# Homework Help: Drawing digital word

1. Jun 19, 2006

### electronic engineer

How to draw signal waveform for this digital word {11001011} ,using these codes:

-OOK(ON-OFF Keying)
-PSK(phase shift keying)
-ASK (amplitude shift keying)
-FSK(frequency shift keying)

I need the guide to go through this problem or at least a useful link.

2. Jun 21, 2006

### Staff: Mentor

Well, draw the carrier first, and then change (modulate) the carrier according to the encoding scheme. Like, OOK will turn the carrier on and off, right? And ASK changes the amplitude, right (hence the name). FSK is not much more complicated, and for PSK just use +/- 45 degrees or so to avoid confusing it with the other modulation schemes.

3. Jun 23, 2006

### electronic engineer

could i use 0/180 degress as BPSK suggests, so that "1" represented by 0 degree and "1" represented by 180 degress

4. Jun 23, 2006

### Staff: Mentor

Yes, but 180 degree BPSK has very high harmonic content. Look at the sharp edges when you turn around the waveform at the zero crossing. Yikes. Have you started to learn about bandwidth tradeoffs and information throughput yet for the various modulation schemes?

5. Jun 24, 2006

### electronic engineer

<<Have you started to learn about bandwidth tradeoffs and information throughput yet for the various modulation schemes>>

No, whar are you aiming to?

6. Jun 24, 2006

### Staff: Mentor

I asked because as you get farther into learning about communication systems and channel characteristics, you will see that there are tradeoffs and optimizations with the various modulation schemes and various kinds of communication channels. You will learn why QPSK may be better than BPSK in some situations, and you will learn that 180 degree BPSK has a very broad power spectral density, which can be bad for several reasons. In general, the minimum phase shift that you can use is the best, as long as you get above the noise.

Communication theory is a pretty interesting field. It involves a lot of different kinds of math (probability, calculus, random variables, etc.), and has very practical implications. Code Division Multiplexing still amazes me, for example.

One of my old textbooks is "Introduction to Communication Systems" by Stremler. Check it out in the library when you get a chance, and I think you'll see what I mean.