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How would a schematic for a typical RF modem look like?

A

AznMaverick

Platinum Member
i'm thinking Transceiver > (De)Modulator > Processor?

Also, my dad was trying to explain it to me, but isn't modulation like an digital to analog conversion? Could you explain both A/D and Modulation to me...

Thanks
 
ok...a/d as i understand it is a process that converts a voltage value into a digital numerical value. and modulation, there are numerous kinds of modulation frequency, analog, QAM, PWM, etc.

but technically...modulation is using a digital signal, modulating it with a carrier signal which is analog..then transmitting it and doing the reverse process to get it back...so isn't that sort of an analog to digital conversion?
 
Originally posted by: AznMaverick
ok...a/d as i understand it is a process that converts a voltage value into a digital numerical value. and modulation, there are numerous kinds of modulation frequency, analog, QAM, PWM, etc.

but technically...modulation is using a digital signal, modulating it with a carrier signal which is analog..then transmitting it and doing the reverse process to get it back...so isn't that sort of an analog to digital conversion?
Click to expand...

In a kinda sorta way, I guess it CAN be described as an analog to digital conversion but when people say A/D and D/A, they are describing a conversion of magnitudes without any frequency or phase transformations.
 
Originally posted by: AznMaverick
i'm thinking Transceiver > (De)Modulator > Processor?
Click to expand...

Antenna --> Bandpass Filter --> LNA (low noise amplifier) --> Mixer (brings down to an intermediate frequency) --> Filter (at the IF) --> Another Amplifier (operating at the IF) --> Demodulator --> A/D

The mixer typically has the demodulator for the various phase/frequency schemes.

 
Modulation is very different from D2A or A2D.

Without modulation, it will be very difficult to transmit (or receive) a message.
A message can be a voice message say in a cell phone. Voice contains a wide frequency range. But, voice of all people pretty much falls in the same frequency band (300Hz - 4kHz). Of course some people have a higher voice and some have a lower voice. But, that is not enough for separating multiple channels in air.

With modulation, the frequency that you transmit is almost independent of the frequency of the message. For example, you may have a cell phone that operates in the 1.9GHz band. Modulation is the conversion of the message from the 300Hz-4kHz frequency to the 1.9GHz frequency. While you are talking on your cell phone, your radio signal may be at 1.940GHz. Someone else in the same room may be also talking on his cell phone. His radio frequency will not be in that same frequency. His will be at say 1.9402GHz. That is enough to allow filters to separate the two radio signals and therefore your two conversations from each other.

In the receiver, the radio channels are separated and the channel of interest is demodulated to get to the message that it was originally modulated with.

Very different from A2D or D2A.

Edit:
An important distinction between a modulated signal and a signal before modulation is the relationship between the bandwidth and the center band (carrier) frequency. Voice bandwidth is about 4kHz (this is true for phone; for an expensive sound or music system the frequency of interest may go up to 20kHz). The center frequency is 2kHz.

The same voice message having been modulated onto a 900MHz radio carrier can have a 900MHz center frequency with the same 4kHz bandwidth. The result is that now we have selectivity after modulation.
 
You explained the A2D perfectly.
Just keep in mind that not all communications systems are necessarily digital.

You will need modulation in every transmitter system. You do not necessarily need A2ds or D2As in all of those.
In a digital communications system, you first convert the message to digital. Then, you only have to transmit two types of signals (in the simplest form); one for logic 1 and one for logic 0. Then, you put all of these digital bits in a frame to make it possible for the receiver to put it back together. Then, you modulate these bits in order to make radio transmission possible.

There are types of radio transmission that are specially suited for digital code.

The advantage of digital over analog is the immunity to noise and ease of storage and reconstruction without significant distortion.
 
Another way to think of it is modulation creates a carrier frequency. You can't just send an analog signal and expect the reciever to be able to lock on to it. Instead your modulation creates a predictable curve to carry the signal to reciever which demodulates it to get back the original signal. I guess it could be analogous to a packet, or package. You are enclosing what you want to send to that in can arrive at the right place.

For digital signals, the most simple type of modulation is OOK or On\Off Keying. The idea is simple. For 1's the carrier frequency is transmitted. For 0's its not. As you can see there is no A/D or D/A conversion.

A/D conversion is the conversion of an analog signal to data. Sound is captured through a microphone, but it is still in an analog form. At different minute time intervals the voltage is at different levels. An A/D converter converts the analog signal to digital data. Ofcourse they have a level of accuracy, so there is always some round off error. The same occurs with D/A conversion.
 
Originally posted by: TuxDave
Originally posted by: AznMaverick
i'm thinking Transceiver > (De)Modulator > Processor?
Click to expand...

Antenna --> Bandpass Filter --> LNA (low noise amplifier) --> Mixer (brings down to an intermediate frequency) --> Filter (at the IF) --> Another Amplifier (operating at the IF) --> Demodulator --> A/D

The mixer typically has the demodulator for the various phase/frequency schemes.
Click to expand...
That's the gist of it, though homodyne receivers (that don't have any IF) are becoming increasingly popular since they are more integrated (heterodyne receivers generally require off-chip SAW filters).

Another thing to consider is that the order doesn't necessarily have to be exactly that one, it's depending on the order of filters, mixers, and ADCs you change the balance of linearity, power usage, noise, etc. For example, it's much easier to make a good digital filter than it is to make an analog one so you might choose to do some filtering on the digital side of things. In exchange you'll probably need a better ADC to avoid aliasing.

RF electronics are generally a mess to design because there are so many issues to contend with. You're usually dealing with a low-power signal with a ton of interferers. We had to design a mixer in the RF class I took last semester, and it took a group of three several days of fiddling and computer simulations to meet spec
 
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