RF receiver question

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Do all wireless devices have there own oscillating crystal at their own very specific frequency, each device being slightly different? If so, how is this level of accuracy when fabricating the crystals possible? Is it easier than I am assuming?
 

Papabravo

Joined Feb 24, 2006
12,693
Do all wireless devices have there own oscillating crystal at their own very specific frequency, each device being slightly different? If so, how is this level of accuracy when fabricating the crystals possible? Is it easier than I am assuming?
More likely they have one of a couple of standard values as a reference frequency and then use a synthesizer to generate whatever other frequencies they need.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Ok I see. Another slightly unrelated question. What is the most common digital modulation technique used in modern wireless systems like WiFi?
 

BR-549

Joined Sep 22, 2013
4,938
Yes....very easy and mass produced. But in the future we won't be using crystals. We are building high stability osc. in our chips now. They are getting more actuate all the time.........any strict timing or tuning can be done with time sync satellite signals.

In the future......microprocessors will have built in gps.....just like an adc is now. It will be a common peripheral.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Ya I have heard of QPSK, its basically phase shifting using 90 degree angles to achieve 4 possible states.

So what are things like CCK (complementary code keying) and Barker code? That is what i got when looking up modern digital modulation methods, not QPSK.
 

Papabravo

Joined Feb 24, 2006
12,693
Ya I have heard of QPSK, its basically phase shifting using 90 degree angles to achieve 4 possible states.

So what are things like CCK (complementary code keying) and Barker code? That is what i got when looking up modern digital modulation methods, not QPSK.
Check out page 6 of the following paper for a diagram of how a Barker code is used to encode data bits
http://www.ece.mtu.edu/ee/faculty/rezaz/wlps/papers/temp/packetstructure.pdf
In this case it is a Barker Code of length 11
 

BR-549

Joined Sep 22, 2013
4,938
I believe the most popular right now is DSSS. Using analog/digital QAM....which is like a combination of PSK and ASK.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
So is all modulation just a form, or combination, of PSK, ASK, and FSK? Are all those other things (CCK, DSSS, Barker Code) just ways to "encrypt" the binary data being sent?
 

BR-549

Joined Sep 22, 2013
4,938
"So is all modulation just a form, or combination, of PSK, ASK, and FSK?"

No.

"Are all those other things (CCK, DSSS, Barker Code) just ways to "encrypt" the binary data being sent?"

Yes.
 

Papabravo

Joined Feb 24, 2006
12,693
So is all modulation just a form, or combination, of PSK, ASK, and FSK? Are all those other things (CCK, DSSS, Barker Code) just ways to "encrypt" the binary data being sent?
I don't think "encrypt" captures what is going on. I think "encoding" and "decoding" are closer to the mark. When digital data is put onto a physical media there are certain desirable properties it needs to have. A known spectrum is one thing and a reliable decoding method is another. These topics are the subject of entire books, and a series of forum posts is not a great place to convey the required background information.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Is all digital communication required to be encoded like this? Or are there some systems that just represent the binary data as is?

Also, for something like my phone connecting to a new WiFi, how does it know what code it will be communicating with? Is that what is happening when the phone is "connecting" with the WiFi? Are the codes randomly generated, or is there a finite list of them for the WiFi system?

Sorry for the high question density haha
 

BR-549

Joined Sep 22, 2013
4,938
You are mis-understanding terms. You will find it very difficult to understand the answers to your questions. You have to study and understand several concepts.......before it will make any sense to you.

Encryption is a process done on the encoded data(to hide it), before the data is modulated.
 

BR-549

Joined Sep 22, 2013
4,938
UN-encrypted encoded data usually defines the type of data. Encoding assigns a bit pattern to the data. For instance the ASCII code. There is audio encoding....video and data.

Modulation is varying some characteristic or property of a radio wave in accordance with the encoded data Pattern.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
Ok. So for a system transmitting ASCII coded data, why does it need a separate coding and decoding scheme to transmit wirelessly?
 

BR-549

Joined Sep 22, 2013
4,938
Right now we are encoding and decoding ASCII. Encode comes from my keyboard. The encoding tells my computer and your computer.....so it can put the right letter/character on your screen.

Different bit patterns represent the keyboard/screen characters.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
I understand that. But if you have a wireless keyboard, would the wireless data be encoded in ASCII? Or is a separate coding scheme used?
 

Thread Starter

jaydnul

Joined Apr 2, 2015
151
yes........but encoding ASCII is not modulation.
It seems modulation encoding would be as simple as "this phase represents 00, this phase represents 01..." Why does there have to be complicated encoding schemes for wireless digital transmission?
 
Top