Digitally controlled FM transmitter with 2 line LCD display

Digitally controlled FM transmitter with 2 line LCD display. (Part I)

Background
All over the net are descriptions of different types of FM transmitters.
You can find all kinds of constructions. Some are more cryptical than others.
One common thing about them is that they have poor explanation about function and assembly.
Most of them don't explain the involving coils or how to make them.
How about performances and stability?

Why can't there be a construction that are easy to build and gives great performance at the same time?
We do have been able to send people to the moon!

I finally got tired of all crap and created this page myself. A page I always wanted to find out there.
I will show you a construction which will have great performance and still is very easy to build.
Even if you are a fresh born homebrewer, you will be able to follow my footsteps in this project.
I will support you with schematic , descriptions, software, components and PCB….how about that!
Enough blurring, let's get down to business.

Project start
A transmitter needs several units to work properly, so instead of mixing all into one huge project, I will divide the project into 4 steps.
Each step will result in a self functional unit. In the end of this journey, you will be able to link the steps together into a very powerful FM transmitter.

Part I (Digitally controlled FM transmitter with 2 line LCD display)

Part II (FM PLL controlled VCO unit)

Part III (1.5W power amplifier)

Part IV (7W power amplifier)

Part I (one)
This part will explain the main controlling unit for the FM transmitter. This part is very important since the transmitter frequency is digitally controlled and thereby very stable.
The hart of this unit is a PIC processor called PIC16F870, a 2 x 16 char display and four buttons.
Let's look at the schematic now.
As you can see I have chosen a PIC16F870 because it is easy to find and cheap to buy.
If you want more info about programming, you can look at my link:
Introduction to microcontroller PIC16F870 and PIC16F84.
If you can't find this circuit or have difficulty to program it, I can support you with this circuit pre-programmed. Mail Me for further information.
At the schematic you will also find a 2 line 16 char display.
This display is a " HD44780-based LCD Modules" which is very common.
Most LCD displays are based on this circuit. You will have no problem to find it.

The display will show you information about the transmitting frequency.
Four wires goes to the transmitter board to control the PLL circuit called "INTERFACE OUT", (coloured in green left side).
When you press any of the four buttons the PIC16F870 will update both the transmitter board and the display with new data.

The frequency of this transmitter can be changed in +/-50kHz steps with two buttons and also in +/-1MHz steps with two other buttons.
The frequency limit is not set by software, only by the VCO of the transmitter board.
This means that you can use this software from 0 to 999MHz, still the transmitter board in this project will be dimensioned to work from 88 to 108 MHz.

What else do we need to say?
Well, at pin 15 you will find a piezosummer that give a short "beep" when ever you press a button.
The PIC16F870 runs with a crystal which should be from 2 to 6 MHz….not critical at all..
The PIC16F870 will always remember the last frequency you used and when the power is connected, the first thing that happens is that the transmitter PLL and display will be uppdated.

Download PIC16F870 programs (INHX8M format)
The zip file contains hex file made for this project.
When you program the PIC16F870 there are 3 bytes in the EEPROM memory which are interesting for you.
You can find them in EEPROM address 0, 1 and 2.
These 3 bytes set and remember the last used transmitting frequency.
If you change the frequency with the buttons, these 3 register will be updated.
In the software (transm_LCD.hex) I have made the initial frequency is set to 90MHz.

If you wish the initial frequency to be something else than 90MHz you can easy change the 3 bytes during programming, se table below.

Frequency (MHz)	Address 0	Address 1	Address 2
88.0	00h	37h	00h
89.0	00h	37h	a0h
90.0 (default)	00h	38h	40h
91.0	00h	38h	e0h
92.0	00h	39h	80h
93.0	00h	3ah	20h
94.0	00h	3ah	c0h
95.0	00h	3bh	60h
96.0	00h	3ch	00h
97.0	00h	3ch	a0h
98.0	00h	3dh	40h
99.0	00h	3dh	e0h
100.0	00h	3eh	80h
101.0	00h	3fh	20h
102.0	00h	3fh	c0h
103.0	00h	40h	60h
104.0	00h	41h	00h
105.0	00h	41h	a0h
106.0	00h	42h	40h
107.0	00h	42h	e0h
108.0	00h	43h	80h

Option:
(If you never will change the frequency with the buttons and don't want a LCD display, you can set the initial frequency and still use this controling circuit.)

PCB
This first part of the project involves just a PIC16F870 a display and a few components.
It is not complicated to build and therefor I have not made a PCB.
If many asks for a PCB I will make it and put it here.

Final word
In this part, I describes the digital controlling unit of the transmitter.
Again, this is a strictly educational project explaining how a RF amplifier can be built.
According to the law it is legal to build them, but not to use them.

Part II
Next part will be the RF transmitter unit.
Click here to go to FM PLL controlled VCO unit (Part II)

FM PLL controlled VCO unit (Part II)

Background
Many people have asked me for this project and specially support about components and PCB.
At the bottom of this page you find all info about my support, so let's start.
All receiver and transmitter needs some kind of oscillator.
The oscillator needs to be voltage controlled and it needs to be stable.
The easiest way to make a RF oscillator stable is to implement some kind of frequency regulating system. Without any regulating system, the oscillator will start to slide in frequency due to temperature shift or other influences. A simple and common regulating system is called PLL.
I will explain it later.

To understand this unit I suggest we look at a block diagram at right.
At the left side you find the interface from the controlling unit Part I :
Digitally controlled FM transmitter with 2 line LCD display

There are 3 wires and ground. The 3 wires goes to the PLL circuit.
In the right corner (Xtal) is a crystal oscillator.
This oscillator is very stable and will be the reference of the regulating system.

The main oscillator is printed in blue and is voltage controlled.
In this construction the VCO range is 88 to 108 MHz. As you can see from the blue arrows, some energy goes to an amplifier and some energy goes to the PLL unit.
You can also see that the PLL can control the frequency of the VCO.
What the PLL do is that it compare the VCO frequency with the reference frequency (which is very stable) and then regulated the VCO voltage to lock the oscillator at desired frequency. The last part that will affect the VCO is the audio input.
The amplitude of the audio will make the VCO change in frequnecy FM (Frequency Modulation).
I will explain it all in detail under section Hardware and schematic.

It is not good to load or "steal" to much energy from the oscillator because it will stop oscillating or give bad signals.
Therefore I have added an amplifier.
The oscillator give about 15mW of energy and the following amplifier will bring up the power to 150mW.
The amplifier can be pressed a bit more (maybe 300mW) but that is not the best solution.
In Part III of this project I will describe a 1.5W power amplifier and in Part IV you will find a 7W power amplifier.

For now, this unit will deliver about 150mW.
150mW doesn't sound much, but it will let you transmit RF signals 500m easy.
In one of my experiments I had 237mW output power and I could transmit 4000m in open field using a dipole antenna.
In city environment I got 3-4 blocks. Concrete and buildings damp RF really much.

First some words about synthesiser and PLL
Before I go any future I will explain the regulating system of a PLL. Some of you are familiar with PLL and other are not familiar.
Therefore I have copy this section from my RC receiver which explain PLL system.
(Synthesizer and PLL can be broke down into complex regulating system with lot of math. I hope all PLL experts have indulgence with my simplyfied explanation below. I try to write so even fresh born homebrewers can follow me.)

So what is a frequency synthesizer, and how does it work?
Look at the picture below and let me explain.

The hart of the synthesizer is something called phase detector, so let's first investigate what it does.
The picture above shows you the phase detector. It has two inputs A ,B and one output. The output of the phase detector is a current pump. The current pump has three states. One is to deliver a constant current and the other is to sink a constant current. The third state is a 3-state. You can see the current pump as a current delivery of positive and negative current.

The phase detector compares the two input frequencies f1 and f2 and you have 3 different states:

 If the two input has exact the same phase (frequency) the phase detector will not activate the current pump,
so no current will flow (3-state).

 If the phase difference is positive (f1 is higher frequency than f2) the phase detector will activate the current pump
and it will deliver current (positive current) to the loop filter.

 If the phase difference is negative (f1 is lower frequency than f2) the phase detector will activate the current pump
and it will sink current (negativ current) to the loop filter.
As you understand, the voltage over the loop filter will vary depentent of the current to it.

Okay, lets go futher and make a Phase loocked loop (PLL) system.

I have added a few parts to the system. A voltage controlled oscillator (VCO) and a frequency divider (N divider) where the divider rate can be set to any number. Let's explain the system with an example:

As you can see we feed the A input of the phase detector with a reference frequency of 50kHz.
In this example the VCO has this data.
Vout = 0V give 88MHz out of the oscillator
Vout = 5V give 108MHz out of the oscillator.
The N divider is set to divid with 1800.

First the (V_out) is 0V and the VCO (F_out) will oscillate at about 88 MHz. The frequency from the VCO (F_out) is divided with 1800 (N divider) and the output will be about 48.9KHz. This frequency is feeded to the input B of the phase detector. The phase detector compares the two input frequencies and since A is higher than B, the current pump will deliver current to the output loop filter. The delivered current enters the loop filter and is transformed into a voltage (V_out). Since the (V_out) start to rise, the VCO (F_out) frequency also increases.

When (V_out) is 2.5V the VCO frequency is 90 MHz. The divider divides it with 1800 and the output will be = 50KHz.
Now both A and B of the phase comparator is 50kHz and the current pump stops to deliver current and the VCO (F_out) stay at 90MHz.

What happends if the (V_out) is 5V?
At 5V the VCO (F_out) frequency is 108MHz and after the divider (1800) the frequency will be about 60kHz. Now B input of the phase detector has higher frequency than A and the current pump starts to zink current from the loop filter and thereby the voltage (V_out) will drop.
The reslut of the PLL system is that the phase detector locks the VCO frequency to desired frequency by using a phase comparator.
By changing the value of the N divider, you can lock the VCO to any frequency from 88 to 108 MHz in step of 50kHz.
I hope this example gives you understanding of the PLL system.
In frequency synthesiser circuits as LMX-serie you can program both the N divider and the reference frequency to many combinations.
The circuit also has sensitive high frequency input for probing the VCO to the N divider.
For more info I suggest you download the datasheet of the circuit.

Hardware and schematic
Please look at the schematic to follow my function description. The main oscillator is based around the transistor Q1. This oscillator is called Colpitts oscillator and it is voltage controlled to achieve FM (frequency modulation) and PLL control. Q1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor which works great.
The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with the varicap D1 and the two capacitor (C4, C5) at the base-emitter of the transistor. The value of C1 will set the VCO range.
The large value of C1 the wider will the VCO range be. Since the capacitance of the varicap (D1) is dependent of the voltage over it, the capacitance will change with changed voltage.
When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function.
You can use many different varicap diod to get it working. In my case I use a varicap (SMV1251) which has a wide range 3-55pF to secure the VCO range (88 to 108MHz).

Inside the dashed blue box you will find the audio modulation unit. This unit also include a second varicap (D2). This varicap is biased with a DC voltage about 3-4 volt DC. This varcap is also included in the LC tank by a capacitor (C2) of 3.9pF. The input audio will passes the capacitor (C15) and be added to the DC voltage. Since the input audio voltage change in amplitude, the total voltage over the varicap (D2) will also change. As an effect of this the capacitance will change and so will the LC tank frequency.
You have a Frequency Modulation of the carrier signal. The modulation depth is set by the input amplitude and therefore I have added a potentiometer (P1). Now you wonder why I don't use the first varicap (D1) to modulate the signal?
I could do that if the frequency would be fixed, but in this project the frequency range is 88 to 108MHz.
If you look at the varicap curve to the left of the schematic. You can easily see that the relative capacitance change more at lower voltage than it does at higher voltage.
Imagine I use an audio signal with constant amplitude. If I would modulated the (D1) varicap with this amplitude the modulation depth would differ depending on the voltage over the varicap (D1). Remember that the voltage over varicap (D1) is about 0V at 88MHz and +5V at 108MHz. By use two varicap (D1) and (D2) I get the same modulation depth from 88 to 108MHz.

Now, look at the right of the LMX2346 circuit and you find the reference frequency oscillator. This oscillator is based on the transistor (Q2) and a crystal of 13MHz. Together they form a very stable oscillator. Most crystals are very exact but some needs tuning. A variable capacitor (C25) in serial with the crystal give you the ability to change the frequency +/- a few kHz.
In most cases the crystal will be so good that you wont need to tune it, but since I wanted high quality transmitter I implemented this option to fine tune the reference frequency.

L2 is connected to the PLL LMX2346 circuit and will induce some of the energy from the VCO. The PLL circuit is very sensitive so, you need only 3 to 4 turns to get enough energy to get it working. The PLL will then use the VCO frequency to regulate the tuning voltage.
At pin 5 of LMX2346 you will find a PLL filter to form the (V_tune) which is the regulating voltage of the VCO.
The PLL try to regulate the (V_tune) so the VCO oscillator frequency is locked to desired frequency.

The last part we haven't discussed is the RF power amplifier (Q3). Some energy from the VCO is taped by (C6) to the base of the (Q3).
Q3 should be a RF transistor to obtain best RF amplification. To use a BC817 here will work, but not good.
The emitter resistor (R12) set the current through this transistor and with R12=100 ohm and +9V power supply you will have 150mW of output power into 50 ohm load. So don't overload this poor transistor, it will be hot and burn up…

PCB

Above you can download a (pdf) filer which is the black PCB. The PCB is mirrored because the printed side side should be faced down the board during UV exposure.
To the right you will find a pic showing the assembly of all components on the same board.
This is how the real board should look when you are going to solder the components.
It is a board made for surface mounted components, so the cuppar is on the top layer.
I am sure you can still use hole mounted components as well.

Grey area is cuppar and each component is draw in different colours all to make it easy to identify for you.
The scale of the pdf is 1:1 and the picture at right is magnified with 4 times.
Click on the pic to enlarge it.

Assembly
Good grounding is very important in a RF system. I use bottom layer as Ground and I connect it with the top layer at several places ( five via-holes) to get a good grounding.
Drill a small hole through the PCB an solder a wire in each via-hole to connect the top layer with the bottom layer which is the ground layer.
The five via-holes can easy be found on the PCB and in the assembly pic at right, they are labelled "GND" and marked with red colour.

Powerline:
Next step is to connect the power.
Add V1 (7805), C13, C14, C20, C21

Reference crystal:
Next step is to get the 13MHz reference crystal oscillator running.
Add the Xtal (13MHz), C22, C23, C24, C25, R4, R5, R6 and Q2.
Test:
Connect the main power and make sure you have +5V volt after V1.
Connect an oscilloscope or frequency meter to the emitter of Q2 and make sure you have an oscillation of 13MHz.
If you have an accurate frequency counter you can tune the oscillator to 13.0000MHz by turning C25.
If you don't have an accurate frequency counter or any counter at all, you can easy tune this unit later by using any FM radio receiver.
More about that later.

VCO:
Next step is to make sure the oscillator start to oscillate.
Add Q1, Q3,
L1, L3, L4,
D1, D2,
C1, C2, C3, C4, C5, C6,C7, C8, C9, C10, C11, C12, C18, C19,
R7, R8, R9, R10, R11, R12, R13, R14, R15

Now, connect a 50 ohm resistor from RF-out to ground as "dummy" load.
If you don't have a dummy load or an antenna the transistor Q3 will break easy.

When you connect the main power, the oscillator should start oscillate.
You can connect an oscilloscope to the RF output to probe the signal.
Make sure you have 3-4V DC at the junction of R13-R14.

TP is a "test point" which voltage (V_tune) will be set by the PLL circuit. You can use this output to measure the VCO voltage to test the unit. Since the PLL circuit hasn't been added yet, we can use this TP as input for testing the VCO and the VCO range.
The voltage at TP will set the oscillating frequency.
If you connect TP to ground, the VCO will be oscillating at it's lowest frequency.
If you connect TP to +5V, the VCO will be oscillating at it's highest frequency.
By changing the voltage at TP you can tune the VCO to any frequency in the VCO range.
If you have a radio in the same room you can use it to find the VCO frequency.
At this point there is no modulation of the transmitter, but you will still find the carrier with the FM receiver.

The inductance of L1 will affect the VCO frequency and VCO range very much.
By spacing/compressing L1 you will easy change the VCO frequency.
In my test I temporary connected TP to ground and used my wireless frequency counter to check
which frequency the VCO was oscillating at. I then spaced/compressed L1 until I got 88MHz.
Since TP was connected to ground I know 88MHz will be the lowest oscillating frequency of the VCO.
I then reconnected TP to +5V and checked the oscillating frequency again. This time I got 108MHz.
If you don't have a frequency counter you can use any FM radio to find the carrier frequency.
At this point the reference oscillator works and so do the VCO.
It is time to add the last components.

PLL:
Add the LMX2346 circuit, P1, L2, C15, C16, C17, R1, R2, R3
The LMX circuit is small so you must be careful soldering it.

Extra:

 It is important that you use a dummy load of 50ohm when you test the unit.

 It is important that the varicap is mounted in right direction (see schematic).

 It is important that you are careful and accurate when you solder the componets.

 Make sure you don't have any tin/lead bridges which short-circuit strip-lines to ground.

The RF unit is now ready to be connected to the Digitally controlled FM transmitter with 2 line LCD display

Component support
This project has be constructed to use standard (and easy to find) components.
People often write to me and ask for components, PCB or kits for my projects.
This is the only project I can offer you a start kit for a small fee.

The kit cost 15 Euro (18 USD) and includes:

 1 PCB (etched and drilled vias)

 2 Varicap (sm1251)

 1 Reference crystal (very accurate and match PCB)

 1 PLL circuit

 2 wires for the air coils

 1 BFR 93 NPN transistor

 2 BC817-25 NPN transistor

 1 variable capacitor C25

 2 100k ohm (0603 size) works equal good

 2 100 ohm (0805 size)

 10 100nF (0603 size) works equal good

Click here to mail me for more information about kit.

Final word
This part II describes the FM PLL controlled VCO unit.
Again, this is a strictly educational project explaining how a RF amplifier can be built.
According to the law it is legal to build them, but not to use them.

Part III
Next part will be the 1.5W Power amplifier unit.
It will soon be presented on my homepage.

1.3W Power Amplifier (Part III)

Background
Some amplifier is based around a RF transistor.
In my case I will use a common NPN RF transistor called 2SC1970.
You can also use other transistors like 2N4427 and some others.
Check datasheets to se how much power the transistor can handle.
2SC1970 will handle 1.3W and 2SC1971 up to 6W.
Most transistor has a power gain of 10dB meaning 10 times power amplification.
This is the reason why you must use several stages to achieve a strong transmitter.
In this construction we are getting higher power then ever and you must be careful with the transistor.
A 50 ohm dummy load MUST be used while testing, else the final transistor will break.
An antenna can be used but the antenna must be properly made else the transistor will break.
More about that later.

Hardware and schematic
In all RF system and specially in RF amplifiers, it is very important to have a stable power supply and making sure you won't get any RF out on the power line. The Capacitor C12 and C13 will stabilise the DC power supply. L1, C10, C11 and L3 with C8, C9 will also prevent RF from leaking out to the powerline and cause oscillation or disturbances. L1 and L3 should be ferrite chokes or inductance's about 1 to 10 uH.

Transistor Q1 will act as a buffer amplifier, because I don't want to load the previous stage to much.
The input RF signal is passin C1 and F1 which is a small ferrite pearl where the wire just passing through.
F1 with C2 will act as an impedance matching for Q1.
F1 can be substituted with a coil as L4, but in my test I found that the ferrite pearls gave best performances.
L2 is nit a critical component and any coil from 2-10uH will do the job. Q1 will amplify the input signal from 50mW to about 200mW.
Q1 can amplify much more, but It doesn't need to do that because 200mW is good for the final transistor.
If you want higher power you can decrease the resistor R2.

If you look at Q2 you will also find a ferrite pearl F2 at the base to emitter. This ferrite pearls is to set the DC voltage to zero and be a high impedance for RF signals. I wounded the wire 4 times around this small ferrite pearl. You can substitute it with a coil of 1uH or more.

C4, C5 and L4 forms an input matching unit for the transistor. Not much we can do about that…
At the output of the final transistor Q2 you will find 2 coils L5 and L6.
Together with C6 and C7, they form an impedance unit for the antenna and also for the transistor.

PCB

Если кому ни будь нужна прошивка на микроконтроллер

пишите мне на Email: evg-bubnov@yandex.ru .