Proper circut for 4MHz FSK modulation.

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AdamTH

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Proper circut for 5MHz FSK modulation.

Hello,

I have an issue with finding proper circuit/solution for FSK with continuous phase modulation with center frequency 5MHz (4,75MHz logic low and 5,25MHz logic high) . Most of VCO that I've found is working with frequencies 20MHz+. Oscillators like LTC1799 or LTC6900 have issues with precise frequency (LTC1799 has frequency error up to 2,5%) where I'm looking for few kHz precision. Do you have any other solution that could work out (not too expensive).
 
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If you're looking for few kHz precision, you will either use a PLL or 2 Crystals switched between HIGH and LOW frequencies.
I think there are precise Crystals around 5MHz.All you have to design a good Xtal oscillator and switch the Xtals via a high speed switch circuit.
 
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    AdamTH

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Is there a way to use two different Xtal oscillators and keep the the same phase of external signal (CPFSK)? Signals from two Xtals almost all the time will have different values, and switching between them will change phase of signal, or am I wrong?
 

AdamTH said:
Is there a way to use two different Xtal oscillators and keep the the same phase of external signal (CPFSK)? Signals from two Xtals almost all the time will have different values, and switching between them will change phase of signal, or am I wrong?
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You do not Phase Modulation so the Phase is not your concern.You will simply shift the Frequency between 4.75MHz and 5.25MHz depending on logic level.
But the answer is yes, if the oscillators are controlled by single PLL, the phases can be synchronized.It takes a bit time ( settling time) but it's possible.But it's not easy..
 
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    AdamTH

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If you want smooth phase transitions, then simply switching between two oscillators isn't going to work. One approach is to use a PLL with a low loop bandwidth and inject your message into the charge pump output. A similar method is described here.

If doing it digitally is an option, then a low cost DDS chip could do this trivially.
 
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Re: Proper circut for 5MHz FSK modulation.

But for for 4,75MHz logic low and 5,25MHz logic high you do not need a kHz precision. An old CD4046 (or 74HCT4046, 74HCT9046) should be OK. You can always use dedicated chips like NE564.
 
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Re: Proper circut for 5MHz FSK modulation.


You could use a 20 MHz vco, and divide the output by 4, for instance....
 

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