LCD FM Transmitter

Radio Transmitter for FM Broadcast

Here’s easy to build high-quality PLL Radio Transmitter for FM broadcast, with typical output power of 5 W and no-tune design. The transmitter includes RDS/SCA input and Audio/MPX input with optional preemphasis. It can be used with or without stereo encoder. Tuning over the FM band is provided by two buttons that control dual-speed PLL. The transmitter can work also without the LCD display.

The characteristic of PLL Radio FM Transmitter:
Supply voltage: 11-13.8 V (stabilised or from a battery)
Supply current: up to 1.2 A
Standard frequency range: 87.5-107.9 MHz
Audio/MPX input sensitivity: 2 V pp (for 75 kHz freq. deviation)
RDS/SCA input sensitivity: 0.2 V pp (for 7.5 kHz freq. deviation)
Board dimensions: 109 x 54 mm

PLL Radio FM Transmitter Schematic Diagram

Transmitter Schematic

PLL Radio FM Transmitter Printed Circuit Board

Transmitter Component Layout

Transmitter PCB-Top

PLL Radio FM Transmitter Part list

Q1 – BF240
Q2 – BFG135 (BFG235)
Q3 – 2SC1971 (2N3553) + heatsink
Q4 – BC547B
D1 – SB260 (1N5822, 1N581x)
D2, D3 – BBY40 (BBY31)
D4 – LED 5mm

U1 – 78L09
U2 – TSA5511 (TSA5512, SDA3202)
U3 – PIC16F627A (programmed)
U4 – 78L05

R1, R2, R11, R17, R20 – 10k
R3, R21 – 270R
R4, R15 – 33k
R5, R7, R12, R13, R16 – 680R
R6, R14 – 18k
R8 – 47R (33R if Q2 is BFG235)
R9 – 18R
R10 – 4k7
R18 – 3k3
R19 – 100k smd 1206
R22 – 91R
R23 – trimmer 5k mini

C1, C4, C9, C12, C13, C14, C15, C30, C31, C32, C33, C35 – 10n smd 1206 (C)
C2, C17, C20 – 15p (C)
C3 – 10p (C) (15p if the PCB is single-sided)
C5 – 1n (C)
C6, C28, C29, C34 – 100u/10V (E)
C7, C26 – 10u/35V (E)
C8 – 22p (C)
C10 – 47p (C) (33p if Q3 is 2N3553)
C11, C27 – 100n (C)
C16, C36 – 33p (C)
C18 – trimmer 50p
C19 – 470u/16V (E)
C21 – 4u7/50V (E)
C22 – 330p (C)
C23, C24 – 47p (C)
C25 – 3n3 (P)

L1 – 3.5 turns on 7 mm diameter
L2 – 1uH/815mA choke, or about 10 turns of thin wire on mini ferrite core
L3 – 2.5 turns on 6 mm diameter (4.5 turns if Q3 is 2N3553)
L4, L5 – 3.5 turns on 6 mm diameter

Y1 – crystal 6.4 MHz or 3.2 MHz
TR1 – rf ferrite transformer 2:1 (3:1 if Q3 is 2N3553), see text
SW1, SW2 – button mini
J1, J2, J3 – BNC connector 90 deg.
J4 – power supply connector
J5 – HD44780 LCD standard connector, 2×8 or 2×16 characters
J6, J7 – jumper

Additional information for PLL Radio FM Transmitter

For 0.05 MHz step tuning a 3.2 MHz crystal is required. In other case a 6.4 MHz crystal will make good work (step tuning 0.1 MHz).

Wind all coils (except the L2) by a 0.8 mm wire.

The Y1 package must be tied to ground!

Make sure the Q3 terminals are as short as possible (about 2-3 mm above board). The 2N3553 case/heatsink can’t be tied to ground!

To make the TR1 transformer, use specified number of turns on primary side and one turn on secondary side. The secondary wire should be quite thick but the primary can be as thin as you want. Wind on a 2-hole ferrite (material 61 or N1).

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