Can Digital Signals Be Transmitted Over FM for FSK Modulation?

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Discussion Overview

The discussion revolves around the feasibility of transmitting digital signals using frequency modulation (FM) for frequency-shift keying (FSK). Participants explore the implications of bandwidth and signal characteristics in the context of real-world applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a digital signal can be directly fed into an FM transmitter for FSK, noting that FSK involves modulating the carrier with two frequencies.
  • Another participant asserts that the digital signal is a real signal with finite bandwidth, countering the idea of infinite bandwidth.
  • There is a discussion about the decomposition of digital signals into sine waves using Fourier Series, with a participant suggesting that infinite sine waves would be needed for perfect replication.
  • Concerns are raised about the optimum slew rate and shape of the signal transitions in binary frequency-shift keying (BFSK) to comply with RF spectrum regulations.
  • A later reply emphasizes the distinction between ideal digital signals and their real-world counterparts, which have finite characteristics.
  • One participant references MFSK as a potential resource for understanding wave-shaped digital modulating signals.

Areas of Agreement / Disagreement

Participants express differing views on the bandwidth of digital signals and the implications for FM transmission. There is no consensus on the optimal characteristics for the modulating signal or how to calculate them.

Contextual Notes

Limitations include assumptions about the nature of digital signals, the real-world effects on signal transmission, and the lack of clarity on how to determine the optimum slew rate and wave shape.

engcon
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Hi,

I'm thinking about transmitting digital signals with FM. The effect I'm trying to achieve is FSK.
Can I just feed the digital signal to the input of the FM transmitter?
With FSK the carrier is frequency modulated with 2 frequencies, but since the digital signal has infinite bandwidth, will the resulting frequency modulated carrier have just 2 frequencies?

Thanks
 
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engcon said:
Hi,

I'm thinking about transmitting digital signals with FM. The effect I'm trying to achieve is FSK.
Can I just feed the digital signal to the input of the FM transmitter?
With FSK the carrier is frequency modulated with 2 frequencies, but since the digital signal has infinite bandwidth, will the resulting frequency modulated carrier have just 2 frequencies?

Thanks

The digital signal is a real signal, so it does not have infinite BW. Furthermore, for optimum use of the RF spectrum (and to help you avoid violating FCC rules with your transmissions), there is an optimum slew rate and shape between the two different frequencies in BFSK. Can you tell us a bit about how you can determine this optimum transition waveshape?
 
Thanks for your reply. I must first say that I'm fairly new to this kind of stuff.

berkeman said:
The digital signal is a real signal, so it does not have infinite BW.

If you had to decompose the digital signal into sine waves using the Fourier Series, wouldn't that require an infinite number of sine waves for perfect replication?

Regarding the optimum slew rate, I have no idea how to calculate that. However, the maximum bps to be transmitted is very low (less than 100).
 
engcon said:
Thanks for your reply. I must first say that I'm fairly new to this kind of stuff.

If you had to decompose the digital signal into sine waves using the Fourier Series, wouldn't that require an infinite number of sine waves for perfect replication?

Regarding the optimum slew rate, I have no idea how to calculate that. However, the maximum bps to be transmitted is very low (less than 100).

Don't confuse the ideal/surreal world of infinitely sharp digital signal representations with the real world, where digital signals have finite slew rates and finite bandwidths. You're asking about FM in the real world (I think), so the digital data stream will have a finite bandwidth.

And for the optimum wave-shaped digital modulating signal characteristic, check out MFSK here:

http://en.wikipedia.org/wiki/Frequency-shift_keying
 

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