(Improved)
187
KHz RF Source
An oscillator, a small power stage, some modulation, and a tiny loop antenna make RF for experiments on at 187.5 kHz on the United States' FCC Part 15 Lowfer band (1600-1750 meters).
The oscillator board is not much more than the 74HC4060 oscillator/divider. The crystal is in a socket. made by cutting down an IC socket.
The charging current for the 10 uf capacitor in the output stage immediately after switching on the power can damage the LED. The LED in this circuit has an internal 100 Ohm resistor. If you use an LED without an internal resistor, use a 120 Ohm resistor in series with the LED instead of the 10 Ohm shown here. Most LEDs don't have internal resistors.
An oscillator, a small power stage, some modulation, and a tiny loop antenna make RF for experiments on at 187.5 kHz on the United States' FCC Part 15 Lowfer band (1600-1750 meters).
Notice: Before operating a radio
transmitter, find out what kind of transmitter operation,
if any, is
permitted in your locality. Radio transmitter operation is
a serious
legal matter. In the
United States, operation of unlicensed intentional
radiators is covered
by Part 15 of Title 47 of the Code of Federal Regulations.
This design
can
be readily adapted to different frequencies and different
power levels.
If you choose to build and operate the transmitter
described here, you
do
so at your own risk. I'm only publishing this as an
example of what can
be done.
The oscillator board is not much more than the 74HC4060 oscillator/divider. The crystal is in a socket. made by cutting down an IC socket.
This is a low power signal source I put
together
one evening
to provide 187 KHz RF signals for an anticipated receiver
investigation. Under
Federal Communications Commission rules inside the United
States, one
is
allowed to operate a transmitter without a license under
certain
conditions. Here, I have copied the relevant section of the
most recent
version of the Code of Federal Regulations that I could find..
TITLE 47--TELECOMMUNICATION
CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION
PART 15--RADIO FREQUENCY DEVICES--Table of Contents
Subpart C--Intentional Radiators
Sec. 15.217 Operation in the band 160-190 kHz.
(a) The total input power to the final radio frequency stage(exclusive of filament or heater power) shall not exceed one watt.
(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.
If you are thinking seriously about building a Part 15 transmitter, I suggest you read through Part 15 to make sure you are fully compliant. The FCC provides for very heavy penalties for those who step out of line.
Here is where I found this copy of Part 15:
I have known about the provision that allows 1 watt license free operation on this band for a long time, but didn't really take it seriously. The maximum antenna length is only 15 meters. This is electrically very short compared to the 1600 meter wavelength and is therefore be a very poor radiator.
However poor the efficency of the antennat some people using sophisticated coding and dedecoding techniques have managed to communicate over thousands of miles in this band (160 to 190 kHz), all operating within the scope of Part 15. Amazing.
In order to get started tinkering with circuits on this band, I put together a low power signal source.
This signal source only supplies a few milliwatts to the output stage when the +8V input is supplied by a 9V transistor radio battery. The low power in the output stage results from the collector load being a parallel tuned stage, thus it is a high impedance at resonance.
TITLE 47--TELECOMMUNICATION
CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION
PART 15--RADIO FREQUENCY DEVICES--Table of Contents
Subpart C--Intentional Radiators
Sec. 15.217 Operation in the band 160-190 kHz.
(a) The total input power to the final radio frequency stage(exclusive of filament or heater power) shall not exceed one watt.
(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.
If you are thinking seriously about building a Part 15 transmitter, I suggest you read through Part 15 to make sure you are fully compliant. The FCC provides for very heavy penalties for those who step out of line.
Here is where I found this copy of Part 15:
I have known about the provision that allows 1 watt license free operation on this band for a long time, but didn't really take it seriously. The maximum antenna length is only 15 meters. This is electrically very short compared to the 1600 meter wavelength and is therefore be a very poor radiator.
However poor the efficency of the antennat some people using sophisticated coding and dedecoding techniques have managed to communicate over thousands of miles in this band (160 to 190 kHz), all operating within the scope of Part 15. Amazing.
In order to get started tinkering with circuits on this band, I put together a low power signal source.
This signal source only supplies a few milliwatts to the output stage when the +8V input is supplied by a 9V transistor radio battery. The low power in the output stage results from the collector load being a parallel tuned stage, thus it is a high impedance at resonance.
The charging current for the 10 uf capacitor in the output stage immediately after switching on the power can damage the LED. The LED in this circuit has an internal 100 Ohm resistor. If you use an LED without an internal resistor, use a 120 Ohm resistor in series with the LED instead of the 10 Ohm shown here. Most LEDs don't have internal resistors.
Circuit
Description
Oscillator and divider
The 74HC4060 oscillates at 6.000 MHz, and is divided by 32 to get 187.5 kHz. Pin 5 of the CD4060 is a 732 Hz square wave which can be used to modulate the output stage rather than using a micro controller, but in this instance, the modulation is supplied by a micro controller that I programmed to send Morse code to the modulation input, with the Morse code modulated by an audio frequency square wave, resulting in tone modulated AM (Mode A2) being transmitted.
Output Sage
The output stage is a gated current source driving a resonant loop antenna. The emitter current of the 2N4401 causes a voltage drop across the 330 Ohm resistor. The 2NSC2458Y reduces the amplitude of the 187.5 kHz carrier signal on the base of the 2N4401 so that the maximum voltage across the 330 Ohm resistor is 0.6 volts, the base-emitter voltage of the 2NSC2458Y. The 0.6 volt drop across the 330 Ohm resistor means that peak emitter current for the 2N4401 is about 1.8 milliamps peak emitter current. Since the collector current is approximately equal to emitter current, this results in the resonant circuit on the collector of the 2N4401 being driven by a 1.8 milliamp square wave.
On each cycle, the 2N4401 puts some energy into the resonant circuit in the form of collector current. Some of the energy is dissipated in the resistance of the coil and a very tiny bit of it is radiated away, but most of the energy continues to slosh back and forth between the magnetic field around the coil and the electric field in the capacitor's dielectric. In the circuit I built, I measured about 30 milliamps peak-to-peak in the coil. The collector wave form is a nice sine wave. Using a Tektronix TDS-2002 in the FFT spectrum analyzer mode to look at the collector voltage, the second harmonic was in the noise and the third harmonic was 32 db below the carrier.
Proof that the 2N4401 is acting as a gated current source is that the collector voltage is 12 volts peak-to-peak when power from either an 8.4 volt battery or a 15 volt power supply. The collector current and therefore the voltage across the tank circuit, is independent of power supply voltage.
The current from the output stage is modulated by shutting of the 2N4401 by injecting current into the base of the 2NSC2458Y through the 4.7 k resistor and the 1N4148 diode. The 1N4148 is not necessarily required. I used it so that I wouldn't have to take the drive capability of the output pin of the AT90S2313 (or ATtiny13) micro controller into account when calculating the emitter current. A theoretical advantage of the 1N4148 is that it provides some isolation between the gating signal and the output stage since the diode is reverse biased, or at least nor forward biased, during the time the 2N4401 is sourcing current.
The transistor in the output is a 2N4401 because it has pretty clean switching characteristics and has plenty of current gain and a moderate cutoff frequency. In other words, it makes a pretty good current source and cuts off cleanly. The current feedback transistor, the 2NSC2458Y, was selected because I have thousands of them and am looking for a way to work them into my projects. It could also be a 2N4401 or a 2N2222, for that matter.
Firmware
By request, a link to the AVR Studio assembly lanugage source code for the firmware for the AT90S2313 was placed at the top of this page. The firmware can also run on an ATTINY2313 without modification. The source code was only commented for my own use, and the code itself is not pretty.
While you can use the AT90S2313 firmware included on this page as a Morse code generator, you might prefer to use the more finished version, the Low Cost Telemtery Device, which was written for the ATMEGA8, and is basically a plug-and-play implimentation. Just load the code into and ATMEGA8 and program the output strings and baud rate with an ASCII terminal. The basic operation of the Morse code generation is the same for both projects. Here is a link to the Low Cost Telemetry Device.
http://www.cappels.org/dproj/morbcn/morbcn.html
Earlier versions
One reader of this page has performed an extensive investigation of the earlier circuit, the one in which the 2N4401 is saturated rather than driven as a current source, to show that since the base drive for the 2N4401 is a square wave, care must be taken to assure that the transistor does not saturate if a very clean sine wave is desired in the inductor. After a couple of years, I got around to changing the circuit to being current driven so that a low harmonic sine wave can be produced without hand tweaking the base drive.
Oscillator and divider
The 74HC4060 oscillates at 6.000 MHz, and is divided by 32 to get 187.5 kHz. Pin 5 of the CD4060 is a 732 Hz square wave which can be used to modulate the output stage rather than using a micro controller, but in this instance, the modulation is supplied by a micro controller that I programmed to send Morse code to the modulation input, with the Morse code modulated by an audio frequency square wave, resulting in tone modulated AM (Mode A2) being transmitted.
Output Sage
The output stage is a gated current source driving a resonant loop antenna. The emitter current of the 2N4401 causes a voltage drop across the 330 Ohm resistor. The 2NSC2458Y reduces the amplitude of the 187.5 kHz carrier signal on the base of the 2N4401 so that the maximum voltage across the 330 Ohm resistor is 0.6 volts, the base-emitter voltage of the 2NSC2458Y. The 0.6 volt drop across the 330 Ohm resistor means that peak emitter current for the 2N4401 is about 1.8 milliamps peak emitter current. Since the collector current is approximately equal to emitter current, this results in the resonant circuit on the collector of the 2N4401 being driven by a 1.8 milliamp square wave.
On each cycle, the 2N4401 puts some energy into the resonant circuit in the form of collector current. Some of the energy is dissipated in the resistance of the coil and a very tiny bit of it is radiated away, but most of the energy continues to slosh back and forth between the magnetic field around the coil and the electric field in the capacitor's dielectric. In the circuit I built, I measured about 30 milliamps peak-to-peak in the coil. The collector wave form is a nice sine wave. Using a Tektronix TDS-2002 in the FFT spectrum analyzer mode to look at the collector voltage, the second harmonic was in the noise and the third harmonic was 32 db below the carrier.
Proof that the 2N4401 is acting as a gated current source is that the collector voltage is 12 volts peak-to-peak when power from either an 8.4 volt battery or a 15 volt power supply. The collector current and therefore the voltage across the tank circuit, is independent of power supply voltage.
The current from the output stage is modulated by shutting of the 2N4401 by injecting current into the base of the 2NSC2458Y through the 4.7 k resistor and the 1N4148 diode. The 1N4148 is not necessarily required. I used it so that I wouldn't have to take the drive capability of the output pin of the AT90S2313 (or ATtiny13) micro controller into account when calculating the emitter current. A theoretical advantage of the 1N4148 is that it provides some isolation between the gating signal and the output stage since the diode is reverse biased, or at least nor forward biased, during the time the 2N4401 is sourcing current.
The transistor in the output is a 2N4401 because it has pretty clean switching characteristics and has plenty of current gain and a moderate cutoff frequency. In other words, it makes a pretty good current source and cuts off cleanly. The current feedback transistor, the 2NSC2458Y, was selected because I have thousands of them and am looking for a way to work them into my projects. It could also be a 2N4401 or a 2N2222, for that matter.
Firmware
By request, a link to the AVR Studio assembly lanugage source code for the firmware for the AT90S2313 was placed at the top of this page. The firmware can also run on an ATTINY2313 without modification. The source code was only commented for my own use, and the code itself is not pretty.
While you can use the AT90S2313 firmware included on this page as a Morse code generator, you might prefer to use the more finished version, the Low Cost Telemtery Device, which was written for the ATMEGA8, and is basically a plug-and-play implimentation. Just load the code into and ATMEGA8 and program the output strings and baud rate with an ASCII terminal. The basic operation of the Morse code generation is the same for both projects. Here is a link to the Low Cost Telemetry Device.
http://www.cappels.org/dproj/morbcn/morbcn.html
Earlier versions
One reader of this page has performed an extensive investigation of the earlier circuit, the one in which the 2N4401 is saturated rather than driven as a current source, to show that since the base drive for the 2N4401 is a square wave, care must be taken to assure that the transistor does not saturate if a very clean sine wave is desired in the inductor. After a couple of years, I got around to changing the circuit to being current driven so that a low harmonic sine wave can be produced without hand tweaking the base drive.
Remember, read that Part 15! Read the
Liability Disclaimer at the bottom of this page, too.
HOME
Contents ©2003, 2004, 2005, 2007 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
Dick Cappels' web version first posted in December, 200; updated February, March 2004, October, 2004, and November, 2004, April and November, 2005, and May, 2007.
You can send email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.
HOME
Contents ©2003, 2004, 2005, 2007 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
Dick Cappels' web version first posted in December, 200; updated February, March 2004, October, 2004, and November, 2004, April and November, 2005, and May, 2007.
You can send email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.