TX-M404A Circuit FM Transmitter Based On GFT809

Table of Contents

TX-M404A circuit FM transmitter based on GFT809. The GFT809 is a high-performance FM transmitter designed for portable multimedia systems. It includes stereo modulator and FM transmitter incorporated with oscillator circuits. This device employs sophisticated stereo modulation circuitry to ensure quality audio characteristics. The GFT809 is fabricated using advanced CMOS process and available as 24-pin MLF package.

Features GFT809 IC

2.7 to 3.7V operation
Low current consumption 10mA @ 3.0V (Typical)
4 bit switch control
Selectable frequency band (US/JPN)

Applications
MP3, MP3 phone, CD changer, notebook PC, car TV, electronic dictionary, portable DVD, portable multimedia players (PMP), Navigator/DMB and portable game devices

Pin description

Pin No.	Symbol	Description	Pin No.	Symbol	Description
1	FLT	Filter	13	OSCI	Crystal oscillator in
2	MPX	MPX signal out	14	D0	Data 0
3	Vcc	Vcc, RF	15	D1	Data 1
4	PFD	Phase frequency detector	16	D2	Data 2
5	OPI	OP amp in	17	D3	Data 3
6	OPO	OP amp out	18	Pilot	Pilot tone adjustment
7	RF_OSC	RF Oscillator	19	LPF_L	Low pass filter, left
8	RF_Out	RF signal out	20	Pre_L	Pre-emphasis, left
9	Vcc	Vcc, analog	21	LJn	Audio input, left channel
10	Band	Frequency band selection	22	R_ln	Audio input, right channel
11	Vcc	Vcc, crystal	23	Pre_R	Pre-emphasis, right
12	OSCO	Crystal oscillator out	24	LPF_R	Low pass filter, right

This solution is a FM radio designed using GFT809, very simple, no need to write programs to control.

Schematic TX-M404A GFT809 FM transmitter module datasheet

Schematic Tx-M404A Circuit Fm Transmitter Based On Gft809 — Schematic

Module purchase please search for “TX-M404A” on AliExpress.

The module pinout and application circuit are shown in the following figure:

Power supply with 3.3V DC, it is recommended to connect a capacitor in series with the headphone end, and the crystal oscillator is recommended to use 21.25MHz. The crystal oscillator frequency will directly affect the transmission frequency of FM.

The transmission frequency of FM is controlled by the levels on pins Band D0 D1 D2 D3, the frequency can be adjusted by pulling up and down these pins.
As shown in the table below:

Data				Frequency (MHz)
D3	D2	D1	D0	Pin #10 to Vcc	Pin #10 to GND
0	0	0	0	87.7	76.8
0	0	0	1	87.9	77.0
0	0	1	0	88.1	77.2
0	0	1	1	88.3	77.4
0	1	0	0	88.5	77.6
0	1	0	1	88.7	77.8
0	1	1	0	88.9	78.0
0	1	1	1	PLL Off
1	0	0	0	106.7	88.0
1	0	0	1	106.9	88.2
1	0	1	0	107.1	88.4
1	0	1	1	107.3	88.6
1	1	0	0	107.5	88.8
1	1	0	1	107.7	89.0
1	1	1	0	107.9	89.2
1	1	1	1	PLL Off

When the Band pin (Pin10#) is at low level, the module operates at 76.8 MHz – 89.2 MHz, interested parties can refer to the chip data sheet.

Here I use DIP switches to adjust the voltage of these pins.

A metal wire can be soldered to the antenna section, for better performance, it is recommended to add a matching circuit at the antenna end, refer to the figure below:

The reference circuit provided in the chip datasheet is as follows:

BOM TX-M404A circuit FM transmitter based on GFT809

Name	Value	Description	Quantity
Capacitors
C5	100nF	C0805 – SMD capacitor	1
C1,C2	27pF	Ceramic capacitor	2
C4,C3	100nF (104)	Ceramic capacitor	2
Resistors
R1,R2,R3,R4,R5	1k (102)	R0603 – SMD resistor	5
R6	470 (471)	R0603 – SMD Resistor	1
Semiconductors
LED1	LED-0603_R	LED0603 – Red	1
U1	GFT809	TX-M404A module	1
U2	AMS1117-3.3	1A Fixed 3.3V Positive SOT-223 Voltage Regulator Linear Low Drop Out (LDO)	1
Miscellaneous
J1	HDR-M-2.54_2x3	6P 2.54mm Double Row Pin Header	1
J5,J3	ANT	1P Pin Header	2
USB1	USB-Micro_1	1A USB 2.0 5P Female Micro-B SMD USB Connector	1
J6	320B/PJ-001 3.5Jacks	Audio Connector Headphone	1
X1	21.25MHz	HC-49S Crystals	1
SW1	EM-05KP	5 Digit SPST SMD DIP Switch	1