Performance
I think that here I should give a little information about the actual measured values of the prototype. The target was to achiece a clean 100mW of output power at 9v. I also indended the transmitter to be equally stable at 13.8v DC since this is what most constructors seem to want. The target was exceeded on all counts. There are no spurious outputs visible from 500MHz upwards, so this spectrum analyser 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 negligible. I could hardly believe my eyes when I built the first prototype, but after cleaning up the PCB the output was even better! The vertical scale is 10dB per division and the horisontal 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 meticulous 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.
Parameter Supply=9v Supply=12.5v Supply=13.8v Freq range 76 - 116MHz 77 - 119MHz 78 - 121MHz Supply Current (98MHz) 75mA 85mA 95mA Output power (88MHz) 160mW 310mW 370mW Output power (98MHz) 180mW 360mW 420mW Output power (108MHz) 165mW 320mW 380mW Spurious Outputs (DC - 1GHz) -60dBc -60dBc -60dBc RMS AF for /-75KHz deviation 210mV 200mV 195mV AF response 0/-3dB 3Hz - 70KHz 3Hz - 70KHz 3Hz - 70KHz
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 modulate from DC to several thousand kilohetrz, the most obvious being music. It may be, however, that you wish to change the frequency (to somewhere legal?) and use FSK data for moving information between computers. Perhaps you even want to convey DC changes or just stabilise 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 varicap voltage is kept high by R1. Without this resistor the DC voltage on the diode will be zero, causing the oscillator to stop. R1 shall be removed if using external synthesiser control. We can, however, use the CTRL terminal to have a preset "frequency" potentiometer on the outside of the box. All we need is a 500K Linear potentiometer. Nothing else! no capacitors, 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 stabilised 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 asume 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 nicely. 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 impedance of the device. 4K7 is normal for computer and CD LINE-OUT signals.
Frequency Modulation Pre-emphasis
If you wished to use audio directly from a PC or CD-Player, then you should add some form of pre-emphasis. If you are using a stereo 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 synthesiser added to "Ctrl" on the circuit board. This will give the synthesiser a 10MHz tuning range, the preset capacitor determining the centre 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 modulated with NBFM. In this situation one MUST use synthesiser control to stabilise the frequency.
The transmitter is clean enough to drive larger power amplifiers to get much 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 primarily for the 144-146MHz amateur radio band.
Conclusion
I hope that you learn a lot from this project. It demonstrates Varicap diode tuning, decoupling and control. It also shows how to amplify low level signals from the 10mW level to the 150mW level and how DC bias can be applied to make compensations 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 whom have far more experience than I.
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