# FM modulation

## Main Question or Discussion Point

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

## Answers and Replies

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berkeman
Mentor
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.

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).

berkeman
Mentor
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