A discrete component transmitter circuit

Related component PDF download:

WBFM TX V7
By Harry Lythall

Introduction

Ok, so I have done a lot of playing around with the domestic FM bank This will probably be one of the last transmitters for the 88MHz to 108MHz band This particular TX is of special interest to those wishing to build low power Amplifiers for the VHF bands since it used effect matching, power amplifier and antenna filtering, all of which should be used by radio constructors, which it is for amateurradio or any other form of radio The features of this project are:

• Higher output power -150mW min (at 9v) and 300mW+(at 12.5v)
• Very pure output signal due to careful design and filtering
• VARICAP module - possibility to add a synthesizer
• Single side Printed Circuit Board, only 40mm x 72mm
• Covers the domestic FM band -88MHz to 108MHz
• Easy to build, but coil winding experience IS required

NOTE - This project is illegal to use or build in many countries I accept no responsibility what so ever for any illegal use This circuit is provided solid as an educational project Now I have got that off my best, let me get on with the project

The Circuit

The circuit itself is fairly conventional, with a couple of small refinements It all begins with TR1 (BC547) in an inverted Hartley oscillator configuration The feedback to the Base of TR1 is via a small 4.7pf capacitor to help keep the ossifications as weak as possible while allowing the ossifier to be a relevant starter The frequency of the oscillator is determined by L1 and the 22pf millimeter capacitor and functions in the range of about 76MHz to 119MHz using the PCB I have made

The 15pf capacitor couples the top of L1 to the variable diode which serves to add more capacity to the tuned circuit to alter the frequency R1 adds the supply voltage to the variable, with a little noise decoupling (the 0.1uf capacitor) If you are to use synthesiser control then it is important to remove R1 from the circuit, then connect the synthesiser loop filter output to the terminal marked "Ctrl" Audio is coupled to the BB105 via a 47K resistor There is only 47pf of decoupling in order not to restrict the AF bandwidth of the complete transmitter The AF bandwidth is flat from 3Hz to about 72KHz, but if we look beyond these limits, there is anIncreaseOf+6dB at DC This is because the two 47K resisters divide the AF input voltage by 2, but at DC the 0.1uf capacitor has time to charge, the two 47K resisters do not otherwise divide

TR2 (BC547) is both biomass and directly connected to the Emitter of TR1, which is a little unconventional in a VHF circuit I needed to get a good input to TR2 and cut down on components There are already far too many coils as it is in this circuit Remember that the BC547 is an audio transistor but works well at VHF The inductor in the Emitter of TR2 helps to extend the response a little to give a bit more signal to drive the final power amplifier transistor (TR3) TR2 gives no voltage gain; It is current we need to drive TR3 We already have enough volts from the oscillator

22pf and L3 couple TR2 output into the Base of TR3 These components match the impacts so we get the maximum power possible into TR3 Base The signal level, hower is still quite low, so some DC breathing has been added to turn TR3 ON a bit. The transition should draw about 5mA with no signal This is not enough to make it come linear, but it is operating around class "B" This would make a very poor frequency multiplier, so harmonics are also reduced a little by the DC bias Note that NO emitter resistor has been used The prototype units all worked well without one and the drive level is not enough to cause the transition to conduct very much The small standing DC bis of 5mA doesn't even "tick" TR3 In operation the DC voltage on the Base of TR1 will be negative due to the drive level, communication of TR3 Base/Emitter junction and the 22pf capacitor TR3 does NOT need a heatsink

If you want to use a different transistor in place of TR3 then I suggest you remove L3 and substitute a current meter in place of L4 Apply volts to the transmitter The current should be about 5mA Select the value of the 47K resistor if required Any current reading between about 2mA to 8mA"Will do nice sir"(even without your American Express card!)

The collector of TR3 (2N4427) has a big (by QRP standards) choose to pass the supply DC, but presents a high impact to RF The RF signal is then matched to 50 Ohms with the 15p, L5 and 56p The 1nf cap simply blocks the supply voltage that would otherwise pass to the antenna L5 and 56pf form a low pass filter that helps to block harmonics present in the output signal L6 and 47pf are added to further reduce the harmonic levels This filter is an absolute MUST for all transmitters if one does not wish to offer every other user of the radio spectrum L5 and L6 have also been positioned on the PCB so that there is a little coupling between them This coupling services to cancel out any residual signals, not within the passband of the filter, that may be present at the input to L5 It is this effective that was responsible for the unexpected cleaning of the first prototype, and a small layout experiencing has now reduced the 2nd and 3rd characteristics to -60dBc at all supply voltages With 150mW output, this responses to 3rd harmonic of 150 nano watts and a 2nd harmonic level of just 50 nano watts

I have "played around" with the values and take a few freedoms If you want to try to adjust the coils then then will be able to get another 2 to 3dB out of the TX. I have delicately missed a couple of times in order that impacts and relationships will improve at the edges of the bank The result is that the performance of the transmitter does NOT vary (much) no matter which end of the band you are operating at

Coils

As you will adapt, L1 and the tuning capacitors all affect the frequency of the complete transmitter Winding L1 has more NOT been considered This would result in a spring like affair that would cause facility, or more precisely, "microscopic" This is an effect where the coil wobbles about with very small mechanical movements In severe cases you can even talk to the circuit, as any owner of a Marconi TF995 signal generator will test By using a coil etched on the PCB you will find that tiny has been eliminated So has coil expansion with temperature It must be remembered, hower, that this circuit is STILL based on an LC circuit and there object to changes of frequency with changes of supply voltage and "hand capacity", etc. I will cover the supply voltage changes shortly

L2 is wond on small ferrite beans L2 is placed in series with the emitter of the buffer transistor, TR2 In the interests of stability it is very important that this coil does NOT radial like a loop antenna It is composed of 4 turns of 0.15mm Dia Enamelled wire (magnet wire in the US) The grade of fertility is unimportant, as long as it is a grey one One complete turn is formed when the wire passes through the hole in the middle once The ferrite is mounted vertically in the same manager as a resister The ferrite is 3mm outside diameter and the assembled coil looks like this:

L3 is sound using 3 turns of 0.8mm Dia enamelled copper wire with a 6mm inside diameter Wind it on a drill bit to get the inside diameter correct The coil is close won, that is to say that the turns are just about touching and shall not be spaced Form the ends of the coil, binding them out then down so that the leads are 5mm apart The coil should look like this:

L4 compares 6 turns of 0.15mm Dia Enamelled wire (magnet wire in the US) won on TWO ferrite beans, the same beans that were used for L2 The beans are placed side by side as a pair of biniculars One complete turn is when the wire is threaded once thought both beans

L5 is 5 turns of 0.8mm Dia Enamelled copper wire with a 6mm inside diameter Just as L3, wind it on a drill bit to get the inside diameter correct The coil is also close wond Form the ends of the coil, binding them out then down so that the leads are 5mm apart

L6 is 3 turns of 0.8mm Dia Enamelled copper wire with a 6mm inside diameter Just as L3 and L5 the coil is also close wond Form the ends of the coil, binding them out then down so that the leads are 5mm apart You can get a closer view of L5 and L6 in this picture The position of the capacitor between them is very important, equal distance from both coils keeps the 2nd harmonic at the lowest level

NOTE - L5 and L6 MUST be wond in the same direction If you try to wind one of them backwards or in the other direction then the sporadic outputs will increase

Construction

The transmitter is constructed on a single sided printed circuit board I will place the PCB foil pattern onMy DOWNLOAD sectionOf the homepages The board is only 40mm x 72mm If you use any other construction method then you will need to change a feed component values If you use verobard, prototype board then the project will probably not work. The board has been designed to compensate for the layer of a second partner ground land and has also "thermal breaks" around some of the component connections This makes it easier to solver for new starters (for me too, but I should not admit that!)

You will note that the 15pf capacitor coupling L1 to the BB105 variant diode is playing on the board It 磗 legs are so formed that it acts as a link Assembly order is not particularly important, but it is easier if all domestic components are mounted first, then the passive components (resisters/caps), transistors and the coils last Necessity and attention to detail is particularly important The vertically mounted resisters should all be mounted as shown on the component overlay It DOES matter which way round they are This is one of the prices for using a cheap single sided board

Testing

When all the components have been fitted, check your work thoroughly I reccomend you shine a strong lamp behind the board component side and compare the tracks with the PCB foil pattern This will allow you to check for solver bridges between tracks Assumming all is well, connect a 50 Ohm resistor to the antenna (ANT) terminals Two 100 Ohms in parallel will be fine Now connect the board to a 9v supply in series with a 12v 3W torch lamp If the lamp glasses brightly then switch OFF and check your wiring because you have a fault If there is no fault then the lamp should only glow dimly, if it glows at all The complete transmitter should draw less than 100mA

If all is well, switch ON an FM radio set tuned to somewhere around 108MHz Adjust the tuning capacitor on the board so the plates are at around minimum capacity and you will hear the transmitter on the radio With the capacitor plates near maximum capacity you should be able to tune the transmitter to 88MHz

Now couple the AF IN terminals of the transmitter to the LINE OUT of a stero ststem, your computer, or even the headphone terminals of your Sony Walkman I prefer to use headmatter terminals since the volume control will give you some control over the modulation depth You can set the modulation depth by comparing it with another radio channel Set your transmitter A LITTLE LOWER IN VOLUME than other channels, unless you have access to a modulation meter Note that you may have to use a capacitor in series with the AF input wire See the application data further down

If your transmitter is working then you can remove the test lamp and connect the battery supply directly to the transmitter Check that nothing is burning TR3 should get a little warm, but compatible to the touch All other components should remain stone cold TR3 may get a little warmer if you increase the supply voltage to 13.8v but in this case the transmitter will be delivering most half a watt of output power

Performance

I think that here I should give a little information about the actual measured values of the prototype The target was to achieve a clean 100mW of output power at 9v I also led the transmitter to be equally stable at 13.8v DC since this is what most constructors see to want The target was exceeded on all counts There are no sporadic outputs visible from 500MHz upgrades, so this spectrum analyzer view is only from DC to 500MHz It shows that there is a little 2nd and 3rd harmonic outputs, but the levels are so low that they are quite neutral I could hardly believe my eyes when I build the first prototype, but after cleaning up the PCB the output was even better! The vertical scale is 10dB per division and the hormonal scale is 50MHz per division:

As you can see, the worst case is the 3rd harmonic at -60dBc The carrier level was+23dBm (200mW) at 10v supply This falls off a little to about 160mW at the ends of the bands Brief specifications are given below I have not been all that metallic with the figures When I got a reading of 73mA I rounded it up to 75mA to keep the figures simple The figures are only a guide anyway

Alignment

Adjust the variable capacitor to get the transmitter on the frequency you want Is that simple enough?

Applications

There are several different applications since the unit will modify from DC to several thousand kilohetrz, the most objective being music It may be, how, that you wish to change the frequency (to somewhere legal?) and use FSK data for moving information between computers Perhaps you even want to buy DC changes or just stabilize the TX frequency Let us now cover these items, beginning with a recap of the circuit diagram:

Frequency Control

(Not to be used with synthesiser)

As you can see from the original circuit, the variable voltage is kept high by R1 Without this resistor the DC voltage on the diode will be zero, using the oscillator to stop R1 shall be removed if using external synthesizer control We can, how, use the CTRL terminal to have a preset "frequency" potential on the outside of the box All we need is a 500K Linear potentiometer Nothing else! No captors, nothing This will give typically 10MHz tuning range

Frequency Stabiliser

(Not to be used with synthesiser)

Given that R1 is connected directly to the battery supply voltage, if the battery voltage were to vary then so would the TX frequency You should really be using a stable power supply, or a high current battery that has a fairly constant supply voltage If this is NOT the case then you can use the CTRL terminal to bypass R1, without making any modifications to the TX. All we need is an external zener diode and a 6K8 resistor The Zener diode should be as high as possible If you have a 12.5v supply, for example, then a 10v diode would be great With a 9v battery then a 6v8 diode is about the maximum practical 8v2 would be Ok until the battery voltage went down a bit. We will ask that+VE is a 9v battery

Frequency Modulation

If you are using DC modulation then the AF input will allow this This can be used to give low frequency Frequency Shift Keying This should only be done in conjunction with the voltage regulator above If you are NOT wanting to have a DC shift, and your input source has a "DC Continuity" (resistance) then there is problem If you were to connect a magnetic microphone or CD player, for example, to the AF input then the TX frequency would jump You therefore need to add a capacitor to block the DC shift at the input 10uF will do nice A 4K7 resistor should also be added to give a load to the audio source and to help prevent "hum" or "pickup" from 50Hz (60Hz) wiring. The value of the resistor should be selected to match the audio source effect of the device 4K7 is normal for computer and CD LINE-OUT signals

Frequency Modulation Pre emphasis

If you wish to use audio directly from a PC or CD Player, then you should add some form of pre emphasis If you are using a steel encoder or FSK applications then you should NOT use pre emphasis Pre emphasis increases treble a little so that when it is received and turned down again, added noise is also turned down Select Cx (nf) to be 10x the number of Microseconds of pre emphasis you want If you wanted 50uS pre emphasis then Cx=50 x 10 nf=500nf

Possibilities

The resistor R1 may be removed from the circuit and the loop voltage from a syntactiser added to "Ctrl" on the circuit board This will give the synthesizer a 10MHz tuning range, the preset capacitor determining the center frequency The loop frequency control voltage can be increased to about 20v but it should be maintained somewhere around 9v to get repeatable modulation characteristics

The coil of the tuned circuit can be reduced and TR1/TR2 replaced for more suitable RF devices, then the TX can be increased to the 144MHz band and modified with NBFM In this situation one MUST use synthesizer control to stabilize the frequency

The transmitter is clean enough to drive larger power amplifiers to get multiple higher powers I will be working on both 5-Watts and 25 Watts amplifiers at some time in the future, but exactly when is another question The more e-mail I get then the longer it will take My work will be primary for the 144-146MHz amateurradio band

Inclusion

I hope that you learn a lot from this project It demonstrates Varicap diode tuning, decoupling and control It also shows how to sample low level signals from the 10mW level to the 150mW level and how DC bias can be applied to make compensation for low signal levels Have fun with the project If you have any questions then please do NOT e-mail me. I have a message board where you may post questions to many, some of what has far more experience than I

Finally, is there any interest in my producing a kit for this project? I would include the input cap, pre emphasis and Zener on the kit PCB, but there would be details showing how to bypass this for those who wish to do more than just send music Please let me know atMailto: harryvpo@hotmail.com ? Subject=v7_kitKeeping your mail short as possible, thanks

Very best regulations from Harry - SM0VPO, Lunda, Sweden