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Writing to the frequency registers is accomplished using the portw instruction. To write to the frequency or phase registers on the DDS, the second operand of the portw instruction must be 10xxxxxx , where the lower six bits are the address of the specific register to be written to. Theaddress of the most significant frequency register on the DDS is 04h , and the address of the second most significant frequency register on the DDS is 05h (see page 26 in the data sheet). It is important to note that the way our DDS boardswere built, you will not be allowed to make two consecutive writes. To solve this problem, a subroutine called nullop is called to waste some CPU time between writes. nullop does this by simply repeating the nop instruction 128 times.

After the program returns from the setcarrier subroutine, it enters an infinite loop in which it waits for a serial interrupt to occur. The serial interrupt occursevery time a new sample is acquired from one of the two input channels and is transmitted to the DSP via the serialport. When the interrupt occurs, an interrupt service routine called ANALOG_IFC (see core_mod.asm executes and calls the handle_sample subroutine. The handle_sample subroutine reads in the acquired sample from the serial port and scales that sample so thatit can be "mapped" to a frequency in the range of75 kHz

In FM radio, the amplitude of the message signal being transmitted determines the amount offrequency deviation from the carrier frequency of the passband signal.75 kHz is the largest frequency deviation allowed.
. The scaled sample therefore determines the frequency deviation and is added to 6Dh . The last step is to write the result to the second most significant frequency register so that thefrequency of the DDS output can be updated.

Programming the phase

The process for changing the phase of the DDS output is the same as it was for changing the frequency of the DDS output.To change the phase, you need to write a phase word to a phase-adjust register on the DDS. The phase-adjust registeris 14 bits wide and is split up into two smaller registers that you can write to (see page 26 in the data sheet). The upper 6 bits have address 00h , and the lower 8 bits have address 01h . The phase word can be calculated using :

Phase word Carrier phase 2 2 14
Once you've calculated the phase word, you can write it to the DDS using the portw instruction as before. Just make sure you use the correct address for the phaseregister.

Programming the amplitude

The DDS also allows you to program the amplitude, but this functionality is not addressed in this lab. You will beable to implement a digital communication system in ECE320 without having to program the amplitude. Interested readersare referred to the data sheet.

Fsk exercise

Now that you know how to use the DDS in single-tone mode, implement a simple FSK system that uses 2 frequencies: 120.005MHz, and 120.011 MHz. You don't need to encode any data for this exercise. In other words, your DDS output should justcontinuously alternate between the two frequency symbols. Also, the DDS automatically ensures continuous phase, so youwon't have to keep track of it. Use a symbol length of approximately 725s (the same length as your lab 5 symbols). Timer interrupts are an elegant way to control thesymbol lengths, but in this lab we will keep things simple and control the symbol lengths by creating a second (longer) nullop subroutine and calling it between writes to the DDS. The second nullop subroutine should waste approximately 725s worth of time.

Since we're not using input or output from the DSP, you don't need to use the WaitData or WaitAudiomacros.


Note that the corresponding baseband frequencies for 120.005 MHz and 120.011 MHz are 5 kHz, and 11 kHz, respectively.Since these baseband frequencies lie within the 22.05 kHz bandwidth of the DSP, you will be able to view your FSKsignal in real time on the oscilloscope without the contribution from the spectral replicas. Just feed the output of the DDS into asecond DSP (the anti-aliasing filter on the DSP will get rid of the spectral replicas), and pass it through to the outputand the scope. You should be able to verify that there is continuous phase between frequency symbols, and that yoursymbol length is approximately 725s. You should also view the spectrum of your DDS output on the VSA to verifythat your symbols have the correct frequencies.



Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
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Source:  OpenStax, Ece 320 spring 2004. OpenStax CNX. Aug 24, 2004 Download for free at http://cnx.org/content/col10225/1.12
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