How software defined radio differs from traditional one

[davidcopper]

Hi, all.

Im new to telecommunication field. I have a few questions. Hope you can help me.

First, I am not clear how digital signal processing works. My understanding is: baseband frequency is modulated as high frequency on a device. Such device sends the signal to another device, which demodulates the high frequency to baseband for processing.

My first question is: where do ADC and DAC normally happen in this process?

Regarding software defined radio, my understanding is: SDR differs from traditional digital signal processing in that demodulated basedband frequency is processed by software rather than specific circuits. Is it right?

Please point out errors in my understanding. Thank you for what you may do for me.

Kind ragards

Lee

 

[NoTime]

Welcome to PF

Here is some info on digital radio, including audio processing and modulation information.
http://en.wikipedia.org/wiki/Digital_Radio_Mondiale

 

[davidcopper]

Hi, NoTime.

Thanks for the info. Hm... it does not describe how to transform analog signal into digital one in details.

But thank you still for the info.

Waiting for someone to clarify my confusion. Any advice will be appreciated. :)

 

[Averagesupernova]

A software defined radio does NOT necessarily use the traditional digital modulation schemes (FSK, PSK, etc.) A software defined receiver by my understanding directly digitizes the IF and using DSP in some sort of microprocessor comes with the end result. The software defined transmitter does the same thing in reverse. Analog mixers and modulators are all but eliminated. davidcopper, I think you are simply looking for an explanation of a/d and d/a convertors.

 

[berkeman]

Thanks for the info. Hm... it does not describe how to transform analog signal into digital one in details.

Here's a pointer to an intro to ADCs:

http://en.wikipedia.org/wiki/Analog-to-digital_converter

 

[rbj]

A software defined radio does NOT necessarily use the traditional digital modulation schemes (FSK, PSK, etc.) A software defined receiver by my understanding directly digitizes the IF and using DSP in some sort of microprocessor comes with the end result. The software defined transmitter does the same thing in reverse. Analog mixers and modulators are all but eliminated. davidcopper, I think you are simply looking for an explanation of a/d and d/a convertors.

that may be. but if you hang around comp.dsp a while, this topic comes up periodically. you're correct that software radio often assumes that the IF is digitized and usually the analytic signal is computed from the sampled IF (this uses the Hilbert Transform) and then parameters such as instantaneous amplitude or instantaneous frequency (for FM demodulation) are extracted pretty straightforwardly. this is where electrical engineers meditate about the meaning of (and the practical and clean implementation of) the arctan(x) function. doing that without division would be considered very cool.

 

[proxy]

My first question is: where do ADC and DAC normally happen in this process?

The answer here is -- it depends. Depending on the ADC that you are using, and the levels of the signals you are working on, the ADC can digitize a small part of the IF or the entire bandwidth you are looking for. For example, if you are working in the VHF/UHF range you are typically going to need some downconversion to an IF that can be handled by an appropriately specced A/D. If you are working with baseband signaling systems or HF systems, you can typically sample your signal directly. For DAC's the same applies. If you want a cheaper DAC, you can generate a signal at some IF and upconvert it in the analog world, or you can use a higher speed DAC to minimize the need for that.

Regarding software defined radio, my understanding is: SDR differs from traditional digital signal processing in that demodulated basedband frequency is processed by software rather than specific circuits. Is it right?

SDR uses DSP to do signal processing types of things. The signal you are processing can be RF/IF/or baseband. They can also be modulated signals slated for demodulation. However you are right that it's processed by software rather than specific circuits.

Hope that helps!