I've constructed a RF transmitter circuit and have been getting numerous problems. after modelling the circuit on a breadboard and testing it we got unexpected readings. these include

  • 411.5hz output from the timer instead of 350hz but correct waveform (square wave)
  • 17Mhz at the base of BJT Q2 instead of 27Mhz and closer to a saw tooth wave rather than the expected square wave enter image description here

the circuit diagram included shows the layout and pin connection but we used a 7555 timer instead of a 555 and a 27Mhz crystal was used instead of the 25Mhz

  • \$\begingroup\$ Are you aware square waves produce lots of harmonics? A carrier wave must be as close as possible to a sinusoid to prevent polluting nearby frequencies. \$\endgroup\$
    – user59864
    May 25, 2015 at 19:29
  • \$\begingroup\$ Welcome, good question. There's a lot of capacitance between adjacent tracks on a breadboard. At RF you need to be more careful - all the low-pF capacitors could be changed by 20% by the breadboard. Things must be very wrong for the crystal oscillator to operate at the wrong frequency. Try building the crystal and transistor part by soldering it onto strip board, or just on a piece of grounded PCB with the wires in the air, keeping everything as tidy as possible. \$\endgroup\$
    – tomnexus
    May 25, 2015 at 20:49

1 Answer 1


You have at least two "sources" for the "discrepancies." The first is the variability (tolerance) of the components, and the second is "parasitic" capacitance. I recommend adding a "trimmer" so that you can adjust the frequency of the 7555, Also, it seems you are overloading the crystal oscillator. You can try a smaller coupling capacitor and/or more amplification.


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