DIY low-power transmitter projects: Microprocessor-Controlled FM Stereo Transmitter for 88-108 MHz MPX2000

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Theory of Operation

The MPX2000 is a full-featured FM stereo transmitter that offers LED frequency display, keypad frequency entry, a calibrated LED bar graph modulation meter with overmodulation and lock indicator LEDs, and full coverage of the 88-108 MHz band in 100-kHz steps. This unit is intended for use by hobbyists, experimenters, and others needing a low-power, short-range FM stereo transmitter. It is intended for music and/or speech. The left and right audio channels may be used independently to transmit two separate audio channels. This is useful for school and church use, when multilingual capability is desirable. The MPX2000 is also suitable for hobby Part 15 operation when a transmitter with digital frequency readout and keypad entry of frequency is desirable. The MPX2000 is fully crystal controlled, and the pilot and sub carrier signals are also generated from a quartz crystal, ensuring stability and accuracy. A phase locked loop (PLL) system ensures frequency accuracy and crystal stability, and the use of a keypad and microprocessor permit ease of frequency entry without having to set jumpers or switches, and ensure that the programmed frequency is correctly produced. For a tutorial on FM stereo signal generation, please refer to Section 7.

MPX2000 Circuitry

We now discuss the MPX2000 circuitry. Refer to the schematic and the block diagram (Figures 1, 2, and 3) for this discussion. Audio input at line level (1 volt peak to peak at a 500-10,000 ohm impedance) is fed into jacks J5 and J6, which are left and right audio input channels, respectively. R1 through R4, with R7, make up an input network, and R7 is used to obtain an equal audio gain for both L (left) and R (right) channels. Q1 and C1, R5, R8, R10, and R14 make up the L audio preamp, with a gain of about 5 at 1000 Hz. Gain is shaped to produce a 75-microsecond pre emphasis that is standard on FM receivers used in North America. The R channel is identical (Q2 and associated components). C5 and R16 feed amplified audio into a clipper consisting of diodes D7 and D8. These are biased by voltages from network R74, R75, R76, and R77. Audio is limited to 3 volts peak-to-peak by these diodes.

The R channel uses the same circuitry. The purpose of the clipping is to prevent exceeding the maximum deviation (75 kHz) with excessive audio input. The clipper acts as an audio limiter. Next, both channels are fed into a two-pole active filter consisting of a quad op-amp IC1, a TLO84N, resistors R20, R22, R24, and R26, and capacitors C7 and C9 (left channel). The right channel has identical circuitry. The active filters cut off at 15,000 Hz and reduce aliasing distortion.

FIG. 1 Schematic of the MPX2000 PLL Synthesized Low-Power FM Stereo Transmitter

FIG. 2 Block Diagram of the MPX2000 Low-Power FM Stereo Transmitter

FIG. 3 Schematic of the MPX2000 Display Board

Next, the audio signals are fed to the matrixing network using the other two sections of IC1. In one amplifier, the L and R channels are summed to produce the (L+R) signal and the (L-R) signal. Both of these signals are necessary for stereo generation. The (L+R) signal is the main monophonic channel that occupies the band 20-15,000 Hz and is the signal received in a monophonic FM receiver. This allows stereo to be received as mono on an FM receiver without a stereo decoder, ensuring compatibility. The (L+R) signal is fed to summing amplifier IC3 via R57, where it is combined with two other signals, the (L-R) subcarrier signal and the 19-kHz pilot. The (L-R) signal is the difference between the channels. This audio signal has the same frequency components as the main (L+R) signal and cannot be transmitted in the same channel as the (L+R). To solve this problem, the (L-R) signal is then modulated on a subcarrier at 38 kHz. The result is a double sideband AM suppressed carrier signal occupying a band from 23-53 kHz (38 kHz plus and minus up to 15 kHz). It is produced with a balanced modulator circuit IC2 and its associated components. (L-R) audio is fed into IC2 via R35, R36, and C11. Pins 1 and 4 of IC2 are fed DC bias via R37, R38, R39, and R41. R40 is used to balance the voltages at pins 1 and 4. Balance occurs when they are identical. R44 sets the circuit gain, and R42, R43, R45, R46, and R47 are biasing resistors for the input and output of IC2.

The 38-kHz subcarrier is inputted at pin 8, and output appears at pin 12. In the absence of input audio, output is zero, and in practice R40 is adjusted to null the 38-kHz output with no (L-R) input. The 38-kHz signal is obtained from frequency divider IC5, a CD4040BE.

This IC is driven by a crystal oscillator/buffer at 4864 kHz, made up from two sections of a 74C00N NAND gate. C14, C15, and IC1 make up the oscillator circuit.

In addition, a 19-kHz signal for the pilot carrier and a 1187.5-Hz signal useful for testing is obtained from the CD4040BE at pins 13 and 1, respectively. The 1187.5 Hz signal is taken off through a series resistor R49 and can be used to test the audio section, but is otherwise not used in the system. The subcarrier output is taken off through C13 and level control R55 and is fed to summing amplifier IC3. The output of IC3 is the sum of (L+R) and the (L-R) signals. It is fed to network C17-R61 via R60. In addition, the 19-kHz pilot signal from pin 13 IC5 is fed to this point via attenuator R51, R53, and R54, where it is reduced to a level 7-10 percent of the peak audio level. This 19-kHz signal is used by the receiver to allow it to generate a 38-kHz subcarrier for recovery of the (L-R) audio component from the 38-kHz subcarrier signal received along with the main (L+R) signal. By adding and subtracting the (L+R) and (L-R) signals, the result is 2L and 2R, which are the individual L and R audio channels. The signal appearing across network R17-C61 is called the base band or composite stereo multiplex signal. This signal is fed to the transmitter modulator. In order to ensure good audio quality on the transmitted signal, some audio level monitoring is desirable. The MPX2000 uses a 10-segment bar graph level indicator, which consists of 10 LEDs mounted side by side to simulate a solid-state meter movement. In addition, another LED is provided, which lights when the peak audio level that would result in overmodulation is reached.

Audio from the summing amplifier is taken via R59, fed to an op-amp IC3 section, and then rectified to produce a DC level corresponding to the peak audio signal.

R65 sets the gain of this stage to unity, and C19, D13, D14, and C20 make up a half wave voltage doubler detector. D11, D12, and R64 are used to improve the linearity of the circuit at lower signal levels by increasing the gain of IC3 to permit low-level signals to overcome the 0.6-volt drop that is normally encountered in silicon diodes.

The audio components are removed by R67 and C21 and applied across potentiometer R68. The DC voltage is fed to the bar graph level display via R78. A part of this voltage appearing from the wiper of R68 to ground is fed to the base of Q3. The emitter of Q3 is biased at 1.5 volts via R72 and R73. Potentiometer R68 is adjusted so that when the audio signal is sufficient to light all 10 segments of the bar graph display, plus 10 percent extra, sufficient voltage (2.2 volts) is available at the base of Q3 to cause it to conduct. This turns on Q4 via R70 and R71, causing voltage to appear from the collector of Q4 to ground. This is fed to an LED on the display, lighting it, and indicating a condition of overmodulation. (The display panel is described separately in another part of this discussion.) Next, an RF carrier that can be frequency modulated is required. It should be variable from 88-108 MHz to cover the entire FM broadcast band, and it must be stable within 10 kHz or better. The MPX2000 uses a PLL frequency synthesizer to generate all 200 channels at 100-kHz spacing, between 88.1 and 107.9 MHz, the standard FM broadcast band. The synthesizer is frequency modulated by applying audio to the voltage-controlled oscillator (VCO) in the synthesizer. This process is a contradiction in a sense because we want the VCO to be stable and yet be able to vary the frequency. With proper design, this goal can be accomplished without any serious compromises in transmitted audio quality or synthesizer performance. The VCO is the heart of the transmitter, and the synthesizer circuitry keeps it exactly on the desired transmit frequency. Free-running oscillators were used in the past for this application and are still used in low-end, low-cost FM transmitters.

It is difficult to keep these transmitters on frequency, and therefore they can be hard to use, especially with digitally tuned receivers. Stability of better than 100 kHz is difficult to come by with this approach; however, the use of frequency synthesis eliminates this problem. The VCO output must be buffered to eliminate frequency pulling caused by varying antenna loads, proximity effects, and so forth. In the MPX2000, FET Q8 is the VCO, and L1 and the combined capacitances of tuning varactor D4 and modulator varactor D3, together with inductor L1, make up the LC tuned circuit that determines the transmitter frequency. VCO FET Q8 is biased by R129 and R136. The drain of Q8 obtains DC from R136. Feedback is from a tap on L1 and is fed to the source of Q3 via C42. C40 and R130 couple some oscillator out put from the VCO to buffer amplifier circuit Q9-Q10, which is made up of R132, R133, R134, and capacitors C34 and C35. L2, C33, C32, L3, and C31 make up an RF output network and harmonic filter for the buffer amplifier. R139, R140, and R141 ensure proper termination of this network and feed RF to the antenna. IC12, R135, R138, and IC12 make up a regulator circuit to feed regulated 5.6 volts to the buffer amplifier. In case of synthesizer malfunction, a DC level from the synthesizer IC is fed to R131 and cuts off the buffer, killing the RF output and reducing the possibility of transmitting outside the FM band.

The VCO is tuned by a voltage from the PLL chip phase/frequency detector. The PLL chip (IC9) is a Motorola MC145170-2. Inside this chip, a sample of the VCO signal is compared with a reference frequency; if they are not in phase and frequency agreement, an error voltage is generated. This technique is used to change the tuning voltage on the VCO, such as to achieve frequency and phase agreement (lock). R126 and C43 feed a sample of the VCO signal to amplifier Q7. Collector resistor R115 biases Q7, and the signal to drive IC9 appears across it. C44 couples this signal to IC9. IC13 generates a clock signal at 4.000 MHz via XTAL1, trimmer C49, and C50. C49 is used to adjust the frequency to exactly 4.000 MHz. The 4.000-MHz signal is used both as a reference signal for the PLL synthesizer and for microcontroller IC8. Two sections of IC13 are used as buffers to provide this signal to IC8 and IC9.

The synthesizer frequency depends on several data words programmed into IC9.

The needed data words are provided by IC8, a PIC16F84 microcontroller. This microcontroller is programmed with the necessary software to program IC9 and to manage other tasks, such as the control of the frequency display and scanning the keypad for frequency entry. Other functions necessary are system shutdown in case of malfunction, rejection of out-of-band frequency entries, and remembering and setting previously used frequency on powering up of the transmitter. When a valid frequency is entered, a voltage appears at the phase detector output pin (13) of IC9.

This voltage is fed to network R118, R119, R120, and C46. This network deter mines some of the loop characteristics of the synthesizer. The output of this network is fed to op-amp IC10 and then to tuning varactor D4. At lockup, this will be a steady DC voltage, varying from 3-4 volts at the low end to as much as 10 volts at the high end of the FM band.

In case of loss of lock, IC9 produces pulses at pin 11. These pulses are integrated by R114 and C48. The DC voltage turns on Q6, placing 5 volts across R113 and R112, and D6. This sends a voltage to the unlock indicator LED D201 and cuts off the RF buffer via D6 and R131. In addition, the rising voltage is coupled to the base of Q5 via R142 and C51, turning on Q5 momentarily. This step causes Q5 to con duct, resetting the microcontroller via R108 and resetting network D5-R107. Capacitive coupling is used to couple signal to Q5 so as not to permanently cause a reset of the microcontroller, otherwise a lockup condition will occur. Normally, this reset ting process is a sufficient cure, if the problem is incorrect frequency entry or a "glitch" because resetting the microcontroller reprograms IC9 with the correct data.

During this process, the RF output is disabled.

Modulation is achieved by applying baseband audio from R61-C17 to both varactors D3 and D4. This allows better modulation and fewer compromises than if audio were applied to the tuning varactor D3 alone. IC7 provides a regulated 12 volts to the PLL and helps filter out any noise disturbances appearing on the power supply.

The audio section is supplied with 12 volts regulated from IC6, and IC11 supplies 5 volts to the microcontroller, display logic, and PLL synthesizer IC. The use of separate regulators for audio and digital functions helps reduce circuit noise. DC input to the MPX2000 should be between 15 and 20 volts to allow sufficient headroom for the regulators, yet keeping dissipation within reasonable limits. (This can be reduced a few volts with low dropout regulators.) Care must be taken to use a wall transformer that is adequately filtered so that the input voltage waveform to the MPX2000 never gets below 15 volts. With less than 15 volts input, DC voltage is insufficient to allow full VCO control voltage swing, and some of the 88-108 MHz tuning range will be lost at the high-frequency end. Also, adequate RF decoupling of the DC supply is needed to reduce the possibility of RF-induced ground hum on the transmitted signal.

RF chokes may be needed in both power leads in certain situations.

Next, the display board circuitry is discussed ( FIG. 3). This board contains a 4-by-3 matrix of touch switches, a four-digit LED multiplexed display, and the bar graph LED and its associated driver, an LM3914 (IC204). Additionally, three other LEDs that serve as overmodulation (D202), PLL unlock (D201), and SCA subcarrier ON (D203) are also included on this board. The keyboard is polled periodically by the microcontroller for a switch closure by applying a logic level to a row and looking to see if this level appears on one of the three columns. Each switch has a unique row and column location, and the switches are scanned sequentially, 1 through 10 (10 is represented by 0), and two other keys, E (enter) and CE (clear entry). The desired frequency is entered, with the most significant digit first.

Because we have four digits, for frequencies below 100.0 MHz, the most significant digit is 0. This 0 does not really have to be entered, although it is recommended to do so. This eliminates possible "glitches" or entry errors and fully clears the key board memory. When the first entry is made, two zeros appear on the display to the left of the entered digit. The leftmost is blanked on leading zeroes. For example, 99.5 MHz is displayed as 995 instead of 0995, but no changes are made in the PLL programming or the transmit frequency until the enter (E) key is pressed. The digits appear on the display as they are entered and shift right to left. Four digits are to be entered, and if more than four digits are entered, the leftmost is shifted out. After the display shows the desired frequency, the enter key is pressed, and if it is a valid (legal FM channel) entry, the display will retain it, the PLL will shift the transmitter frequency to it, and it will also be stored in memory. This frequency will come up when the MPX2000 is powered up the next time. If an illegal entry (lower than 88.1 or higher than 107.9) is made, the microcontroller will reject it and simply revert to and display the current frequency. If an error is made during entry, the CE key can be pressed, and the current frequency will once again appear. When the MPX2000 is powered down, the current frequency is retained in memory and reappears on the next power up. No memory backup battery is needed for the microcontroller.

The display section is a conventional four-digit multiplexed display using four 7 segment common anode LED digits, driven by a 7447N TTL driver IC. R209 to R215 are current limiting resistors for the individual segments. No decimal point is used in this display. There are eight logic inputs from the microcontroller, which are feed a 74HC573N eight-bit latch IC201. The digit data is latched into the 74HC573N by a strobe pulse from the microcontroller at the appropriate time, and this data contains the binary value of the particular digit and its position on the display. The digit select information is decoded by a 74HC138N decoder, and its output turns on one of four 2N3906 switching transistors Q201-Q204 via bias resistors R201-R208. L201, C206, and L202 are noise suppression chokes to reduce switching noise. The display segments are operated at 20 mA each, and if an 8 digit is shown, 140 mA must be switched by the 2N3906 associated with that digit. Chip capacitors C202-C205 slow down switching speeds to further reduce noise spikes.

Although an LED display can be noisy and consumes a lot of current, it is much brighter, has more eye appeal, and is easier to read than an LCD. LED readouts need no illumination. The display is shut down by the microcontroller about 15 seconds after the last keypad press, cutting off the display multiplexing and leaving a few segments lit on the least significant digit to serve as a power-on indicator. This approach eliminates residual switching noise generation and conserves current. The display can be awakened by pressing the 0 key to check the current frequency setting.

It will stay on for 15 seconds and go back to sleep.

The display and multiplexing could have been handled directly by the microcontroller without the three ICs, but the software overhead would be larger and the cur rent of the LED display would be too much to be directly handled. This approach was tried, but the display was too dim, performance of the microcontroller was a little slow, and some additional "glue" circuitry was needed because of the limited number of pins that were available on the PIC16F84. In cases such as this one, a hardware versus software tradeoff has to be made. The hardware multiplexing approach was used here because it gave the best results. A larger microcontroller could also have been employed, but this option was not investigated.

The bar graph display is conventional and uses an LM3914 linear bar graph driver (IC204). DC input from the main board is applied to pin 5 of IC204, and the sensitivity is determined by the setting of R218. Approximately 0.8 volts DC is needed to light all 10 segments of LED display DS202. R218 is set so that 10 segments light when the main board audio system is producing full audio level just short of limiting.

This represents 100 percent modulation. R220 and R221 limit power dissipation in IC204, and C207 bypasses the Vcc line to the display. D201 is fed from the unlock detector on the main board and, when illuminated, indicates that the PLL is unlocked. D202 is powered from the main board and is illuminated when audio clip ping occurs, indicating overmodulation. D203 is used to indicate that the optional SCA audio subcarrier system is activated.

The MPX2000, when once programmed to a desired frequency, operates without the display board because its functions are mainly supervisory. The board may be disconnected during operation with no effect on the transmitted FM signal; however, it is needed for reprogramming of frequency.

Overall power requirements of the MPX2000 are 15-20 volts DC at 350 mA. The display consumes much of this current. If the display is asleep, this is about 160 mA. When the display is disconnected from the main board, current consumption drops to 125 mA. Operation below 15 volts is not recommended unless low-dropout regulators (LM2930, etc.) are substituted for the LM7812s used here. Operation above 20 volts may cause overheating of the 5-volt and 12-volt regulator ICs. Heatsinks should be fitted to these ICs if operation over 20 volts is expected. The DC input is polarity protected by D1, and accidental polarity reversal will do no harm; the MPX2000 will simply not operate and draw no current until the supply polarity is corrected.

Construction of the MPX2000

PC Board Assembly

The MPX2000 PC boards (see Figures 4 through 7), while having nothing very critical or difficult to handle by someone with a little PC board assembly experience, require a certain assembly sequence in order to avoid mistakes that lead to elusive problems. In particular, several through-hole connections (vias) are required to connect traces on both sides of the board.

Plated-through boards are great for mass production of PC boards. They reduce assembly cost and facilitate soldering; however, the use of homemade boards generally precludes plated-through holes. If you must remove a component from a plated through board, you will have a difficult time and will probably ruin the component and the PC board as well, unless you have a specialized workstation with a vacuum desoldering setup. It is normal and expected that you will make a few assembly errors in constructing any new project because you are doing the task for the first time and you are inexperienced with the assembly. With a plated-through board, you may need a replacement part or PC board if you make an error. Therefore, we do not recommend the use of plated-through PC boards in this project.

The boards are best assembled and tested circuit by circuit. First, the main board can be prepared, jumper vias installed, and a few parts for the power distribution circuitry assembled. Then, the board should be powered up and checks made for various voltages. A power supply of 15-20 volts DC and a DC voltmeter are needed for these tests-a VOM or a DVM will do. Next, the audio (MPX) circuitry can be assembled and tested. If it works, the display board should be assembled next. Then the microcontroller section is installed on the main board, which can be tested together with the display board if desired. After this step is done, the RF circuitry can be assembled and the entire MPX2000 can be checked out. At this time, the project is operational. No critical adjustments are needed, and it should work the first time with the default settings given in the assembly procedure that follows, assuming that you have made no mistakes. Although it is possible to simply "stuff " the PC boards and wait until after completion of assembly for testing, this is really not recommended unless you are very experienced.

FIG. 4 Component Side Parts Layout of the MPX2000 Main PC Board

FIG. 5A Component Side Parts Layout of the MPX2000 Display Board

FIG. 5B Chip Capacitor Placements Main and Display Boards Foil Sides

FIG. 6 Solder Side of the Main Board of the MPX2000

FIG. 7 Solder Side of the Display Board of the MPX2000

First, install all vias and any parts that connect to them, the regulators and DC power supply filtering, and bypassing components. Check your work and make sure that all connections are soldered. Connect +15 to +20 volts DC to D1 (positive lead) and the negative lead to ground on the PC board. Check for the voltages on the following list. (All voltages assume that the regulator ICs supply an exact 5 or 12 volts.

Because they have 5 percent tolerance, which is acceptable for this application, remember to allow for this variance if your voltages are a little low or high because they depend on exact regulator voltage.) Consult the parts placement diagram (FIG. 4) for the physical location of the test points as needed.

Jct. C26, C27, D1 (Input): +15 to +20 VDC

Jct. C28, IC6, IC11, IC12, IC7: +14.4 to +19.4 VDC

V31: +5.0 ±5% VDC

V1: +12.0 ±5% VDC

V6: +6.0 VDC

Wiper R40: +6.0 VDC

Jct. R74, R75, C53: +7.25 VDC

Jct. R76, R77, C55: +4.75 VDC

Jct. IC7, C30: +12.0 VDC

Jct. IC12, C36: +5 to more than +12.0 VDC (Should vary with R138. Set for +5.6.)

Next, inspect all voltage points (see parts placement diagram) to make sure they are soldered, and contact the traces on both sides of the board. These tests confirm all work so far and ensure that all sections of the PC board will get DC power and that all signal traces are intact. Then install the audio and MPX generator circuit components.

Audio and MPX Generator Checkout

Apply +15 to +20 volts as before to the DC input and check for the voltages on the following list. (It is assumed that all voltages that you obtained were as specified in step 1 checkout of the PC board, before the audio components were installed. Using 10 percent tolerance is acceptable.) Preset all potentiometers to the center of rotation, except R138.

Collector Q1 and Q2: +3.6 VDC

Pin 4 IC1: +12.0 VDC

Pins 1, 7, 8, 14 IC1: +6.0 VDC

Pins 1 and 4 IC2: +3.1 VDC, varying with R40

Pins 2 and 3 IC2: +2.4 VDC

Pins 8 and 10 IC2: +5.9 VDC

Pins 6 and 12 IC2: +8.5 VDC

Pin 5 IC2: +1.2 VDC

Pins 1, 7, 8, 14 IC3: +6.0 VDC

Collector Q4: 0 VDC

Collector Q3: +12.0 VDC

Emitter Q3: +1.5 VDC

Jct. R67, R68, R78, C21: 0 to +0.2 VDC

Pin 16 IC5: +7.5 to +9.0 VDC

Voltage Settings and Display Assembly

Next, set R40 so that the voltage between pins 1 and 4 of IC2 is zero. Make this adjustment with the most sensitive scale on your meter, to as low as 1 mV if possible.

If you have or can borrow a scope and audio generator, you can apply a 1-volt peak to-peak audio signal to the L and R inputs and trace the signals through the circuitry.

This technique is excellent for uncovering any errors before you proceed further and ensures that all is well so far. You can also use a source of stereo audio, such as a CD player, cassette player, or stereo receiver, and use an audio amplifier and speaker to trace the signals through the circuitry. You will not be able to hear the pilot and the subcarrier signals (unless you are a cat or a dog) because they are above the audible range of frequencies. Refer to the waveform diagram for audio waveforms. Adjust R7, R40, and R55 as needed to get these waveforms. If you cannot do this procedure because of a lack of test equipment, then leave all potentiometer adjustments where they are. If the DC voltages were all correct and your assembly is error free, all should be okay so far.

FIG. 8 Front Panel Assembly Details of the MPX2000

The display board consists of three separate circuits: the keypad, LED display circuitry, and the bar graph metering and status LEDs (see FIG. 8). These circuits should be tested in conjunction with the microprocessor section, but some initial tests can be made on this board without it. This board is single sided.

Insert the 12 touch switches that make up the keypad. If you like, you can check out the keypad with an ohmmeter to see if continuity exists between a row and column when the corresponding switch is pressed.

Next, assemble the display section (center of display board). Make sure that you first install the jumpers that are located below where the LED displays are later installed. You can use low-profile DIP sockets here if you wish to avoid soldering the ICs directly onto the PC board. Now install LED assembly DS201. This consists of two identical two-digit subassemblies. Next, fabricate and install hash chokes L201 and L202 (see FIG. 9 for details). These are not critical regarding inductance, and 15-20 turns are sufficient. Install a 20-pin socket where DS202 is to be located.

This socket must be used so the top of DS202 is at the same height as DS201. Then plug DS202 into the socket, making sure that the rounded corner or pin 1 indicator faces the corner of the PC board. No harm will be done if DS202 is inserted back wards, but DS202 will fail to light. Next install IC204, the LM3914N bar graph driver chip. You can use a DIP socket if you wish, as before. Install LEDs D201, D202, and D203, making sure to observe polarity. Also make sure that the tops of these LEDs are the same height as DS201 and DS202. This step completes the display board assembly.

Microcontroller and Logic

First, install 10-pin header (J1) and 8-pin header (J2) as shown in the display board parts placement drawing. Make sure V32 and V33 are well soldered because you will not be able to access them after this step. Next, apply DC power to the board as before and check for +5 volts on the J1 pin shown. Remove DC power and allow a minute for the capacitors to discharge. Next, install the programmed PIC16F84 microcontroller and all related parts. The microcontroller must be programmed with the appropriate software in order to operate properly. If you are building this project from scratch, you will have to do this step for the project to work. We refer you to the books and data sheets published by Microchip Corporation regarding how programming is done. You can write your own software or, if you prefer, a preprogrammed microcontroller can be purchased from the source mentioned at the end of this article.

Make sure to install an 18-pin socket at IC8. This is required to allow easy removal of IC8 so that future changes can be made to the internal operating software if needed. You may also install a DIP socket for IC13, but this step is optional. The display board can be connected to the main board using ribbon lead cable of the kind used in PC internal cabling, which is recommended. You can use Molex or similar connectors with 0.100-inch spacing to plug directly into the headers on the main board; however, installing these connectors generally requires a special crimp tool.

Alternately, you can use solder-type connectors or "press-on" types. Check out a computer parts catalog for suitable connectors. Connectors are not absolutely needed, although they do make testing easier; however, installing them takes time and may not be worth the effort. Once the MPX2000 is assembled and packaged in a case, these leads do not usually have to be disconnected again. You can check out the microcontroller section now, but it requires wiring it to the display board and disconnecting it again. If the wiring is correct, little can go wrong here.

PLL and RF Section

This section is the last to be assembled, and when completed, the MPX2000 PC boards will be ready for final testing and installation in a case of your choosing. First, install all resistors, then install capacitors, followed by transistors Q5 through Q10, and all diodes. Next, install the remaining ICs, IC9 and IC10. Again, you can use low-profile DIP sockets for IC9 and IC10 if you wish. Make sure to observe correct IC orientation.

The final step is the fabrication of L1, L2, and L3 (see FIG. 9 for coil details). L1 is five turns of #18 tinned wire wound around a 1/4-inch mandrel. L1 is installed in the PC board with the turns spaced evenly so the coil fits the PC board. A tap consisting of a short wire lead soldered to the appropriate point on the PC board is then connected to the first turn of the coil as shown in FIG. 9. L2 and L3 are made from four turns of #22 bare tinned wire wound using the threads of an 8-32 screw as a mandrel. L2 and L3 will look like small springs when made; the screw controls the dimensions, so you cannot go wrong. Shape the leads as shown in FIG. 9.

FIG. 9 MPX2000 Coil Winding Information

Remove the screw and install the coils in the PC board. Make sure no adjacent turns short together on L1, L2, or L3. Next, check over all work done so far for any errors. You are then ready for final testing. The display board must be connected to the main board for final testing.

After installing all components, make sure that the ICs, diodes, and electrolytic capacitors are correctly oriented. Power up the board as before and note the following: Bar graph display DS202 should momentarily flicker and then go out; this is normal and a good sign that things are working fine. The frequency display should light and show either a valid FM station frequency or three zeroes. The left-most (MSB) digit is zero blanked and will not display a zero. Next, examine the keyboard layout in the display board parts placement diagram; it is fairly standard. Now, enter an 8. The right digit should show this entry. Enter another 8, and now the two right digits should show "88." Enter a 1. The display should show "881." Now press the enter key. The display should still show "881." Now remove DC power, and after about 10 seconds, reconnect power. The display should light up "881." Now try entering each digit 1 to 9 in sequence. The newly entered digit should appear on the right and move left as new digits are entered; the fourth digit will disappear off the left side as new digits are entered. Now press the enter key, and unless the display is showing a frequency between 881 and 1079, the display will revert to 881 or what ever valid frequency it previously showed.

To reject an entry, press the clear entry key, and the display will show the previous valid frequency. Try entering a few valid FM frequencies, each time pressing the enter key afterward. Removing power and repowering should result in retention of the frequency in memory. If the frequency displayed differs, try reentering all four digits (i.e., 0995 for 99.5 MHz instead of 995). The leading 0 is needed and does not show. A 1 is sometimes retained in memory when a frequency of 100 MHz or more was previously entered. Entering a new frequency less than 100 MHz may not erase the fourth digit, and the 1 will be retained, confusing the microcontroller. This is not a fault but rather a result of suppressing the leading 1 for purely aesthetic and appearance reasons, making the operator forget that it is still really there. It is best to enter 0883 rather than 883, for example, especially if the previous frequency was 100 MHz or higher. When you are done, enter "0981" because this frequency is needed for later testing. Leave this frequency in memory. If everything works as specified, you can assume that the microcontroller and display circuit are correct.

Next, apply an audio signal of about 1-volt peak-to-peak to the L input. The bar graph indicator should show several lit segments. Adjust R218 on the display board so all 10 segments light. Then increase the signal about 10 percent and adjust the pot R68 on the main board so that overmodulation LED D202 on the display board (to the right of DS202) just lights. Reduce audio drive and D202 should extinguish, with DS202 showing fewer segments lit as the audio input is decreased. This step checks out the metering circuit. Now repeat that step with audio connected to the R input.

No difference should be noted, and no readjustment should be needed.

Final Test and Setup

You will need a VOM or DVM, an FM stereo receiver of some kind, and a source of line-level stereo audio. The MPX2000 is designed so that 1-volt peak-to-peak (0.316 volts RMS sine wave) at the input produces the required deviation, and the audio input level should not exceed this figure. The input impedance is approximately 10k ohms. Power up the board and check for the following voltages:

Pin 16 IC9: +5 VDC

Pin 7 IC10: +12 VDC

Pin 6 IC10: +3 to +12 VDC

Collector Q5: +5 VDC

Collector Q7: +1.5 to +3.8 VDC

Center pin D4: same as pin 6 IC10

Jct. D2, D3, R128: +12 VDC

Collector Q8: +12 VDC

Base Q8: + 5 to 7 VDC

Emitter Q8: + 4.5 to + 6.5 VDC

Collector Q10: +1.4 VDC

Emitter Q9: +0.7 VDC

Collector Q9: +5.6 VDC (adjust R138 as required)

After this test, remove power from the MPX2000. If any of the voltages were incorrect, the reason should be determined before proceeding further. If these voltages check out, you may proceed. Power up the MPX2000. Frequency 981 should appear in the display; if not, enter this frequency as described previously.

On power up, bar graph display DS202 and unlock LED D201 should flicker and go out. Measure the voltage at TP1, pin 6 IC10. It should be more than 3 and less than 11 volts. Now enter a frequency of 107.9 MHz. When the enter button is pressed, D201 should flicker and go out, indicating lockup of the PLL. The unlock LED may take several seconds to extinguish. This delay is normal because of the long loop time constants used. If DS201 stays lit, check the voltage at TP1; it may be 11 volts or higher.

Spread the turns of L1 until this voltage drops below 10 volts and D201 goes out.

Next, program in a frequency of 88.1 MHz. Check to see if D201 flickers and goes out. Measure the voltage at TP1; it will be around 3-3.5 volts. If D201 is still lit, compress the turns of L1 slightly until the voltage rises slightly and D201 goes out.

Recheck at 107.9 MHz and 88.1 MHz until lock is obtained at both frequencies- indicated by D201 extinguishing. It is normal for D201 to flicker slightly when changing frequency. If you are unable to get both 88.1 and 107.9 to lock up with one setting of L1, check to make sure you have at least 12 volts at the output of regulator IC7. If not, you can replace it or add a diode in the common leg of IC7 to raise the voltage slightly.

Once the PLL is adjusted, set up an FM receiver on 98.1 MHz or somewhere near this if 98.1 MHz is busy in your area. Enter this frequency into the MPX2000. LED201 should extinguish after a few seconds, and you should hear a carrier in the receiver, as evidenced by the quieting of the receiver. Momentarily power down the MPX2000 to confirm that the carrier is coming from it and not from somewhere else. Restore power, and the carrier should reappear in a few seconds when D201 extinguishes. Next, connect audio to the L and R inputs. The bar graph display should indicate something, and you should hear the audio in the FM receiver. The receiver's stereo indicator should be on, and the audio should sound like any other FM station. Make sure you do not apply excess audio because this causes distortion and degrades stereo separation. Adjust R55 for best separation. If you have access to an audio generator and scope, you can get an exact alignment by adjusting for the waveforms shown in the waveform diagram. Adjust the input audio level for best sound in the receiver without distortion and clipping. With this audio input level, set R218 on the display board so that the LED bar graph shows all 10 segments lit on the loudest audio peaks. Slightly increase input audio and adjust R68 so that the over modulation LED D202 just flashes. Then back off the audio input level so that it barely flashes on the loudest audio peaks. This step sets the proper audio drive level.

FIG. 10 Photo of the Main Board of the MPX2000

FIG. 11 Front Panel View of the Completed MPX2000 Packaging

The MPX2000 may be packaged in any suitable metal or plastic case (see Figures 8, 10, and 11). Remember that this audio device generates RF signals, so the use of shielded cables for audio input and RF output is recommended. Keep the display board as far as possible from audio lines and the main board because it does generate some switching noise, which could appear as buzz or hum on the transmit ted signal until the display goes to sleep. Once programmed, the MPX2000 will operate without the keyboard and LED frequency display as long as the frequency is not changed. We recommend that accessibility to the keyboard be limited to keep curious individuals from playing with the MPX2000 and inadvertently changing frequency. This can be done with a removable panel or cover over the display.

The LEDs indicating lock and modulation should be kept visible at all times to signal improper operation. A good RF ground and antenna system is essential in reducing RF ground-induced hum, which is a problem with low-power FM transmitters.

Simply using a wall transformer and a whip antenna plugged into the RF output jack might not provide adequate RF grounding. To check if hum is RF induced, power the MPX2000 from a battery. If this cures the hum, the problem is either a poorly filtered DC supply or RF-induced hum. Filtering is built into the MPX2000, so a pretty poor DC supply will be tolerated, but this is still not recommended. If improving the power supply filtering does not help, the hum is most likely RF induced. In this case, you may have to experiment with antenna placement, grounding, and RF chokes in the power leads. A metal case is a help in some cases because it provides some shielding and grounding via its self-capacitance.


In keeping with Part 15 FCC requirements, the radiated field must be kept below 250 microvolts per meter at 3 meters (about 10 feet) from the transmitter. The use of a 6-inch whip antenna is recommended. If the receiver is close to the MPX2000 and a plastic case is used for packaging, no antenna is needed because sufficient signal will radiate from the PC board itself to be detectable. In practice, this 250 uV/meter at 3-meter limit permits good reception at 50-500 feet from the transmitter, depending on the receiver sensitivity and its antenna system. Do not connect any more antenna than you need, or you may receive an FCC citation.

Parts List

Resistors 5% 1/4 W unless noted


Capacitors Diodes, Transistors, and LEDs

Integrated Circuits


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