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The circuit below is a radio transmitter circuit of a typical RC car. The circuit makes use of IC TX2 encoder.

The output of IC ("SO" pin) is a series of pulses at low speed (84 Hz). This signal will be modulated by a crystal, onto an RF carrier (27 MHz) in order to be transmitted. Transmission is at 27 MHz.

Is it safe to assume that modulation is AM (as frequency remains the same)? Does circuit indicates the type of modulation? I know that part of the transmitter circuit is the modulator circuit, but I can't find which part is exactly.

Any help would be appreciated.

enter image description here

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  • \$\begingroup\$ Note "AM or FM" is a false dichotomy. There are many kinds of modulation that are neither, or both. \$\endgroup\$ – Phil Frost Dec 1 '13 at 3:45
  • \$\begingroup\$ That is correct. Fixed. \$\endgroup\$ – dempap Dec 1 '13 at 17:41
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This appears to be on-off keying which can be thought of as amplitude modulation where the modulating signal (SO) has just two levels, 0 (no carrier) and 1 (full carrier).

enter image description here
(source: sitelec.org)

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  • \$\begingroup\$ If I understood, when SO output is high (base of the right transistor gets biased) it allows 27MHz signal to be transmitted, otherwise not. What is the purpose of the left transistor? It is part of the oscillator circuit? Osillator circuit except crystal, includes any other component? \$\endgroup\$ – dempap Dec 1 '13 at 18:25
  • \$\begingroup\$ The left transistor is part of the oscillator circuit (one must have gain to have an oscillator so an active device, e.g., a BJT, is required). The right transistor ia a common-emitter amplifier, coupling the output of the oscillator to the antenna, that is either on or off according to the state of SO. \$\endgroup\$ – Alfred Centauri Dec 2 '13 at 1:50
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Yes, it's an AM transmitter. When the SO output is high the rightmost transistor C1815 gets biased into conduction. When SO is low the transistor's base will be pulled low, and the transmitter will be off. It's called CW (continuous wave) transmission.

Note: I would place a coil in series with the 6k8 resistor, so that the SO output doesn't load the oscillator.

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  • \$\begingroup\$ Both transistors are tagged C1815. \$\endgroup\$ – jippie Nov 30 '13 at 16:07

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