Plz help me in understanding this RF reciever

[nikhil arora]

this was used in car alarm reciever 433Mhz, plz help me how this thing work

View attachment 39139

 

[audioguru2]

It is a regenerative receiver that is connected so it nearly oscillates due to positive feedback through the 3.3pF capacitor.
The transistor is a common-base oscillator, but with not quite enough positive feeddback to oscillate. Its gain at the tuned frequency is extremely high.
It might also be a super-regenerative receiver where a signal causes its output level to build up until it oscillates and it cuts itself off at a fast rate called its "squegging" frequency which keeps its gain at its most sensitive.
Google has many examples of regenerative and super-regenerative receivers.

 

[nikhil arora]

and the first L and 5pf capacitor makes the tank ckt to select 433Mhz

 

[nikhil arora]

now after this stage in the ckt we have a stage using opamp this configuration of opamp is also confusing can u help me on this.

View attachment 39156

 

[audioguru2]

The opamp circuit doesn't make sense as you drew it. If the opamp's inputs are reversed like I draw it, then it would be a Schmitt Trigger and would have a square-wave output.

View attachment 39157

 

[nikhil arora]

and what is use of capacitors
and when and how it will give outputs.

i.e when it recieves 433mhz signals

 

[audioguru2]

The capacitors in the opamp circuit are filters. The larger capacitor following the 100k resistor smooths the output of the receiving transistor to DC. The 1nF capacitor has a small value and smooths the 433MHz but allows the signal's modulation to pass to the opamp.

Look up Super-regenerative Receiver in Google. It is a very simple AM receiver that gets cutoff when it receives a carrier. When it is cutoff, its output voltage goes high which causes the output of the opamp in my drawing to also go high.

 

[nikhil arora]

this RF reciever was in car alarm,
how unreliable can be this reciever,
i have got a 433Mhz module which is PLL and crystal controlled

actually i want to ask how reliable this reciever can be...

 

[audioguru2]

What 433MHz receiver?
You posted the schematic of a very simple regenerative receiver that has only a single transistor. It doesn't use PLL and doesn't have a crystal. Better circuits are used in cheap remote controlled toys.

 

[trigger]

If it has a crystal, it is a superheterodyne receiver with local oscillator, typically 10.7MHz.

It is much stable than the super-regenerative receiver.

But I can tell if the layout and circuit is fine-tuned for the regenerative circuit, it won't do any significant differences in performance with heterodyne one.