Hi,
This is for your website, right? Who is Mike? (Mike for Michael and mic for microphone)
Both designs are horrible!
If you want a simple modulated supply, use an LM317 in the supply line, use a 240 Ohm resistor between its output and 'adj' pin and from there to ground use a 910 Ohm resistor. Apply the audio across the 910 Ohm resistor so it modulates the output voltage. The impedance at the input point will be about 150 Ohms so driving it will be easy.
Do note though that modulating a crystal oscillator like that is prone to all kinds of problems, keep the modulation level low or it will stop running on negative cycles and clip hard on positive ones.
Brian.
The 240 Ohm should stay the same, it satisfies the load current requirements for the LM317. The 910 Ohm is to set the output voltage to half the input so that value is to give 6V out based on the 12V in your schematic. Change the value so you get about half of whatever supply you choose. The idea of half voltage is so you get most chance of the voltage swinging above and below that point, in other words the carrier with no modulation is at 50% of the peak which is where it should be for AM. You should place a decoupling capacitor at the input of the LM317 as normal but at the output use something smaller than usual, maybe 100nF, so it decouples RF but not the audio frequencies.
Brian.
Both designs are horrible!
Do note though that modulating a crystal oscillator like that is prone to all kinds of problems, keep the modulation level low or it will stop running on negative cycles and clip hard on positive ones.
Brian.
Do note though that modulating a crystal oscillator like that is prone to all kinds of problems, keep the modulation level low or it will stop running on negative cycles and clip hard on positive ones.
Brian.
Hihi! I like the overcooking partNeazoi is using a quartz crystal to stabilize the frequency but 'overcooking' it massively.
Brian.
Hihi! I like the overcooking part
Seriously now, using old large crystals OR a few modern crystals in parallel helps keeping the crystals cool even at high powers. One reason is that I am not feeding the whole signal back to the crystal, like done on pierce oscillators, but through a capacitive divider, so it helps a bit. See my construction here **broken link removed**
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Here is the modulator. Not currently optimized, this circuit achieved the highest modulation and trouble-free operation of the TX on AM.
Note that the VCC could be anything from a few volts to 20v or so. But I have only tested it at 20v.
Shall I better connect the base resistor to the adjust pin instead of the VCC for that reason (or any other benefit)?
Shall I try to remove the 10k at the C-E at all and leave only the transistor there?
If I wanted the circuit to operate on different voltages, which resistor should I make variable (to adjust modulation at different voltages). Right now I adjust the 470k, but adjusting the 10k could be better?
It is because the circuit is being used as a current booster, the current being the supply to your oscillator and dependent on the voltage it produces. The LM317 always tries to keep 1.25V between it's output pin and 'adj' pin but where you benefit here is the current at the 'adj' pin is very low so it is easier to apply audio to it. You are using the error amplifier inside the voltage regulator as an additional amplifier stage. The output pin voltage varies according to the voltage on the 'adj' pin so if you use it to bias the transistor you get negative feedback which improves the quality. More output voltage = more bias current = lower output voltage.UPDATE2 I have just realized that I had make a mistake. I originally connected the base to the output of the regulator (modulated) instead of the input.
I have found it works both ways, but the modulation with the "mistake" circuit was cleaner and the carrier was kept at higher levels! Why is that?
It is because the circuit is being used as a current booster, the current being the supply to your oscillator and dependent on the voltage it produces. The LM317 always tries to keep 1.25V between it's output pin and 'adj' pin but where you benefit here is the current at the 'adj' pin is very low so it is easier to apply audio to it. You are using the error amplifier inside the voltage regulator as an additional amplifier stage. The output pin voltage varies according to the voltage on the 'adj' pin so if you use it to bias the transistor you get negative feedback which improves the quality. More output voltage = more bias current = lower output voltage.
Brian.
If you mean in the LM317 circuit, that would limit the lower output voltage to VCEsat + Vf so you would lose around 2V of modulation audio. Remember that the idea of modulating the 'ADJ' pin is it is a high impedance point, the current flowing there is quite low, a few uA from the LM317 and some current through the 240 Ohm resistor.I tried a led at the collector of a transistor and tried to drive the led brightness by the music in the base.. It worked but the led did not switch completely on or off, so the modulation was low.
There is an even simpler implementation of this concept but it only works up to about 2MHz and produces REALLY bad spectral output. You can generate about 5W of RF using only 4 components. I leave it up to you to work out the details.....
Brian.
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