Three :If you feel that the RF output signal is too low or that the frequency changes too much when somebody approaches the Antenna ("Frequency Pulling") , then you need an extra transistor acting as an Amplifier/Buffer.

Three Transistor "FM Bug" Courtesy Harry Lythall SM0VPOVisit Harry for more details, PCB layout, Performance specs

The last stage offers about 10dB of gain, but more importantly, provides a degree of isolation between the Oscillator and the Antenna. This makes the circuit a bit more stable against frequency changes when somebody comes near the antenna.

Cana Kit's CK111:The CK111 that Cana Kit sent me is a two transistor kit. The kit does not have the first Audio stage, they drive the Oscillator directly from the Electret Microphone. An Amplifier/buffer stage feeds the antenna.

The Cana Kit came with excellent documentation in English and Francais. It showed all components pictorially for ease in identification and even listed the color bands on each resistor used. It had basic tips on kit building , step by step instructions on what to solder in next and a complete circuit diagram. I wish the documentation had a little bit of theory or explained circuit operation. The PCB and all components were of high quality. Thankfully the capacitors were very easy to identify.

My 6 year old daughter built the kit in about 45 minutes, under my supervision. It worked first time.

The CK111's main tuning coil is 6 turns of copper wire wound around a small ferrite slug. It comes pre-built, even with the tap on the third turn. All you need to do is plonk it in and solder it. As the coil was correctly designed, we could immediately tune to the recommended 90 MHz without any fuss. We could get a range of about 30 meters around our building ( some turns and one wall) which makes me believe the 200 mtr range claimed for open spaces. As the kit uses up only 25mA at 6V, I am sure that it can be easily altered to get the 500-1000 mtr open space range that Harry gets out of similar circuitry biased to use 50 mA at 9V. It might need a different output stage though !

The suggested antenna is a piece of wire a quarter of a wavelength long , about 83 cm ( supplied with the kit)

We found the Electret mike to be very sensitive and it was very easy to overmodulate the transmitter by speaking close to the microphone. This justifies Can Kit's decision not to

use an Audio stage. Looks like an Audio Limiter or Automatic Gain Control (AGC) is totally necessary for a bug so that it can pick up whispers at 3 metres, without overmodulating when someone speaks 10 cm away from the Mic.

As the kit does not have pre-emphasis, the radio receiver unnecessarily de-emphasizes all the high frequency content leading to a slightly boomy bass voice. On top of that, the Electret mike is fairly directional so if you do not speak directly into it ( when used as a room bug, for example), it catches a rebound from a wall which makes the voice boom even more. Besides this boominess, the Audio was very clean with no noticeable distortion.

Cana Kits inform me that they have just released a new version of this kit, which features an on-board Mic as well as an RCA input to connect to a Walkman. One can choose between the two inputs by a Jumper. I hope that kit has pre-emphasis as that's totally necessary when transmitting music.

The Frequency of operation shifts about 100 KHz initially till circuit reaches Thermal equilibrium, after which it is quite stable. We have not yet run its battery down but I am sure its frequency will change slightly as the batteries go weak. If you change the antenna characteristics a bit by folding up the antenna wire, the transmitted frequency can shift by as much as 400 KHz.

All in all, an excellent kit for a rank beginner who will be amazed when he firsts conquers the air waves.

DIY Electronics' Kit 32:Don't be fooled by the title of this kit "Two stage FM Transmitter". While it does have two RF stages ( A Colpitts Oscillator and an Amplifier/buffer), it also has an Audio stage. So it could possibly lay claim to being a three stage circuit, very similar to Harry's 3 transistor circuit shown above. The only difference is that Harry uses a broadband last stage while the Kit 32's final stage has to be tuned.

The Kit 32 fits all the three stages unto a small, well designed PCB measuring only 17mm x 70mm. The resistors are stood up on one end to save on PCB real estate.

I found the Kit 32 quite easy to build as the instructions are sufficient. To aid novices, the documentation lists the color codes of each resistor used. The documentation included some basic description of the circuit and a complete circuit design. DIY posts the PDF of their kit's documentation online so you can check out exactly what you are buying into before you put your money down :) I wish more kit manufacturers would follow DIY's lead. ( Kit 32's PDF)

The DIY Kit 32 is well constructed and designed. All necessary components are supplied except the two optional capacitors needed for changing FM frequency range and for loose coupling the Antenna. It took me about an hour to build. It worked first time. The coils needed are pre-wound but you need to attach a center tap to the tank inductor yourself.

Though I'm OK on soldering, it took me about 15 minutes to accomplish that simple task to my satisfaction. Maybe if I had a conical tip soldering rod instead of a wedge.....

The Audio stage makes Kit 32 so sensitive that it can easily over-modulate if you whisper closely, while it also picks up quite a bit of unwanted noise. Any slight over-modulation results in extreme distortion in the FM receiver. Maybe a variable gain control pot or instructions on how to adjust the Audio gain would have been nice.

Lack of Pre-emphasis : As with most other simple FM bug circuits, the DIY Kit 32 did not have Pre-emphasis, leading to a boomy bass voice.

Range : DIY claim that upto 1 Km range can be achieved with 9V and the supplied half wave antenna. They claim even more range with a 15V supply and a Dipole Antenna.

Tuning the Kit 32 is an interesting exercise because of many factors -

Tuning Range : The Kit 32 cannot tune to the whole FM band as built. Components are supplied to tune from 98 to 105 MHz. You can, of course, change this a little by compressing/expanding the tuning coil but to reach the lower ends of the FM band, you need to replace the 10pF tank capacitor with a 27pF ( Not supplied, I wish it was)

Sensitivity of the trim cap : I found that moving the trimmer cap plates by 15 degrees shifted the frequency by about 3 MHz. That is very sensitive indeed and you need a very steady hand to reach your desired frequency of operation.

Thermal stability : As with all free running oscillators, the Kit 32 shifts frequency of operation till it reaches thermal stability. Here is my test data -

Cold Start 104.10 MHzAfter 2 Min 104.00 MHzAfter 8 Min 103.95 MHzAfter 15 Min onwards 103.90 MHzAfter blowing on the circuit 103.95 MHz

Frequency Pulling : Though the kit does have an amplifier stage which also provides a measure of buffering, one can easily see the effects of Antenna loading as explained in point C above.

DIY suggests a piece of wire that is half a wavelength. This works out to be a 160 cm long wire (supplied with the kit) which makes it pretty hard to conceal if used as a bug.

As an aside, some kits suggest a quarter wave antenna which is only about 83 cm long making it much more manageable. The half wave and quarter wave antenna actually need a good ground plane to operate ( 3-4 radial wires about quarter wavelength long), which such kits do not have so I am uncertain on which antenna is better when used in this way.

Other kits suggest any odd piece of wire, maybe only 20 cms long, like what you see at the base of wireless mikes. The idea is that you do not want a well matched antenna as better match = more effects of Antenna loading.

Here's my data on Frequency Pulling on the Kit 32 -

Kit on table, Thermally stable and Antenna wire in straight line 103.90 MHzHolding the unboxed kit in the hand 103.45 MHzKit on table, Thermally stable, Antenna wire folded up a bit 103.80 MHz

Above was done with the 1/2 wave antenna as suggested. When I cut the Antenna wire to a more manageable 1/4 wave, I found that folding the Antenna wire would change the frequency by upto 2.5 MHz !!!!

The documentation suggests that a 10pF coupling capacitor ( Not supplied, I wish it was) be connected in front of the Aerial. This would make the coupling of the last stage to the antenna "loose", giving you greater stability at the expense of range.

Frequency Pushing : Though the kit comes with a snap-on for a 9V battery, DIY suggests that you can use upto 15 V to get maximum power. As with all such circuits, the frequency of operation will shift with power supply.

Here's my data :

New 9V battery delivering 9.4V and 25 mA 103.90 MHzOlder 9V battery delivering 5.5V and 11 mA 102.20 MHz

From the Cc versus Vcb graph above, we see that lower Collector-Base voltage = more Collector capacitance. We know that more capacitance in tank circuit = lower frequency of operation.

As Kit 32 opted for a tuned final stage, it can only output maximum power when the output stage is tuned to the exact frequency of the oscillator stage. DIY includes a "Peaking Circuit kit" along with Kit 32. This consists of diodes and capacitors to rectify and sample the RF voltage at the output. One of the inductors on the last stage has to be expanded/compressed for maximum voltage readout on a Voltmeter connected to the peaking circuit. DIY recommend this is done with the peaking circuit in place instead of the real antenna. When the circuit is peaked, the antenna wire can be replaced.

As the last stage has to be tuned for the correct frequency of operation (which shifts with power supply, temperature and Antenna loading), it is best to attempt tuning after the kit is boxed and correct power supply used. You need to wait for thermal stability, make a little change to the Inductor, move away from the kit, wait 2-3 minutes, look at the Voltmeter and decide what to do next. All the while, you have to have a radio receiving the kit's signal so that you know you are peaking the right signal.

Oh yes, I forgot to tell you, changing the inductance on the last stage alters the operating point of the last stage, which now presents a slightly different impedance to the oscillator stage which, you guessed it, makes the oscillator output a slightly different frequency.

And when you connect the real antenna back again, .............

All that reminded me of one of Einstein's theory about the very act of trying to determine where an atom is at a given moment changes where the atom was. Tuning such kits is like hitting a moving target with the added problem that when you hit the target, you move it even further and then they tell you it was the wrong moving target anyway !!

I took the easy way out. The kit's documentation said that "almost certainly the (last stage's) coil turns have to be spread very far apart" so that's exactly what I did. One day I will alter the final stage's tuning circuitry and replace it with a resistor, making it more like Harry's last broadband amplifier/buffer stage shown above. I am not sure that the output power at desired frequency will be much different between the two options, I only expect the tuned last stage to have much less harmonics than the broadband stage.

Bottom Line : Both the Cana Kit CK 111 and the DIY Electronics Kit 32 are well designed kits, easy to assemble and appropriate technology for rank beginners. In the end, if well built and boxed, you have workable FM bugs with atleast 30 mtrs of very clear reception inside buildings and maybe 200-1000 mtrs out in the open. Getting to your desired frequency of operation and staying there will be a major challenge.

They performed exactly as can be expected from circuits of their class and are guaranteed to raise further interest in features most noticeable by their absence in such simple kits.

I would think such kits make ideal presents for all the 6-16 year old nephews and nieces you have. If you also help them build the kit, you have a great excuse to spend some "Quality time" with the kids. And who knows, you just might have started one of the kids on the road to become an RF Engineer ?

Wanna have your own page here, write about your own experiences, own review, refute existing review,

add comments to existing review, publish your circuits ?Contact me

 

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4 Watt FM Transmitter

more info / buy kit: www.smartkit.gr

 

Copyright of this circuit belongs to smart kit electronics. In this page we will use this circuit to discuss for improvements and we will introduce some changes based on

original schematic.

 

General Description

This is a small but quite powerful FM transmitter having three RF stages incorporating an audio

preamplifier for better modulation. t has an output power of 4 Watts and works off 12-18 VDC

which makes it easily portable. It is the ideal project for the beginner who wishes to get started in

the fascinating world of FM broadcasting and wants a good basic circuit to experiment with.

 

Technical Specifications - Characteristics

Modulation type: ........ FMFrequency range: .... 88-108 MHzWorking voltage: ..... 12-18 VDCMaximum current: ....... 450 mAOutput power: ............ 4 W

How it Works

As it has already been mentioned the transmitted signal is Frequency Modulated (FM) which

means that the carriers amplitude stays constant and its frequency varies according to the

amplitude variations of the audio signal. When the input signals amplitude increases (i.e. during

the positive half cycles) the frequency of the carrier increases too, on the other hand when the

input signal decreases in amplitude (negative half-cycle or no signal) the carrier frequency

decreases accordingly. In figure 1 you can see a graphic representation of Frequency Modulation

as it would appear on an oscilloscope screen, together with the modulating AF signal. The output

frequency the transmitter is adjustable from 88 to 108 MHz which is the FM band that is used for

radio broadcasting. The circuit as we have already mentioned consists of four stages. Three RF

stages and one audio preamplifier for the modulation. The first RF stage is an oscillator and is

built around TR1. The frequency of the oscillator is controlled by the LC network L1-C15. C7 is

there to ensure that the circuit continues oscillating and C8 adjusts the coupling between the

oscillator and the next RF stage which is an amplifier. This is built around TR2 which operates in

class C and is tuned by means of L2 and C9. The last RF stage is also an amplifier built around

TR3 which operates in class C the input of which is tuned by means of C10 and L4. From the

output of this last stage which is tuned by means of L3-C12 is taken the output signal which

through the tuned circuit L5-C11 goes to the aerial.

The circuit of the preamplifier is very simple and is built around TR4. The input sensitivity of the

stage is adjustable in order to make it possible to use the transmitter with different input signals

and depends upon the setting of VR1. As it is the transmitter can be modulated directly with a

piezoelectric microphone, a small cassette recorder etc. It is of course possible to use an audio

mixer in the input for more professional results.

 

Construction

First of all let us consider a few basics in building electronic circuits on a printed circuit board. The

board is made of a thin insulating material clad with a thin layer of conductive copper that is

shaped in such a way as to form the necessary conductors between the various components of

the circuit. The use of a properly designed printed circuit board is very desirable as it speeds

construction up considerably and reduces the possibility of making errors. Smart Kit boards also

come pre-drilled and with the outline of the components and their identification printed on the

component side to make construction easier. To protect the board during storage from oxidation

and assure it gets to you in perfect condition the copper is tinned during manufacturing and

covered with a special varnish that protects it from getting oxidised and also makes soldering

easier. Soldering the components to the board is the only way to build your circuit and from the

way you do it depends greatly your success or failure. This work is not very difficult and if you

stick to a few rules you should have no problems. The soldering iron that you use must be light

and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at

all times. For this purpose come very handy specially made sponges that are kept wet and from

time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate

on it. DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it.

There are many different types of solder in the market and you should choose a good quality one

that contains the necessary flux in its core, to assure a perfect joint every time. DO NOT use

soldering flux apart from that which is already included in your solder. Too much flux can cause

many problems and is one of the main causes of circuit malfunction. If nevertheless you have to

use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after

you finish your work. In order to solder a component correctly you should do the following:

- Clean the component leads with a small piece of emery paper.

Bend them at the correct distance from the components body and insert the component in its

place on the board.

- You may find sometimes a component with heavier gauge leads than usual, that are too thick to

enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes slightly.

- Do not make the holes too large as this is going to make soldering difficult afterwards.

- Take the hot iron and place its tip on the component lead while holding the end of the solder

wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly

above the p.c. board. - When the solder starts to melt and flow wait till it covers evenly the area

around the hole and the flux boils and gets out from underneath the solder. The whole operation

should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally

without blowing on it or moving the component. If everything was done properly the surface of the

joint must have a bright metallic finish and its edges should be smoothly ended on the component

lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you

have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo

it.

- Take care not to overheat the tracks as it is very easy to lift them from the board and break

them.

- When you are soldering a sensitive component it is good practice to hold the lead from the

component side of the board with a pair of long-nose pliers to divert any heat that could possibly

damage the component.

- Make sure that you do not use more solder than it is necessary as you are running the risk of

short-circuiting adjacent tracks on the board, especially if they are very close together.

- When you finish your work cut off the excess of the component leads and

clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain

on it.

This is an RF project and this calls for even more care during soldering as sloppiness during

construction can mean low or no output at all, low stability and other problems. Make sure that

you follow the general rules about electronic circuit construction outlined above and double-check

everything before going to the next step. All the components are clearly marked on the

component side of the P.C. board and you should have no difficulty in locating and placing them.

Solder first of all the pins, and continue with the coils taking care not to deform them, the RFCs,

the resistors, the capacitors and finally the electrolytic and the trimmers. Make sure that the

electrolytic are correctly placed with respect to their polarity and that the trimmers are not

overheated during soldering. At this point stop for a good inspection of the work done so far and if

you see that everything is OK go on and solder the transistors in their places taking grate care not

to overheat them as they are the most sensitive of all the components used in the project. The

audio frequency input is at points 1 (ground) and 2 (signal), the power supply is connected at

points 3 (-) and 4 (+) and the antenna is connected at points 5 (ground) and 6 (signal). As we

have already mentioned the signal you use for the modulation of the transmitter could be the

output of a preamplifier or mixer or in case you only want to modulate it with voice you can use

the piezoelectric microphone supplied with the Kit. (The quality of this microphone is not very

good but it is quite adequate if you are interested in speech only.) As an antenna you can use an

open dipole or a Ground Plane. Before you start using the transmitter or every time you change

its working frequency you must follow the procedure described below which is called alignment.

Parts List

R1 = 220KR2 = 4,7KR3 = R4 = 10KR5 = 82 OhmR = 150Ohm 1/2W x2 *VR1 = 22K trimmer

C1 = C2 = 4,7uF 25V electrolyticC3 = C13 = 4,7nF ceramicC4 = C14 = 1nF ceramicC5 = C6 = 470pF ceramicC7 = 11pF ceramicC8 = 3-10pF trimmerC9 = C12 = 7-35pF trimmerC10 = C11 = 10-60pF trimmerC15 = 4-20pF trimmerC16 = 22nF ceramic *

L1 = 4 turns of silver coated wire at 5,5mm diameterL2 = 6 turns of silver coated wire at 5,5mm diameterL3 = 3 turns of silver coated wire at 5,5mm diameterL4 = printed on PCBL5 = 5 turns of silver coated wire at 7,5mm diameter

RFC1=RFC2=RFC3= VK200 RFC tsok

TR1 = TR2 = 2N2219 NPNTR3 = 2N3553 NPNTR4 = BC547/BC548 NPND1 = 1N4148 diode *MIC = crystalic microphone

Note: Parts marked with * are used for the tune-up of the transmitter in case you have not a stationary wave bridge.

 

Adjustments

If you expect your transmitter to be able to deliver its maximum output at any time you must align

all the RF stages in order to ensure that you get the best energy transfer between them. There

are two ways to do this and it depends if you have a SWR meter or not which method you are

going to follow. If you have a SWR meter turn the transmitter on, having connected the SWR

meter in its output in series with the antenna, and turn C15 in order to tune the oscillator to the

frequency you have chosen for your broad casts. Then start adjusting the trimmers C8,9,10,12

and 11 in this order till you get the maximum output power in the SWR meter. For those who dont

have a SWR meter there is another method which gives quite satisfactory results. You only have

to build the little circuit in Fig. 2 which is connected in the out put of the transmitter and in its

output (across C16) you connect your multi-tester having selected a suitable VOLTS scale. You

tune C15 in the desired frequency and then adjust the other trimmers in the same order as it is

described above for the maximum output in the multitester. The disadvantage of this method is

that you do not align the transmitter with a real antenna connected in its output and it may be

necessary to make slight adjustments to C11 and C12 for a perfect antenna match.

Do not forget to align your transmitter every time you change your aerial or your working

frequency.

WARNING: In every transmitter there are present apart from the main output frequency various

harmonics that usually have a very short range. In order to make sure you havent tuned on one of

them do the tuning as far as possible from your receiver, or use a Spectrum Analyser to see your

output spectrum and make sure that you tune your transmitter on the right frequency.

 

Warning

Smart kits are sold as stand alone training kits.

If they are used as part of a larger assembly and any damage is caused, our company bears no

responsibility.

While using electrical parts, handle power supply and equipment with great care, following safety

standards as described by international specs and regulations.

CAUTION

All the RF kits are sold for experimental and laboratory use only. Their possession and use are

limited by laws which vary from state to state. Please get information about what you can or can

not do in your area and stay within the legal limits. Make sure you do not become a nuisance to

others with your experiments. Smart Kit has no responsibility whatsoever for any misuse of its

products.

 

If it does not work

- Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues

that usually cause problems.

Check again all the external connections to and from the circuit to see if there is a mistake there.

- See that there are no components missing or inserted in the wrong places.

- Make sure that all the polarised components have been soldered the right way round.

- Make sure that the supply has the correct voltage and is connected the right way round to your

circuit.

- Check your project for faulty or damaged components.

If everything checks and your project still fails to work, please contact your retailer and the Smart

Kit Service will repair it for you.

 

Electronic Diagram

IF YOU ARE GREEK READ : PAGE1 & PAGE2

Also check this site for additional information

 

 

Improvements on original desing

Thomas [thomasciciyan @ yahoo.com] made some improvement on original design:

Ability for capacitor microphone

Preamplifier for the mic

External audio input jack, for example: to transmit sounds from an computer

audio card

A selector switch to select microphone or input jack

PCB redesign

 

New PCB design - Download PCB in Protel 2.7.1 format 

New components layout

Parts added:

C17 (1uf) C18 (4uf7)

C19 (4uf7) C20 (100NF)

H1 (13*13*10MM) JACK1 (STEREO)

LED2 (RED) MIC (CAPACITIC)

R6 (1M8) R7 (1K5) R8 (560K) R9 (820R) R10 (4K7) R11 (4K7) R12 (1K) R13 (1K) R14 (2K2) R15 (680R) TR5 (BC547\BC548) VR1 (50K)

 

Downloads

  Download this project in Persian language

 With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo meters. The transistor Q1 and Q2 forms a classic high sensitive preamplifier stage. The audio signal to be transmitted is coupled to the base of Q1 through capacitor C2. R1, R3, R4, R6, R5 and R9 are the biasing resistors for the preamplifier stage comprising of Q1 and Q2. Transistor Q3 performs the collective job of oscillator, mixer and final power amplifier.C9 and L1 forms the tank circuit which is essential for creating oscillations. Inductor L2 couples the FM signal to the antenna.

Circuit diagram.

Assemble the circuit on a good quality PCB. The circuit can be powered from anything between 9 to 24V DC. Inductor L3 can be a VK220J type RFC. For L1 make 3 turns of 1mm enamelled copper wire on a 10mm diameter plastic

former. On the same core make 2 turns of 1 mm enamelled copper wire close to L1 and that will be L2.

Frequency can be adjusted by varying C9. R9 can be used to adjust the gain. For optimum performance, value of C8 must be also adjusted.

Using a battery for powering the circuit will reduce noise.

Single Chip FM Transmitter, For Short Range ApplicationPosted on May 28, 2009 by Adam Sponsored Links

The FM transmitter circuit here works in the broadcast band 88 to 108MHz, and can be used to broadcast audio signals for remote listening.  The output power is around -21dBm, and the circuit operates with 3V power supply. Here is the schematic diagram of the circuit:

If you want to carry out your portable sound system around the house, this simple FM transmitter can be applied to links your mp3 player or your home-entertainment system. For example, if you want enjoy your favorites song in the backyard while doing barbecue, you can play your CD or MP3 player in your room and broadcast with this FM transmitter. All you need in the backyard is  tuning your portable FM radio receiver to the music.  [Source: MAXIM Integrated Product Application Notes]

This entry was posted in RF and Video and tagged FM Transmitter. Bookmark the permalink.,

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