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There are a scheme of Hartley oscillator and Hartley oscillator with varicap diode. Frequency of "1" and "0" are almost identical (f1=2.212MHz, f0=2.201MHz). Does anyone know how to set the difference frequency is about 200-400kHz?
What is the value of D2?
C5 is very small.
Do some math. 50pF is in parallel with (2pF//D2). Because D2 is only 2pF you can't change the frequency much. If D2=0pf to 100000pF then you will have a combined effect of 0pF to 2pF. The total capacitance will be 50pF to 52pF.
Please check my math.
If D2=300pF, 300pF and 2pF=1.98pF
If D2=20pF, 20pF and 2pF=1.8pF
So the combination of D2 and C5 is 1.8 to 1.98pF.
Now add the 50pF and we get 51.8pF to 51.98pF. This is a very small change in frequency.
If you want a larger change in frequency change C5. Try 20pF.
Thanks ronsimpson. I have a scheme for modulator. frequencies that were obtained from the simulator for "1"=(680hKz - 691kHz), "0"=(669kHz - 675kHz). Frequencies are variable over the time. Now I need scheme for FSK demodulator, which worked for this frequencies. Is it a good solution IC NE564? Scheme for modulator and datasheet for IC NE564 are attached. http://www.datasheetcatalog.org/datasheet/philips/NE564.pdf
Most PLLs can be suitably used for FSK. I am not familiar with th NE564, but have in the past used the 4046 in that role.
Early types of CD4046 would have a maximum frequency of less than 2 Mhz, but newer variants (74HC, 74HCT) go far higher in frequency.
But, before we continue further, let me add that your modulator circuit requires output buffering to be useable. Any amount of loading, including a 10X scope probe, will throw the frequency totally out of whack.
A good buffer for this application is an n-channel JFet.
any reason for the gigantic bandwidth requirement? this isn't going to be an RF device is it? there are other things in that band, and they really won't appreciate having a 400khz wide FSK signal there. it's the upper end of a ham band and the lower end the maritime radiotelephone band. 400khz is a huge chunk of spectrum. what's your intended data rate? there might be a better way to do this....
D1 changes capacitance with voltage, and therefor changes the frequency. if you need something more like 300pf to 600pf, you can use the B-C junction of a power transistor as a varactor diode
this is something like what i was getting at. V2 is a bias supply, and V3 is the audio source. when i ran the sim, the simulator isn't changing the B-C capacitance, but i have used such circuits before, and they do work.a condition you want to avoid is where the negative audio peaks forward bias the junction. the DC bias should be adjustable from about 1.5V-10V or so. as you get close to 0V on the collector of the transistor, the capacitance change vs voltage may get nonlinear, causing distortion of the received signal. the bias voltage can be used to fine tune the unmodulated carrier. this technique works better with a standard LC tuned oscillator, because the change in capacitance has more of an effect on the tank circuit than it does on a crystal.
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