Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Why do we usually use IF(Intermediate frequency) ?

Status
Not open for further replies.

jaehlee99

Junior Member level 1
Junior Member level 1
Joined
Oct 9, 2012
Messages
19
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,401
Hello everyone!!
I am RF newbie.

When we design transceiver unit, we usually use IF .

Why do we have to convert frequency instead of direct conversion?
 

Direct conversion is becoming more popular with DSP techniques. For traditional analogue circuits though, direct conversion has a number of drawbacks that make it more difficult to use than the seemingly more complex super-heterodyne design.

Drawbacks such as:

Lack of ability to decode AM signals at the baseband by envelope detection.
No automatic gain control (implemented at the IF) so the baseband signal varies very widely in amplitude.
Local oscillator energy can leak through the mixer, into the antenna input and back into the mixer, creating a DC offset that can overload the baseband amplifier.
 
Hello FoxyRick,
Thank for your posting.
I have been struggling with AGC, so I was asking this topic.
In case of envelop detection of signal at the baseband, how does Direct Conversion cover the wide range of input signal?
 

DCRs use a method of phase locking the local oscillator to the received carrier in order to properly process AM and FM. It is necessary to eliminate frequency drift. The output of the mixer is then the demodulated signal. It might need a lot of audio amplification if the signal level is low (due to lack of AGC and the need to tolerate strong signals too).

Since the advent of cheap DSP it is simpler to use an I/Q demodulator and then process the signal in software with well-known algorithms.

Here are some useful pdfs:

http://www.sp5ddj.pl/Odbiorniki homodynowe.pdf
www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf
 
Thank for your good information.
I agree with using Direct conversion instead of using single conversion, however, I was missioned to solve out AGC detector circuit.
One more question, do I have to use schottky diode to AM detection while simulating on the MicroSim ?
 

because IF filter is easy to make and wonderful proformance.
 
abs is quite right.
If you use IF, you can build one constant frequency high-Q IF filter,
if you want to use direct conversion, you should design variable frequency high-Q filter, it supposed to be much more complicated design.
 

Thank all guys for your time and good answers.

One more question, do I have to use schottky diode instead of IN4148 for AM detection while simulating on the MicroSim ?
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top