Putting together a serious high power FM transmitter

This page deals with the problems of assembling together a serious high power professional quality FM stereo radio broadcasting transmitter with optional RDS capability. Our (MAX) kits will be used as an example as we feel these make it possible to put together a system with minimal effort. Several configurations will be presented and it is up to you to select the one that fits your needs best.

1.) 25W stereo FM transmitter with RDS and PC remote control
A 25W system will be presented here, with stereo encoder and optional RDS encoder. Optional RS232 remote control function will be included as well. Such system can be a building block of a much larger station. 25W can directly drive almost any high-power FM amplifier out there, up to 5KW and more. If the procedure below sounds too complicated you can also buy assembled ready unit here.

Required hardware components:
a.) 25W FM exciter module - our MAX PRO 4025+
b.) SE5000 DSP+ stereo encoder - you can buy it here.
c.) 10cm Maxlink cable to connect stereo encoder to MAX PRO 4025 - you can buy it here.
d.) RDS daughter board - RDSMAX 4000+ mini
e.) Optional RS232 IO board+ available here.
f.) Stabilized mains power supply (12-15V/5A), a suitable model is available here towards the bottom of the page.
g.) Metal enclosure - your own or buy ours. On this page you can also find a small 40x40 12V fan, optional XLR connectors for stereo, maybe also On/Off switch.
h.) Also needed: some wire, short microphone coaxial cable, soldering iron, patience and a few hours of free time.

Assembly steps:
a.) Mount the components into a metal case. 40x40mm fan, MAX PRO 4025+ exciter board, SE5000+ stereo encoder, IO board. Don't install RDS just yet. You can find manuals for all these boards here.
b.) On SE5000 DSP+ stereo encoder set pre-emphasis jumpers to 50uS (EU) or 75 uS (Americas, Japan). Remove J10 and J14 jumpers. Connect MAXLINK cable from SE5000 to MAX PRO 4025 board.
c.) On MAXPRO4025 set pre-emphasis jumper (J2) to NONE. Install flat cable from MAXPRO4025 to the LCD module. Connect fan to FAN +- ports. Make sure MAXPRO4025+ is fixed to case with all screws and make sure to use METAL distancers, not plastic! You can use some thermal paste under heatsink, if you have it.
d.) If you want XLR audio input connectors, install XLR connectors as well. Note there are numbers 1-3 on the connectors that match numbers on 3-pin header on the PCB.
e.) Install RDS daughter board, if needed (optional). Connect it to the IO board. Pin1 to pin 1 and pin 5 to pin 5. If you can't detect which pin is 1 try with ohm meter. Pin1 is connected to ground.
f.) Install audio coax between MPX out on SE5000 stereo encoder and audio input on MAXPRO4025+.
g.) Check all wiring again
h.) Pour yourself a glass of beer (or milk) and relax. You are done, time for a short break.

Setup and alignment:
a.) Before you turn the unit on make sure all connections are correct. Also make sure your antenna is ok and well tuned. We recommend that you read the manual for MAXPRO4025, there is a section dedicated to antennas, connectors and coaxial cable.
b.) Turn on transmitter and increase power to only 1/4 (25%). You can measure SWR and make sure everything is ok. On the radio you should hear either connected audio or total silence.
c.) There are only two things to align. First turn the audio modulation on MAXPRO4025 all the way to zero (direction -). Now while listening to your signal on the radio (set to same frequency as shown on transmitter LCD) turn modulation/audio trimmer on MAX PRO4025 until stereo indicator turns on. Turn it a bit more towards + and you are done. Connect some audio source and test your audio with ear-meter :)
d.) If you have RDS encoder, do the same thing there. Turn the trimmer to zero (no RDS indicator). Now slowly increase until you see RDS indicator turn on. Increase a little beyond this point and you are done.
e.) If your audio is weird, check pre-emphasis jumper on MAXPRO4025 board. It should be set to OFF!

Quick technical specifications:
- Power: 25W
- Frequency band: 87.500 - 108.000 MHz
- Frequency step: 50KHz, PLL
- Modulation: FM, wideband
- Audio inputs: XLR and RCA
- Output antenna connector: BNC
- Power supply: 12-15V/5A stabilized
- Extremely clean signal, complies and exceeds standards
- Qsonic PLL/VCO for excellent audio

2.) 300W stereo FM transmitter with RDS and PC remote control
A 300W system will be presented here, with stereo encoder and optional RDS encoder. Optional RS232 remote control function will be included as well. If the procedure below sounds too complicated you can also buy assembled ready unit here. or in a package including antenna, coaxial cable and everything else here.

Required hardware components:
a.) 15W FM exciter module - our MAX PRO 3000+ You can also use MAXPRO4025+ for even better audio.
b.) SE5000 DSP+ stereo encoder - you can buy it here.
c.) 10cm Maxlink cable to connect stereo encoder to MAX PRO 3000 - you can buy it here.
d.) RDS daughter board - RDSMAX 4000+ mini
e.) Optional RS232 IO board+ available here.
f.) 300/400W FM amplifier module with integrated filter, metering, protections and DCDC stepdown regulator.
g.) DIGIAMP cable, available here.
h.) Stabilized mains power supply (110-240V to 48V/12A), a suitable model is available here towards the bottom of the page (SE600).
i.) Metal enclosure - your own or buy ours. On this page you can also find 80x80 12V fan, optional XLR connectors for stereo, maybe also On/Off switch and mains filter with switch, they all fit our box exactly.
j.) Also needed: some wire, short microphone coaxial cable, soldering iron, patience and a few hours of free time.

Block diagram of a 300W FM transmitter with stereo, RDS and PC remote control

Assembly steps:
First assemble the exciter and stereo encoder part, which is basically identical to the steps for 25W exciter in example 1 above. Note that you will be using this to drive a 300W amplifier so there are some changes there related to this amplifier and its mains power supply.
a.) Mount the components into a metal case. One fan is recommended for the exciter module, it does not have to be strong, just a simple 60x60mm or 80x80mm type. Another fan is recommended for the mains power supply. You may be able to combine them together and just use one. If possible place MAX PRO 3000 or 4025+ exciter board and SE5000+ stereo encoder in separate compartment, isolated by a metal wall. Don't install RDS just yet. You can find manuals for all these boards here.
b.) On SE5000 DSP+ stereo encoder set pre-emphasis jumpers to 50uS (EU) or 75 uS (Americas, Japan). Remove J10 and J14 jumpers. Connect MAXLINK cable from SE5000 to MAX PRO3000+ or 4025+ board.
c.) On FM exciter MAXPRO3000+/4025+ set pre-emphasis jumper to NONE. Install flat cable from MAXPRO3000/4025 to the LCD module. Connect fan to FAN +- ports. Make sure MAXPRO3000/4025+ is fixed to case with all screws and make sure to use METAL distancers, not plastic! You can use some thermal paste under heatsink, if you have it.
d.) If you want XLR audio input connectors, install XLR connectors as well. Note there are numbers 1-3 on the connectors that match numbers on 3-pin header on the PCB.
e.) Install RDS daughter board, if needed (optional). Connect it to the IO board. Pin1 to pin 1 and pin 5 to pin 5. If you can't detect which pin is 1 try with ohm meter. Pin1 is connected to ground.
f.) Install audio coax between MPX out on SE5000 stereo encoder and audio input on MAXPRO4025+.
g.) Check all wiring again and you can make a small break, the driving part of your 300W transmitter is complete. It is time to move on to the 300W amplifier.
h.) Now install mains power supply in the the rack and be really careful while wiring the 110V/220V mains voltage, this is potentially lethal. Always make sure you do not touch any live wires when equipment is under mains power. Also note that sometimes live voltages can be present even after mains voltage is disconnected. Treat mains power supply with respect.
i.) Wire the 300W amplifier module into the rack enclosure. Wire the +48V terminal to the mains power supply, preferably via a switch on the front panel.
j.) Wire the antenna output from MAXPRO3000+/4025+ to the 300W amplifier module input. You will be connecting the antenna to the antenna output from the 300W amplifier this time.
k.) Connect the DIGIAMP cable from the 300W amplifier module to the MAXPRO3000+/4025+ exciter.
l.) Connect the +12-15V DC power from the 300W amplifier module to the MAXPRO3000+/4025+ exciter.
m.) Pour yourself a glass of beer (or milk) and relax. You are done, time for a short break.

Setup and alignment:
a.) Before you turn the unit on make sure all connections are correct. Also make sure your antenna is ok and well tuned. We recommend that you read the manual for MAXPRO3000+/4025+, there is a section dedicated to antennas, connectors and coaxial cable.
b.) Turn on transmitter and increase power to only 1/4 (25%). You can measure SWR and make sure everything is ok. On the radio you should hear either connected audio or total silence.
c.) There are only two things to align. First turn the audio modulation on MAXPRO4025 all the way to zero (direction -). Now while listening to your signal on the radio (set to same frequency as shown on transmitter LCD) turn modulation/audio trimmer on MAX PRO4025 until stereo indicator turns on. Turn it a bit more towards + and you are done. Connect some audio source and test your audio with ear-meter :)
d.) If you have RDS encoder, do the same thing there. Turn the trimmer to zero (no RDS indicator). Now slowly increase until you see RDS indicator turn on. Increase a little beyond this point and you are done.
e.) If your audio is weird, check pre-emphasis jumper on MAXPRO4025 board. It should be set to OFF!
f.) Note there is no special setup for the amplifier because everything is already taken care of by DIGIAMP interface (SWR protection, ALC, power readout etc)

Quick technical specifications:
- Power: 300W
- Frequency band: 87.500 - 108.000 MHz
- Frequency step: 5 or 50KHz, PLL (depending on module, 3000 or 4025+)
- Modulation: FM, wideband
- Audio inputs: XLR and RCA
- Output antenna connector: BNC
- Power supply: 110-240V mains voltage
- Extremely clean signal, complies and exceeds standards
- Qsonic PLL/VCO for excellent audio


3.) Basic 300W FM transmitter
A 300W system will be presented here, without stereo encoder and without a case. This is about as cheap as you can get while keeping things relatively simple and reaching good quality albeit without stereo operation, but this can be added by purchasing stereo encoder later. If the procedure below sounds too complicated you can also buy assembled ready unit.

Required hardware components:

4.) PC based FM transmitter
A 300W system will be presented here, without stereo encoder and without a case. This is about as cheap as you can get while keeping things relatively simple and reaching good quality albeit without stereo operation, but this can be added by purchasing stereo encoder later. If the procedure below sounds too complicated you can also buy assembled ready unit here.

Required hardware components:

WORK UNDER PROGRESS, LAST UPDATE APRIL 19th, 2010. MORE COMING SOON!

WORK UNDER PROGRESS, LAST UPDATE APRIL 19th, 2010. MORE COMING SOON!

Discuss this article in our Forum!