6
\$\begingroup\$

This is a circuit for a remote control toy receiver. Can someone help me understand this. Connecting the antennae to the collector makes no sense to me. RF is my weakness.

Datasheet where this came from is here, page 617. http://www.bitsavers.org/components/samsung/1990_Samsung_Linear_IC_Vol_1_Audio_CDP.pdf

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
5
  • \$\begingroup\$ First spend some time reading up on the whole idea of regenerative receivers; what they do is a bit odd. You probably wouldn't want to do this today when there are 2.4 GHz digital packet radio modules like nRF24 clones comparable to (and in some cases interoperable with) what high end RC sets use available for $1 each. \$\endgroup\$ Jul 21, 2019 at 22:20
  • 1
    \$\begingroup\$ @ChrisStratton - My brother has a bunch of these from the 90s. I am helping him to get them working again. He says that although they are cheap, they were really fun to race. We believe that caps have degraded. The throttle is frequency modulated and works. The steering is duty cycle controlled and doesn't work. \$\endgroup\$
    – Mattman944
    Jul 22, 2019 at 0:49
  • 1
    \$\begingroup\$ Then the problem is likely with the decoder circuit not the regenerative detector \$\endgroup\$ Jul 22, 2019 at 2:00
  • \$\begingroup\$ @ChrisStratton - The decoder is mostly in the IC. He has replaced the electrolytic caps. Multiple transmitters and receivers have the same issue, the steering doesn't work on any combination of them. Its like something has de-tuned with age, and the steering is more sensitive to the tuning. Anyway, I think that we have enough clues now, thanks to you and Kevin. \$\endgroup\$
    – Mattman944
    Jul 22, 2019 at 8:16
  • \$\begingroup\$ You should probably check the signal into the decoder with a scope and see that it behaves as given in the decoder data sheet; also the same for the transmitter, it's entirely possible that it is the encoder which is misbehaving. Replacing the control electronics is an option if you want to keep the cars, though with servos so cheap it gets tempting to put in proper steering too... \$\endgroup\$ Jul 22, 2019 at 13:45

1 Answer 1

10
\$\begingroup\$

A super-regenerative receiver is actually an oscillator that is arranged to be periodically stopped (or Quenched as it is called) then allowed to build up oscillations again.

The time taken for the oscillations to build up depends upon the signal (or noise) level in the circuit. When there is a signal present at the oscillation frequency the oscillations will start more quickly.

The time taken to start oscillation will affect the average current so by filtering this the modulation can be recovered.

They have extremely high gain and be sensitive to microvolts of signal but have rather broad bandwidth as well as radiating at the operating frequency.

Because they can get such high gains from a single active device they were popular when devices were expensive or for very low-cost applications where the limitations were acceptable. The main application until recently was for remote control of garage doors and toys. Garage door openers now use more sophisticated receivers. More stringent FCC requirements also restrict their use.

They were invented in the 1920's by Edwin Armstrong who also invented the Superhet receiver technique (super-sonic-heterodyne).

In the circuit shown Q1 is configured as an oscillator with the frequency set by L1 and C2 with feedback from c3 from collector to emitter.

Since there is a positive feedback loop the antenna can be connected either at the collector or emitter - most circuits I have seen connect it to the tuned circuit (L1/C2).

Oscillations build up from noise or the signal and cause a rectified version to appear at the base and emitter of the transistor charging up C4 modifying the bias of the transistor. At some point, the bias will be such that the oscillations will stop and C4 will discharge through resistors R2 and R3. Eventually, the oscillations will start again and the cycle will repeat typically at a few 10's or 100's of kilohertz (the quench frequency).

The average collector current is sensed with R4 causing a voltage that varies with the signal level although it also has a high level of the quench frequency. This is filtered by the resistors R5 and R6 and C7 and C8 leaving the modulation as an output.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Thanks. I have been obsessed with the burst behavior (quench), not knowing that it was expected. \$\endgroup\$
    – Mattman944
    Jul 22, 2019 at 1:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.