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# difference between modulating frecuency carrier frecuency sampling frecuency

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##### Newbie level 6
what is the difference and the relationship.... is the same modulating frecuency than signal frecuency the modulating frecuency must be bigger than carrier help me please

if i have a modulating frecuency of 30 Khz what should be my carrier and my sampling f. for voice

Modulating frequency is the carrier frequency. You shift the spectrum of the low pass signal to the required channel by multiplying it with the carrier frequency.

are you talking about digital modulation scheme or analog modulation?

Sampling frequency must be more than twice the bandwidth of the low pass signal. In case of voice signals, it is 8k samples per second, since the voice bandwidth is 3.4kHz.
After you quantize it and digitize it, you use some appropriate keying technique to transfer the data.

please clarify on your question. Sampling frequency doesn't occur in the analog domain. (except for one use, but that is used in measuring extremely high frequency signals by undersampling.)

Modulating Frequency of 30kHz is also the Carrier Frequency. When we talk about CHANNEL, we call it Carrier Frequency. When we talk about TECHNIQUE, we call it Modulating Frequency. There is another term for Modulating called UPCONVERT (in some books).

Voice bandwidth is 330Hz to 3.3kHz. For easy mathematics, we usually band limit it at 4kHz or better known as Fmax
By Nyquist Sampling Theorem, to prevent Aliasing, Sampling Frequency (Fs) must be at least 2 x Fmax, thus Fs >= 2 Fmax or 8kHz.

Modulating (Carrier or Upconvert) Frequency is not the same Sampling Frequency.

Modulating is a technique to shift the baseband signal (unmodulated or original signal) to higher frequency, where it can be transmitted using shorter antenna.
Sampling is a technique to acquire the baseband signal (with minimal loss of the signal fidelity), so it can be reconstructed (if Fs >= 2Fmax). If a signal is undersampled, it is impossible to reconstruct the signal back to its original form.

Hope this helps.

what is the difference and the relationship.... is the same modulating frecuency than signal frecuency the modulating frecuency must be bigger than carrier help me please

if i have a modulating frecuency of 30 Khz what should be my carrier and my sampling f. for voice

You need larger antenna to transmit small range frequency signals .

Both of our ideas are on the same track .For transmitting small frequency signals we need large antenna (what i have already stated).

Yes, I do agree that we are on the same track. :grin: Cheers!

I guess that's because my interpretation is to relate frequency (or wavelength, inversely) to antenna length.
Well it is inevitably that longer antenna has the tendency that require a larger equipment or even a structure!

However it may not necessary be the case because today's technology has in fact tried to use high-K dielectric material, thus higher permitivity allows antenna length to become shorter.

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