[Photo] 40 Watt Frequency Modulation (FM) Broadcast Amplifier

The indicators and characteristics of the circuit:

Component list

Numbering Component parameters and description FEC Model (Note 1) Quantity
C1, C2, C4 5.5-50p small ceramic dielectric trimmer capacitor 148-161 3
C3 100p 50V porcelain dielectric capacitor (NP0) 896-457 1
C5, C6, C7 100n 50V porcelain dielectric capacitor (X7R) 146-227 3
C8 100u 35V electrolytic capacitor 667-419 1
C9 500p 500V high frequency capacitor (metal clad Note 2) 1
C10 1n threading capacitor 149-150 1
C11 16-100p mica trimmer capacitor
(Arco 424)
1
C12 25-150p mica trimmer capacitor
(Arco 423 or Sprague GMA30300)
1
C13 300p 500V high frequency capacitor (metal clad Note 2) 1
C14, C17 25p 500V high frequency capacitor (metal clad Note 2) 2
C15, C16 50p 500V high frequency capacitor (metal clad Note 2) 2
C18 100n 200V ceramic dielectric capacitor 1
L1 64nH inductance is a hollow coil with a diameter of 1.2mm enameled wire and a diameter of 6.5mm 4 turns elongated to 8mm 1
L2 25nH inductance hollow coil wound with 1.2mm diameter enameled wire diameter 6.5mm 2 turns elongated to 4mm long 1
L3 Wrap 2.5 turns with 0.7mm diameter enameled wire on 6-hole magnetic beads 219-850 1
L4 210nH inductance is a hollow coil with a diameter of 1.2mm enameled wire and a diameter of 6.5mm. The 8-turn dense winding is about 12mm long 1
L5 21nH Inductor is made of 0.8mm diameter enamelled wire and the hollow coil is 4mm in diameter and 3 turns elongated to 10mm 1
L6 41nH inductance is made of 0.8mm diameter enamelled wire and the hollow coil is 4mm in diameter and 4 turns elongated to 6mm 1
L7 Two magnetic beads placed on the power lead at C10 242-500 2
L8, L10 100nH inductance hollow coil wound with 1.2mm diameter enameled wire diameter 6.5mm 5 turns elongated to 8mm 2
L9 115nH inductance is a hollow coil with a diameter of 1.2mm enameled wire and a diameter of 6.5mm 6 turns elongated to 12mm 1
R1 10K 0.5W adjustable resistance (potentiometer) 108-566 1
R2 1.8K 0.5W metal film resistor 333-864 1
R3 33R 0.5W metal film resistor 333-440 1
D1, D2 BZX79C5V6 5.6V 400mW Zener diode 931-779 2
TR1 MRF171A (Motorola) RF power MOSFET 1
SK1 BNC socket 583-509 1
SK2 BNC or N-type socket 310-025 1
16 x 60 x 89mm 3.4 ° C / W radiator and fan 170-088 1

Notes:

The FEC model is for reference only. Metal clad high-frequency capacitors have short thick leads with minimal inductance. The inductance coil of the input circuit can be smaller, but for the convenience of production, the same coil as the output circuit is used.

Magnifier photo

40W output power 88-108 MHz frequency range 20dB gain + 28V 3A DC Power Supply high efficiency (70%, MOSFET class AB amplifier) ​​Low component output low pass filter (LPF)

Circuit diagram

Assemble

Prepare a 100 x 85mm double-sided printed circuit board (PCB). Drill a hole through which the FETFET power tube can pass. Drill 4-6 holes in the PCB to fix the PCB in the cast aluminum shielding box. Design how to install, fix the radiator and its fan. Set the socket holes required for power supply, input and output on the cast aluminum shielding box. Refer to the photo and circuit diagram above to cut out the soldering area on the PCB. Most of the area on the back and front of the PCB is ground. Welding components. The output low-pass filter part is shielded from the amplifying circuit part by a copper sheet (or printed circuit board).

The role of the output low-pass filter: The filter is used to filter higher harmonics to reduce the interference to nearby stations, which is very important for high-power RF output. For example, assume that the amplitude ratio of the third harmonic is -30dB. When the amplifier is 1W, the harmonic power is 1mW, which is nothing; but when the amplifier is 40W, the harmonic power is 40mW, and interference is the problem. The harmonic suppression of this circuit is greater than 50dB. If you are not involved in interference, you can not use the output low-pass filter.

debugging

Offset adjustment:

The input terminal is short-circuited with a 50-ohm resistor (note: this circuit will self-oscillate when the input is not connected), adjust all variable capacitors to the middle, and adjust R1 to the minimum voltage. Connect the + 28V power supply, the current should be about 14mA. Adjust R1 to make the supply current 115mA. If you find that the current suddenly increases a lot, it means that the circuit self-oscillates, and you need to add more shielding isolation.

Input and output matching adjustment:

Recommended equipment: Current-limited power supply (+ 28V, 3A) 50W VHF fake antenna load 500mW-1W FM transmitter RF 40dB attenuator RF power meter RF spectrum analyzer and network analyzer

If you have an RF spectrum analyzer and a network analyzer, you can use these instruments to adjust. It doesn't matter if you don't have these instruments; add a 500mW-1W RF signal through the FM transmitter at the input, connect a 50W dummy load at the output, and measure the output power with an RF power meter through a 40dB attenuator. If you do not have an RF power meter, simply use the detection circuit on the right to measure the voltage at the monitoring point. Set the transmitting frequency of the transmitter and adjust the trimming capacitors C1, C2, C4, C11, and C12 to maximize the output power of the amplifier.

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[Photo] 6 Watt Frequency Modulation (FM) Broadcast Amplifier

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Published on 2006-04-15 20:51 • 259 times read
[Photo] 6 Watt Frequency Modulation (FM) Broadcast Amplifier
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