Digital Signal Processing - IP and Algorithms

[vlavruhin]

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arash-rezaee wrote:

I have one question : My clock is 12.160MHz and I want to have FM modulation. Is there any problem That the input of DDS has the sample rate of 3.04MHz and the clock of DDS is 12.04MHz? I think the output is 12.04MHz in sample rate but is it any problem if the input has lower sample rate or not? I hope my question is clear. If it is not let me.

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DDS has clock rate 12.160 MHz (I presume, 12.04 MHz is a misprinting).
Modulating signal has sample rate 3.04 MHz ( f_mod = f_dds / 4 ).

Is it ok or not? Lets see...

If we are going to change phase increment of DDS once in a four cycles (i.e., with sample rate 3.04 MHz, when samples rate of DDS is 12.160 MHz), then it's equivalent to the case when modulating signal is upsampled to 12.160 MHz with zero-order polynomial interpolation (i.e., sample and hold latch). Zero-order polynomial interpolation, of course, produces non-linear distortion as it makes 'steps' in originally 'smooth' signal.

But if frequency band of your input (modulating) signal is much less then original sample rate (i.e., 19 kHz << 3.04 MHz), then steps produced by this interpolation are not high (in time domain) and power of new harmonics (in frequency domain) is relatively low.

So, if you are going to use audio signals ( < 20 kHz) at sample rate 3.04 MHz, then it's ok. But if it doesn't suits you, then you can add additional upsampler from 3.04 MHz to 12.160 MHz (using sinc-interpolation, for example).

[arash-rezaee]

Thanks agian. yes it was misprinting. 

I will go to open new threat to ask about how can I done dynamic range contro for audio processing to not go above 75KHz deviation. if you have some information plaese let me know.

Regards

[arash-rezaee]

Dear Vitaly

I confused about FM modulation. Can we have FM modulation in I/Q mode?! I know there is two kind of FM modulation : FSk and FM modulation. But is it possible to have Fm in I/Q terms modulation? If yes would you please help me how can I implemet it n FPGA.

Regards

Arash

[vlavruhin]

Hi, Arash.

FSK (frequency shift key) is applied only to modulate digital data. FM is, first of all, analog type of modulation.

All types of modulation (digital / analog) can be implemented using I/Q technique.

I/Q means two channels: in-phase and quadrature. Mathematically, I/Q signal is complex signal, i.e. s(t) = I(t) + j*Q(t).

In telecommunications digital I/Q signal is used only in so called "baseband" signal representation. To put it simple: "baseband" means signal with central (carrier) frequency f_c = 0.

When carrier frequency is not equal to zero and lowest frequency of signal is higher than zero: 0 < f_c, 0 < f_lowest, then I/Q representation has no reason, because such signal can be represented only by one channel of I/Q signal (or, mathematically speaking, real signal). It's called 'passband' signal.

So to answer your question: yes, frequency modulation can be done at zero frequency (baseband) using I/Q (complex) representation.

But it's hard topic for exaplanation on the forum. I can suggest you to study complex math, complex signal representation, complex Fourier transform. And after that you will be able to design baseband frequency modulator.

[arash-rezaee]

Thanks. But for FM broadcasting one method can be creating FM with DDS and then use high speed DAC to upconvert it. Am I right?

Regards

Arash

[vlavruhin]

Yes, there are many different ways to design FM transmitter.

For example:

1) Digital passband FM (real) -> DAC -> analog upconverter from IF to RF

or

2) Digital passband FM (real) -> high speed DAC -> amplifier (in case of low RF and high sampling frequency)

or

3) Digital baseband FM (I/Q) -> high speed DAC with embedded interpolation and digital mixer -> amplifier

or

4) Digital baseband FM (I/Q) -> 2-channel DAC (I/Q) -> analog quadrature upconverter from IF to RF

and so on...

[ru551n]

@arash-rezaee:

I don't know if this thread is still open/applicable.

I have made a complete implementation of an FM + RDS modulator complete with interface to DAC/upconverter.

If you still need help, please contact med via PM.

Regards

[denisfk1985]

Hi, ru551n!

Can you help my for fm stereo modulator via FPGA please?