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Baseband processing and Digital to Analog Conversion

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Korada SriHarsha

New Member
Hi Guys,
I have a question on the need for Digital to analog converters in Transmitters. To elaborate bit further:
1) Because of the advantages of the Digital signals, most go the signal processing in wireless communications is performed in baseband. But my question is: Since digital modulation is also a part of baseband processing, how can we still say its baseband as a carrier signal (which has high centre frequency) is used for modulation?
and
2) The output of baseband processing block In the transmitter side, is given to Digital to Analog converter in order to transform the digital signal to analog signal that can be sent over the wireless Channel Here my question is: Anyway due to to digital modulation, analog signal is present at the output of Modulation block and why still the point of DAC?

Even though, these questions seem basic, I need some explanation on this stuff. Hope to hear some valuable answers.

BR,
Sri
 

RadioRon

Well-Known Member
Hi Sri
2) If there is a DAC, then the signal preceding it must be in digital form. So, when you refer to "analog signal is present at the output of Modulation block" then this "analog signal" is not truly in analog form, but is a digital representation of the analog signal. Otherwise it does not make sense.

I hope that you understand that when you create a signal in a digital signal processing, the signal is only a digital representation using numbers. These numbers must be converted to an analog voltage, current or power at some point in order to be transmitted and this is the job of the DAC and circuits following the DAC.

1) signal processing for a transmitter is done at baseband rather than at the transmitter output frequency because the DSP (digital signal processor) electronics would be too expensive and too power hungry (and sometimes not feasible) if the DSP is done at the transmitter output frequency. To make a transmitter in a practical and economical way, we create the signal at baseband knowing that there are various ways to then move this baseband signal up to the transmitter operating frequency. One way to move up to the transmitter output frequency is called "translation". The result of translation is that the baseband spectrum is moved up in frequency without changing the width or relative amplitude of the spectrum. Translation is done using a mixer and a local oscillator and is usually called "up conversion" or "up mixing" or "direct modulation" and some other names. It is most common now that signals are translated using IQ up conversion, wherein the I (in phase) information follows one upconversion path while the Q (quadrature) information follows a different upconversion path. This is also a method of translation but it has the advantage of allowing all of the amplitude and phase information to be controlled at the cost of more complexity in the up conversion circuitry. IQ up conversion is found in most cell phones, WiFi, and other digital wireless systems.
 

Korada SriHarsha

New Member
Hi Radioron,
So by the time the signal is getting translated to the transmitter target frequency, both the I and Q components would be in analog form i.e some kind of voltage or current waveform. Does my understanding is correct?
 

RadioRon

Well-Known Member
In most cases, yes, you are correct.

If the transmitter output frequency is low enough, or if technology advances to a point where it is not so expensive or power hungry to convert from digital to analog form directly at the transmitter output frequency then the need for analog I and Q components would go away and we would see only one DAC followed mainly by amplification.
 
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