EEVblog Electronics Community Forum

Electronics => RF, Microwave, Ham Radio => Topic started by: atmfjstc on August 04, 2018, 11:39:32 pm

Title: Trying to understand how this RC receiver works
Post by: atmfjstc on August 04, 2018, 11:39:32 pm

I've taken apart a cheap RC car and I'm trying to understand exactly how the receiver/demodulator works. I know that the incoming signal is 27MHz with binary amplitude modulation, i.e. pulses on/off, and the receiver is supposed to essentially isolate the 27MHz from the antenna and extract the binary envelope which is then fed as-is to an IC. What puzzles me is the very first stage of the receiver, whose schematic I've attached here (the other stages seem to do filtering mostly).

The topology confuses me and I'm not sure where to begin analyzing it. What is the transistor doing? Is it filtering? Oscillating? If it's an amplifier/filter, what topology is it? The input seems to be going into the collector and the base serves as an output, which doesn't make sense.

P.S. Note that the tank circuit formed by C2 and L1 matches the desired frequency.

Title: Re: Trying to understand how this RC receiver works
Post by: Circlotron on August 04, 2018, 11:47:33 pm

Superregenerative receiver?

https://www.google.com.au/search?q=superregenerative+receiver&rlz=1C9BKJA_enAU745AU745&hl=en-GB&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiX1PmkytTcAhUOAogKHS5sDXEQ_AUIESgB&biw=1024&bih=653#imgrc=XUQrb3HammucmM: (https://www.google.com.au/search?q=superregenerative+receiver&rlz=1C9BKJA_enAU745AU745&hl=en-GB&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiX1PmkytTcAhUOAogKHS5sDXEQ_AUIESgB&biw=1024&bih=653#imgrc=XUQrb3HammucmM:)

Title: Re: Trying to understand how this RC receiver works
Post by: atmfjstc on August 05, 2018, 02:48:39 pm

Interesting. The circuits do look similar. Looks like I have quite a lot to unpack, I never imagined the receiver in a toy car would employ such sophisticated principles.

Title: Re: Trying to understand how this RC receiver works
Post by: iMo on August 05, 2018, 04:27:44 pm

There is nothing sophisticated in that kind of receiver. They are used for last 80 100 years at least.
It is a combination of a "regenerative receiver" and a relaxation oscillator (called therefore "superregenerative receiver").
The relaxation oscillation (10-50kHz) are set by the R1C4 (and isolated from the regenerative part by L2), relaxation sawtooth wobble the regenerative part L1C2-C3 such it passes from pre- to post-regeneration periodically, and that creates an enormous amplification (~millions). With "averaging" this events by the low pass filter R4C6-R6C23 it "demodulates" the RF signal envelope (AM or FM). The relaxation frequency is set such it is above audible spectrum when used for transmitting voice.
It works, but it is not recommended to use this receiver type today as it radiates a lot of EMI crap. Do not use it in Europe..