Synthesized Exciter for LF

Technical Aspects

A high precision, very versatile transmitter or signal source for use in the range 0 to 400kHz or more. It includes advanced keying, sweep and beacon mode options, yet the unit is inexpensive and simple to make and use.

INTRODUCTION

The Exciter is based on a microcontroller which uses a software Direct Digital Synthesis technique. Digital to analog circuitry and a power amplifier provide low distortion sinewave output. The unit is controlled and set up by a host PC or terminal using serial communications, but once set, the unit will continue to operate without intervention. Once set up, the unit can transmit a fixed carrier or multi-mode beacon message with no PC connected, and since the operating parameters are stored, starts up as expected when power is applied. The Exciter has great versatility thanks to a special PC software control and message download program.

There is so much to this unit that a careful description is necessary. The Exciter has few front panel controls - just a few sockets and one LED, plus perhaps a power output level control if you wish. Two separate command interpreters are used to tell it what to do. One command interpreter, which has as many as eleven different commands, operates from an RS232 serial link, and provides maximum versatility of control. It operates a bit like a packet TNC in "KISS" mode. Through this link the user can remote control the unit from a "dumb terminal" or from a special PC control program. The control program may be a simple frequency and power setting program, or a special sophisticated beacon message controller, sweep generator, or whatever you wish.

The second command interpreter works with a "script", a command message stored in the microcontroller EEPROM. Data for transmission and commands to allow changes of mode, shift and speed are stored in the script, allowing the Exciter to operate in a stand-alone manner.



The Exciter can transmit MFSK, including Sequential MT-Hell and graphics, using the internal script!
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EXPLANATION FOR BEGINNERS

This is a VFO controlled low power transmitter for the LF bands, from audio frequencies up to about 400kHz or so. To provide very high performance and at the same time save cost and simplify construction, the VFO is a digital type, using a readily available general purpose microcontroller chip. The unit is set up by a PC or computer terminal, but once set, will operate fine all by itself. It has a sweep generator mode and a built in message keyer (see pictures). The Exciter will cost about $50 to build, and there's nothing tricky about construction. You can have somebody program the controller for you and operate it with a "terminal" program like you would a packet TNC.



The Exciter beacon transmitting QRSS ASK Morse (3 sec dots)
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HARDWARE DESCRIPTION

Take a look at the Block Diagram.

The heart of the Exciter is an Atmel AT90S2313 microcontroller. This is a small 20 pin device which looks just like a conventional logic chip. It operates from 5V and consumes very little power. It is controlled via a serial link, and provides two outputs - a "transmit" LED, and an eight-bit (256 level) signal output. The signal output is a sinewave with low distortion. The sinewave output has a low pass filter to remove the sampling frequency and alias frequency products.

An HEF4060 oscillator - divider provides a stable low frequency reference for symbol (dot) timing. The output is nominally 64Hz.

An optional 1W power amplifier is suggested. This consists of a Philips TDA7052A or similar audio amplifier which has suitably high frequency response. The Philips device also has DC power level control, and can control power output over a range greater than 60dB, without adding distortion. The 1W output of the amplifier is transformer coupled to the antenna or a further power amplifier.

Depending on the application, you may not even need the power amplifier, as the Exciter can generate 5V p-p or drive 250mV p-p into 50 Ohm (-8dBm).



A view inside the Exciter case. Note the amount of spare space!
The 4060 divider is top left of the board, the AVR micro below it, and the tiny TDA7052A
amplifier just above the toroidal output transformer.
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The Exciter is intended to operate from +12V DC. The power supply includes reverse voltage protection and a 5V regulator for the microcontroller. The power amplifier operates from 12V, and is internally protected against thermal mismatch overload. No negative supplies are required as the RS232 driver configuration uses the negative voltage supplied by the host PC or terminal.

The microcontroller draws about 25mA. The optional 1W power amplifier draws a further 100mA, and so the whole unit can be conveniently operated from a small 9 - 12V battery supply. Output is 5V p-p open circuit (direct) or 1W into 10 - 50 Ohm using the power amplifier. Construction cost should not exceed $50. The microcontroller programming software is free and easy to use; a suitable programming cable can be made for $10 or so. Nothing else is required, as the controller is programmed on the Exciter board.

The whole circuit can be constructed on a 100mm x 150mm project board, preferably the type with copper spots rather than stripes, as this makes construction simpler and more compact. Most connections are made by soldering component leads together on the underside of the board, or by using tinned copper wire to "join the dots". Some places where such connections are difficult to achieve (for example connecting the controller to the programming header), the points are connected by fine magnet wire (insulated copper) soldered under the board after all the other connections are made. Since the unit is so compact and there is no need for a front panel, the unit can be assembled in a small plastic or die-cast box. The unit could also be completely waterproof for reliable outdoor use.



A closeup of the Exciter circuit board
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Schematic Sheet 1 contains the microcontroller, +5V regulator, D-A converters and serial port interface. Schematic Sheet 2 has the power amplifier, power supply and output connections. You can see from these drawings and the above pictures that the design is indeed very simple. The Parts List also demonstrates the simplicity of the design (only 59 parts) and the general use of very common components.

FREQUENCY ACCURACY AND STABILITY

Click here to see details.

SERIAL PORT CONTROL

The user can directly command the Exciter frequency, operating mode, control various output ports, and turn the transmitter on and off. The current settings, including some used only for beacon transmission, can be stored in the Exciter for automatic recall when power is reapplied. In other words, once the operating paramaters are set, the unit will continue to operate without the PC or terminal. A built-in user programmable "script" can continue to control the Exciter in a stand alone manner, setting transmission mode, baud rate, output ports and FSK offset as required.

With a more advanced PC program, the unit can also be commanded to transmit a wide range of complex modes, from ASK and DFSK Morse through to Sequential MT-Hell and Jason (synchronous differential FSK). In this advanced scenario there is no lower limit to the data rate, so just about anything is possible. Since the communications protocol is simple and well documented, the user can write an application to achieve exactly what is wanted.

SERIAL COMMANDS

Click here for command table.

SERIAL COMMUNICATIONS PROTOCOL

Each command consists of one command letter (upper or lower case) and zero to six following hexadecimal characters. No "Enter" or other control characters follow. The correct number of characters must be used. Unexpected characters, misunderstood commands and some impossible values will result in the Microcontroller responding "?", and the synthesizer will continue with the current settings. If a command is not completed, the synthesizer will remain stopped. Sending any wrong character (such as <ENTER>) will restore operation.

It is quite possible to send impractical frequencies (even negative frequencies) and impractical sweep step sizes. Negative frequencies (the hexadecimal values are signed, so those from Fffffff downwards are negative) result in reversed table stepping - no difference will be noted except that ADD offsets will be down in frequency, sweeping will be reversed, and FSK and MFSK shifts will be negative. The Nyquist Frequency occurs with a setting of F800000 and would generate meaningless signals if approached.

When the device is first powered up, it issues a message identifying itself, including the firmware version number. If using a dumb terminal, start the terminal before the Exciter. If using the PC software, start the Exciter first.

The A ADD, F FREQUENCY, T TX and A RX commands are used for "normal" CW mode serial port control. With a combination of these commands, just about anything can be transmitted. The maximum sending speed is limited by the serial communications. For example, ASK using the T and X commands can be used up to about 100 baud; FSK using the A command up to about 30 baud. Frequency change using the F command takes about 5ms, limiting the speed rather more. Timing of course takes place in the controlling PC. Remember that this speed restriction is a serial communications limitation - the actual synthesizer can change frequency virtually instantly. For example, in sweep mode the steps take about 2us to effect.

The W WIDTH command is used for sweep generator operation. This sets sweep width (actually the number of frequency steps). There are from 2 to 255 sweep steps. The step frequency increment is set by the K command, with a resolution of about 0.08 Hz (1 to 65535 times the synthesizer step size). The frequency of the first step is given by the F command, and the dwell time (time on each step) by the A command.

An LED and digital output are set when the sweep is at the start frequency. This output is useful for triggering an oscilloscope. Setting W00 turns off sweep mode. The T and X commands also operate in this mode. The beacon mode has three additional commands, BEACON, KEY and MODE. All the other commands also operate while the beacon is running. There are several other useful commands, which apply in all modes. These are HELP, PORT, REPORT and STORE.

To help you work out the numbers for manual control, use the really cool SPREADSHEET provided.

SCRIPT CONTROL

A much simpler command interpreter operates from a 'script" saved in the microcontroller EEPROM memory. The script can contain data to be transmitted, mode commands, shift and speed commands, plus frequency settings and port controls. Commands always have a first character "F", and everything else is considered to be data. Thus you can't transmit data values "F0" to "FF".

There are six script commands:


MODE Fn
Set the mode to n, where n is 1 to 6. In version D5A or earlier, you can't set Mode 0 (F0), as the command is ignored. In version D5b the Mode 0 command is used to halt beacon mode and script processing. Normal operation is restored by resetting the micro. This allows the Exciter beacon to be synchronised to an external timer.
PORT FBpp
Set the three output ports PD2, PD3, PD4 according to least significant three bits of pp. Other bits ignored. See the serial P command for details.
FREQ FChhmmll
Set the frequency to hhmmll (hexadecimal), where the number represents the required frequency multipied by the DDS resolution. Exactly the same as the serial F command.
SHIFT FDnn
Set the carrier offset for FSK/MFSK to nn. See the serial comms A command for details. Fox example a shift of 1 Hz is achieved using FD 0C. Has no effect on ASK (Mode 1).
SPEED FEnnnn
Keying speed 0000 to FFFF, as defined in the serial comms K command. The baud rate is 32/nnnn, or dots of nnnn/32 second duration.
END FF
End of script. Next data read will be at the start of the script.

The commands can appear in any order, distributed through the message. And yes, you can QSY in the middle of a message, so dual frequency beacon operation is a definite possibility! Indeed, just about anything you can think of is possible. Here's a script command example which sets A08 KE M2 (shift 08, keying speed E, mode 2):

	FD 08 FE 000E F2
While the commands are easy enough to understand, loading the data for the script is a little more complex. Fortunately this is not something you need to do on a daily basis! The Exciter does not have code-space for text to Morse or text to graphics conversion, so the message must be hand crafted or generated by an external compiler program. Two different techniques are used -

The ASK, FSK and DFSK modes (M=1, 2, 3) use a standard "Murphy" algorithm, where dots are recorded as "0" and dashes as "1", from LSB upwards, with a trailing "1" added to mark the end of the character and allow precision keying with perfect timing. For each dot or dash to be sent, a following dot space is generated (unless it's the last bit of the character), and then the three-dot character space is sent. This technique is used (rather than sending one dot space at the end of the character followed by a further two for the character space) as it generates cleaner narrow band signals in FSK mode. DFSK is even more complex, as the dot-spaces can be omitted if the following bit (dot or dash) is different from the previous one! A table of "Murphy" Morse characters appears further down this page.

Here's an example which sends "ZL1BPU":

	13 12 3E 11 16 0C
The MFSK and HELL modes works quite differently to the other modes - to allow maximum graphics flexibility, they are fully bit-mapped. All eight data bits in each byte are scanned in exactly the same way, with "1" meaning transmitter on, and "0" being transmitter off, but in MFSK mode the frequency sent is stepped by the shift value for each successive bit in the byte. Thus the first bit is exactly on the set frequency, and the others are 1 to 7 steps higher. This technique allows visible text and simple images to be sent using Sequential Multi-tone Hell (see the picture at the top of this page). The following example sends a smiley face:
	7E 81 95 85 95 81 7E 00
The HELL mode is similar, except a single frequency is used, and data is recovered by precise scanning of a facsimile amplitude display, exactly like Feld-Hell. The dot time needs to be set with some precision, and the mode is very effective at dot times of 125 ms or longer. 125ms is convenient as it allows exactly one second per column. The font can be the same as for MT-Hell, but some tweaking may improve appearance, and text may look better with columns repeated. In the example below, each column is sent four times, and each character takes about eight seconds to transmit.


Weak signal Feld-Hell with 125ms dots
(Generated using the internal beacon.)

Text can also be sent efficiently in "Murray", a fun new MFSK mode I have invented and named in honour of Donald Murray, who developed what has become the ITA2 code used for RTTY. This mode is really simple to code - simply look up the ITA2 code for the character in an ARRL Handbook or other code reference, and type in the values! For example, here's "ZL1BPU " in "Murray":
	1F 11 12 1B 17 1F 19 16 07 04 00
Take a look at the MFSK example at the top of this page - the stuff to the right of the text is "ZL1BPU " in "Murray" mode. Although they look really cool, the MFSK modes are much less sensitive than ASK or FSK modes, since in general less time is spent on each dot. However these modes are MUCH faster - perhaps three times as fast as ASK Morse. MFSK works best at one ARGO speed slower than the other modes.

The Mode commands are very useful for transmitting messages in several different modes, one after another. Remember to set the sending speed as necessary to suit the mode in question (to give best ARGO readability, for example). The message in each mode needs to be recorded separately after the mode command. Periods of key-up or key-down silence can be transmitted using the HELL mode (Mode 5 or F5), using a very slow baud rate for long delay periods.

To use Mode 0 (F0) correctly, utilize it only as the last command in the message, and preferably preceed it with a zero frequency command (FC 00 00 00). Then set the default mode to a beacon mode (anything but Mode 0), set the default speed, frequency and keying speed, and then save them with the S command. Then, whenever the micro is reset, either by pulling the reset pin (pin 1) low, removing and restoring power, or sending a HELP serial command H, the beacon will start automatically, and continue until it reaches the Mode 0 command, when it will halt. The message should preferably be arranged so that it fills most of the time between resets. The Mode 0 (F0) script command only works in firmware version D5b and later.

NORMAL CW MODE

In this mode (M0) the generator acts just like a VFO. It can be used as a signal source, receiver local oscillator, or low powered transmitter. The carrier can be keyed on and off (T and X), and the frequency set anywhere within the operating range with high precision. The PC software supplied allows the frequency to be tuned up and down in 10Hz and 1kHz steps, but also allows incremental tuning right down to the maximum resolution (typically 0.1 Hz).

The communications link operates in hexadecimal, so although you can control the unit quite adequately and very precisely from a simple terminal, setting the frequency takes some thinking about! The number you need to enter must be the frequency you require divided by the synthesizer resolution, and then converted to hexadecimal.

For example, say you want to set the frequency to 181.000 kHz, and you are using a 10.000 MHz reference for the Exciter microcontroller. First, the resolution will be 10000000/150994944 = 0.066227383 Hz. The lowest frequency you can set is this frequency, and all other frequencies are in steps of this amount. So, to set to 181.000kHz, you need to set the synthesizer to 181000/0.066227383 = 2733008.4864. You can't set fractional steps, so you use 2733009, which is 29B3D1 hexadecimal. (A good HEX calculator is a must - I use the Win98 calculator in scientific mode.) If you set the Exciter to 29B3D1, the frequency generated will be 0.066227383 x 2733009 = 181000.0340 Hz, an error of only 0.03Hz. Of course the actual accuracy may be worse than the resolution achievable!

The PC software provided works all this out for you, so once you've set the clock frequency in the setup file, you simply enter the frequency you require in decimal and the program does the rest. Many of the other commands also use hexadecimal, but since they use one or at the most two characters, it's easy to handle these in your head.

The A ADD command, used for adding offsets, is not used much in this mode, although it is used by programs which generate FSK or MFSK. You can use the ADD command to check what the effect of such a shift would be on the CW carrier. The frequency range is 00 to FF, so a shift from zero to 255 resolution steps (typically 16Hz) can be set. Obviously A00 returns the carrier to the frequency set by the F command. Changing the frequency with the F command also removes any previous offset. For chifts larger than FF, you'll need to use the F command.

The T command starts the transmitter. The X command stops the transmitter. If you check the output of the A-D converter with an oscilloscope, you'll see that the output just stops dead, leaving a DC voltage on the output, and when it starts up again, it starts from the same point. The output is DC coupled! The amplifier output is transformer coupled, so you don't notice this effect. Using external hardware, you can experiment with ramping the power down before using the X command, and again ramping the power up after the T command starts the transmitter. This should reduce the keying transients for narrow-band applications, but all control will need to be external. In practice the transients caused by simply turning the transmitter on and off have not caused a problem.

The maximum usable frequency depends on the low pass filter used, and on the synthesizer crystal frequency. A good guideline is to keep the output frequency below 1/30th of the crystal frequency, or about F4C0000 hex (so 333kHz using a 10MHz crystal). Above this the signal quality becomes quite degraded as aliasing occurs due to inadequate sampling, which the low pass filter cannot prevent. At a value of F800000 the generator crosses over into generating "negative frequencies", which are little use except with assymetric waveforms, MFSK transmissions or sweep mode. The FREQUENCY command value is "signed", so using the example above for 181 kHz, you can just as easily generate this frequency with a setting of FD64C2F as F29B3D1, but in this case the offsets set with the ADD command move the frequency down, not up!

The user interface to the Exciter is rather like that from a packet radio TNC - you can talk to it directly since the protocol is reasonably straightworward, and the Exciter talks back to you. The H command will list all the possibilities, and the R command will report the current settings. With a computer control program you don't need to think about these "KISS" mode commands as the program manages them all for you.

SWEEP MODE

Perhaps one of the most useful features of this Exciter, this mode allows you to sweep antennas, filters and antenna tuners for correct adjustment. When used with a Return Loss Bridge (like an SWR meter with no meter), or an Impedance Bridge, you can plot the performance of your antenna and feed system very simply. Setting the sweep mode functions requires care.

The sweep mode is best used with 20 discrete frequency steps (A14), and differs from conventional sweep generators because these are discrete steps, not a smooth sweep, and also in that the frequencies generated by the Exciter are very precise. There can be from 2 to 255 sweep steps. The sweep start frequency is set by the F command (as you would expect), and the number of steps (2 - 255) is set by the W WIDTH command. A setting of W00 turns off sweep mode. The KEYING command K sets the precise step size. The range is W0001 to WFFFF, and since the step size is the same as the synthesizer resolution, you can step from about 0.08Hz per step to over 4kHz per step.

Note: Previous versions of this software had a fixed sweep facility with 20 steps and 5ms dwell time per step. This mode can be replicated by setting A3E W14 and multiplying previous frequency steps by 256, set with the Bnnnn command.
The sweeping occurs continuously, with each frequency generated for the set dwell time, typically 5ms (A3E). The TX LED is turned on for the duration of the first step. Using an oscilloscope, trigger the sweep off the LED output and with the sweep set to 10ms/div, the 20 steps will nicely line up across the screen. A1F works well with a 5ms/div timebase.

You can also make the sweep generator sweep backwards, by setting a negative start frequency, although why one would need to do this isn't obvious! It will also sweep through zero, but once again, that's not much use. For audio sweeping, try setting the start frequency to zero (F000000), and use a narrow sweep increment (e.g. K0498 for 100 Hz/step).

All sweep generator parameters can be adjusted while the sweep generator is running.

BEACON MODES

The internal beacon is not as versatile as generating signals using PC control, where the sky's the limit, but there's still plenty of capability. There are six keying modes - ASK (M1) where the transmitter is fixed in frequency and simply keyed on and off; FSK (M2) where the transmitter stays on, and is shifted back and forth in frequency; DFSK (M3), where dots are sent unshifted and dashes sent as shifted dots; MFSK mode (M4), where eight separate frequencies are generated in turn, and keyed on and off as required to generate a bit-map; Feld-Hell mode (M5), where dots are on the same frequency, and keyed on and off at precise times to create a bit-map; and MFSK/IFK data mode (M6), where frequency steps or increments are transmitted, typically for JASON.

One of the most unusual features of the beacon modes is that the normal mode commands still work while the beacon is running! Thus you can change frequency, experiment with the FSK shift, and change keying speed, without coming out of beacon mode. This is because of the event driven nature of the firmware. Obviously if there are similar commands in the beacon script, any manual changes made cease to affect the output when one of these is read.

The data for beacon mode is stored in the microcontroller EEPROM memory, and so can be changed at will. The message can be compiled and downloaded using the microcontroller development tools. Alternatively, you can load a new message with the B BEACON command, which accepts more bytes of data than you will have patience to enter (around 112). The data includes transmitted Morse data, graphics and data bits, eight per byte, and commands, as outlined in the section on Script Control. You can change the mode, shift and keying speed from within the message script. See the section on Script Control (above) for details of the commands and message data storage.

The data for beacon mode has to be worked out in a very laborious manner, but since this is not done very often is not a big chore. It probably takes 15 minutes to work out a new message by hand. There is a computer message compiler provided, which will handle anything you can visually describe. A table of "Murphy" coded characters for Morse generation is shown below. For MFSK (MT-Hell) and Feld-Hell, use your imagination!

Morse Coding Table
SP  01     !   00     "   52     #   00     $   C8     %   01     &   01     '   5E
(   2D     )   6D     *   01     +AR 2A     ,   73     -   61     .   6A     /   29
0   3F     1   3E     2   3C     3   38     4   30     5   20     6   21     7   23
8   27     9   2F     :   47     ;   35     <   00     =BT 31     >   00     ?   4C
A   06     B   11     C   15     D   09     E   02     F   14     G   08     H   10
I   04     J   1E     K   0D     L   12     M   07     N   05     O   0F     P   16
Q   1B     R   0A     S   08     T   03     U   0C     V   18     W   0E     X   19
Y   1D     Z   13     [   00     \SK 68     ]   00     ^   00     _   6C     `   00

The table values are in Hexadecimal. Characters with entries "00" are not transmitted. By changing keying speed and mode within the script it is easy to send 5 WPM Morse, 3 sec dot Morse, 3 sec dot DFSK Morse and Sequential MT-Hell all in the same message. Periods of carrier-off can be generated by bytes of 00 with a slow keying speed. The example below shows an FSK Morse beacon transmission using the internal script and 3-second dots.



The Exciter transmitting FSK Morse (3 sec dots)
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FIRMWARE DETAILS

The microcontroller code is essentially event driven. After initialization, the DDS loop operates in the main program, looping around only six instructions continuously, providing a loop frequency and therefore output sample rate of over 1 MHz. There are three other code threads - the serial comms and command interpreter (including the script entry code) is in the UART interrupt, which can enable and disable the timers and the two other interrupts on the fly. You can imagine the potential for debugging headaches! The sweep function operates in the T0 interrupt, which is fixed at 5ms, while the beacon modes and script command interpreter operate in the T1 interrupt, which uses an eternal 32 Hz source, and is user settable from 1/32 sec down to a maximum of about 2047 seconds, or about 34 minutes. Unfortunately it is not possible to chain the internal timers to achieve long keying times since the interrupts generated would cause glitches in the synthesizer D-A output. The DDS generator of course stops while interrupts are processed, which can be from under 2us (sweep generator) to many seconds (hand typed commands).

Code was written in assembler, using the free Atmel WAVRASM assembler. Code occupies all of the program memory. 236 words are used for tables, including the 256 byte DDS lookup table. Of the available 1024 words, all but 4 words are used! RAM is not used at all: the whole operation is register oriented, and all but about three registers are used. The source code file is about 30k in size and contains over 1300 lines, although it is quite heavily documented. Default settings are hard coded: user settings are stored in EEPROM, along with RS232 comms data rate and up to about 100 bytes of user script. Code development took about six weeks of evenings and weekends. Development time was much reduced by the similarity in techniques to the AVR Signal Generator already designed. The final fully working and debugged version is about the 50th build of the fourth revision of the fifth version of the code. No processors were damaged and no animal testing was involved during research and development of this product. No lives were lost.


Copyright Murray Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.