Harros said:Well, for the transmitter part, the output swing of the crystal oscillator is around 4Vpp, so, is there any suitable amplifier for this situation? This is because I want to increase the transmit power of the transmitter... Or should I use voltage divider on the output of the crystal oscillator and implement the small signal amplifier (just like the type that i build for the receiver) to increase the transmitter power?
RadioRon said:I would use no voltage divider or perhaps a modest voltage division ratio, but you don't need to attenuate the signal. A better approach is to use the same transistor in your amplifier but alter the bias point by adjusting the resistor values. When you have a lot of voltage swing available like this and you aren't carrying modulation on the carrier, a simple class C bias is appropriate. You can achieve this by removing the base bias resistors and simply letting the input signal turn the transistor on and off. You could also reduce the emitter resistor a little bit too so that more output power can be had.
Harros said:Is there any example of this type of amplifier?
Harros said:I have solved the etching problem, the ground plane in all the circuits are perfect in shape as well as the connectivity... Well, regarding the Class C Amp, I have designed an Class C Amp bu referring to a example model on internet. But the circuit is not working... Did I do any mistakes in designing this amp circuit?
Harros said:Is the RF choke the ferrite bead? By the way, i dont really know the correct method to design class C amp, as i never touched this type of amps before. Well, do you have any idea on the determining the value of the components (inductor, rf choke, and capacitors) used in the amp?
Besides, I can hardly get a exact ferrite bead at my place for the particular ferrite bead used in the simulator, and I am thinking of editing the PSpice model of the ferrite bead... Do you have any idea on the corresponding values defined in the PSpice model of the ferrite bead?
Harros said:Well, I have redesigned this circuitry for the amp. The output is better than before, but the output waveform is not desirable. Any comment on this circuitry?
Harros said:Is it better for me to put the low pass filter after the power amp rather than between the power amp and the crystal oscillator?
Harros said:The output now is not distorted anymore and is desirable... Is it ok for us to amplify the square wave rather than the sine wave (the harmonic-removed siganl)? Or should we put two filter in the circuitry (1 for crystal oscillator harmonic removal, 1 for power amp harmonic removal)?
By the way, i dont know which rf choke i should buy here, could you please give some recommendations of it? (a list of rf chokes is enclosed here, thank you)
I am thinking of power up this transmitter using 2 9v battery. Will the battery provide enough power for the transmitter?
Harros said:Hi, back to the tx loop antenna, i found that both the radiation resistance and loss resistance is very low for the tx loop antenna. I wonder the way that i should start with to get the impedance matching between the tx antenna and the power amp. Do you have idea on it? By the way, there is no network analyzer in my college's lab, this has made the impedance measuring to be difficult.
Should i provide a ground plane to the rod antenna ( i learn from the internet that a ground plane is provided to the monopole antenna usually)? Does the impedance buffer amplifier provide the virtual ground to the rod antenna?
RadioRon said:It is OK to amplify the square wave from the oscillator. At worst, the output waveform before the low pass filter might become a bit more distorted, but it will be operating more efficiently, and we normally trust the filter after the final amp to clean up the waveform.
The RF choke for the base of the final amp can be the 200 uH part in that list. It is normal practice when building experimental circuits to buy several values of a component so that if tuning is necessary, you have some alternates on hand.
I prefer not to use 9V batteries except if the current draw is quite low, like less than 50 mA. I don't know what your current draw is going to be exactly, but it might be more than 50. You get a lot more lifetime out of AA or C alkaline cells, but of course you either have to deal with the lower total voltage available.
RadioRon said:The impedance matching to the loop antenna is probably best done using transformer technique. If you resonate the loop with a capacitor you increase the terminal voltage and the impedance, then you can couple into the this with a small number of turns of a secondary loop on the same form. You can check the effectiveness of your coupling by putting a receiving loop nearby (like 3 feet away) and hook that up to a scope or other 1 MHz receiver and see what matching gives the most signal.
The rod will work better if you provide some sort of ground plane or counterpoise. Any mass of metal is a help, the bigger the better, within reason. No matter what you do though, you will not achieve a resistive impedance match to 50 ohms because the rod is very short, electrically speaking so it will always have a high impedance. The purpose of the impedance buffer amp is to convert this to a low impedance.
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