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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.
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 ?
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.
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