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Would you think my 2.4 GHz audio receiver would work?

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skatefast08

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Attached is the picture of my 2.4 GHz audio receiver. Could there be a problem with this implementation? Thanks
 

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skatefast08 said:
Could there be a problem with this implementation?
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Yes. It will work after a fashion, but what happens if there is a signal at 2.26GHz?
You will need a bandpass filter to filter out only the signals you want at the input, and another after the mixer to filter the IF.
 
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Provided your 100MHz filter has at least +/- 10MHz bandwidth and you are happy with a zero IF output it should work. I'm not sure what the VCO is tuned by though, if this is for FM reception it would be wise to add a phase detector in the VCO loop.

Brian.
 
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betwixt said:
Provided your 100MHz filter has at least +/- 10MHz bandwidth and you are happy with a zero IF output it should work. I'm not sure what the VCO is tuned by though, if this is for FM reception it would be wise to add a phase detector in the VCO loop.

Brian.
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Should further amplification (IF amplifier) be put before the phase detector, after the phase detector, or after the whole PLL? What would work better? Also isn't a phase detector similar to a mixer (I guess with better isolation)? Thanks
 

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A phase detector and a mixer are similar but not the same animals.

It isn't clear what kind of signal you are trying to recover, as shown, the VCO mixes with the IF down to DC or at least less than 20KHz so you have an AM receiver with poor image rejection. I suspect what you are trying to do is accurately tune the VCO to pick out one signal from the 90 - 110MHz range. The best way to do that is with a PLL, the VCO isn't tuned from the LPF as your schematic shows, it does come from a filter (the 'loop' filter) but that is fed from a phase comparator. One side of the phase comparator is a reference frequency which you choose to be the smallest tuning step you need, the other side is the VCO output through a frequency divider. The VCO then tunes to the reference frequency multiplied by the division ratio and it is as stable as the reference.

I'm not sure what would really happen if you tried the existing design, the VCO would be modulated by the recovered signal so I suspect a rather random oscillation would occur.

Brian.
 

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betwixt said:
A phase detector and a mixer are similar but not the same animals.

It isn't clear what kind of signal you are trying to recover, as shown, the VCO mixes with the IF down to DC or at least less than 20KHz so you have an AM receiver with poor image rejection. I suspect what you are trying to do is accurately tune the VCO to pick out one signal from the 90 - 110MHz range. The best way to do that is with a PLL, the VCO isn't tuned from the LPF as your schematic shows, it does come from a filter (the 'loop' filter) but that is fed from a phase comparator. One side of the phase comparator is a reference frequency which you choose to be the smallest tuning step you need, the other side is the VCO output through a frequency divider. The VCO then tunes to the reference frequency multiplied by the division ratio and it is as stable as the reference.

I'm not sure what would really happen if you tried the existing design, the VCO would be modulated by the recovered signal so I suspect a rather random oscillation would occur.

Brian.
Click to expand...
I am trying to create an FM demodulator. After the IF and LPF I have a PLL to demodulate the FM signal at 100 MHz. You say the VCO would be modulated by the recovered signal, but the 100 MHz FM signal isn't recovered until it has passed to the output of the PLL (after the last LPF), isn't this right? Just think that the mixer at the PLL is a phase comparator, not a mixer, I didn't know how to represent a phase comparator (phase detector). Thanks
 

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Make the 100MHz IF amplifier a limiting amplifier so that the input to the mixer/Phase detector is held constant and it should work. Without sufficient noise at the output of the mixer and input tot he VCO it will not find a signal to lock to.
You do need to design the low pass filter after the mixer to set the loop dynamics to be what you want.
 
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A PLL will typically use an integrating loop filter between phase detector/mixer and VCO input respectively audio output.

A mixer respectively multiplier can work as PLL phase detector, but the lock range is small, in a first estimation smaller than the loop filter bandwidth.
 
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