porch.
A note after using this transmitter for four months: The
transmitter is connected to the audio output of my main desktop
computer, and it has been this way for four months. This means that I
listen the alert sounds as well as the rest of the audio from the
computer through an FM radio. The transmitter comes on fast enough that
I catch the system beeps, etc. One of the nicest aspects of
having this transmitter is that if I have to leave the room - go to the
bedroom, kitchen, or even outside - while listening to an internet
radio program (a important source of English language news and
entertainment where I live), I just pick up a battery operated FM
receiver and take it with me. This is one of the most useful
projects I have made.