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Lowering inductor values in this circuit creates poorer results

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mik3ca

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This is my circuit:

circuit.png

In the current state as it is in, it doesn't function correctly, meaning the modulation is not present when I use small value inductors.

When I use 100K resistors instead of 10K resistors, and 1uH inductors instead of 0.1uH inductors, the result at OUT is 3x the power of the result at Q1 collector and the waveform appears to be normal and in-sync with the inputs.

The problem with 1uH is that I want to aim for a transmission frequency of 433Mhz or 900Mhz if 433 is not feasable (depending on local conditions). and if I use 1uH for 433Mhz then I'll have to use ridiculously small capacitor values (under 1pF) which I think is not possible and/or will not work.

One input here is a constant 160Khz signal. Later, I'll change this slightly to allow binary data to change the phase of the signal. I'll use BPSK through xor gates.

The other input right now is a constant 80Khz signal used for troubleshooting purposes. Later, I'll convert it into a switch to turn the transmitter on or off as desired.

Since this circuit can work for lower transmission frequencies and I need something for higher transmission frequencies, How do I properly scale the components to make higher transmission frequencies possible?
 

mik3ca

Member
Thanks but the thing is all circuit ideas you showed me require a SAW resonator for the transmitter. What I want to do is make my transmitter be able to change carrier frequency at any time and I plan to do that with reverse-biased diodes connected across the tank circuits and that idea of mine does work. It is just the matter of being able to change frequency as necessary.

If I were to go by the circuits you showed me, could I somehow use just resistors, capacitors and/or inductors to make my own adjustable SAW resonator and calculate the frequency based on those components?
 

ci139

Active Member
to be honest i'm not the right person to ask - though i've done a lot RF simulations i have successfully built (read got operating in acceptable way) only 3 of 'em !!!
however there are some fuzzy rules - as ? you have to enable (good/sufficient) energy inflow to oscillator yet (if the amount of energy is about to fix a DC bias) the effect is getting opposite again --- it's about your reduced value C2 (in real it might be right but it is much unlikely to simulate with it unless it is tuned to be near right ??? say 1.98889nF ← just grabbed out of the thin air - not computed)

there's no uncoupling from supply --- inserting a large RF choke coil would complicate the simulation (which however would be better in real i assume) --- setting the series RC network - to supply - might improve the situation for simulation (which however would be questionable in real circuit) ← since i don't have UHF range scope in hand and the "learning curve" goes waving through "real" "simulated" the area as of the scientific study makes much no sense to me.
 

Mosaic

Well-Known Member
I think L3 needs to be much larger as it is a choke allowing for DC bias to get class C ops. L1 ids for your oscillator. Place a load of 100 ohms and then 50 ohms on the ouput to get a calc of the actual ouput impedance based on the voltages across the loads. Based on that you can tune the output to get the load impdance to 50 ohms .
 

Colin

Active Member
You may have to put a diode on the base of the op tr to discharge C4.
It seems to have an emitter-to-base voltage of 3v, and it is not suitable for 5v operation.
 
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Mosaic

Well-Known Member
What I want to do is make my transmitter be able to change carrier frequency at any time and I plan to do that with reverse-biased diodes connected across the tank circuits and that idea of mine does work. It is just the matter of being able to change frequency as necessary.
DIY varactor?
 
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