Rev0VTx VCO

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This page describes the Voltage Controlled Oscillator (VCO) for the rev0VTx video transmitter.

Summary

Schematic for the rev0VTx VCO.
Board for the rev0VTx VCO.

The VCO runs from a single 5V supply and spans a frequency range of approximately 1240-1360 MHz. The design is based heavily on the DL1MFK 23cm VCO, found here: 23cm VCO BFP405.pdf The modified design used here uses a BFS17A 2.8 GHz BJT and the BB833 varactor diode. A 15nH wirewound inductor was used in series with a 15pF capacitor in series with the 3-7pF capacitance of the BB833 to form the series resonant circuit. The schematic was simulated in LTSpiceIV using the SPICE circuit model provided by NXP. The LTSpice circuit can be found here: http://www.rev0proto.com/files/23cm_VCO.asc The gerbers, Eagle schematic and board file can be found here: http://www.rev0proto.com/files/rev0VTx_VCO.zip

rev0VTx VCO as simulated in LTSpiceIV.

Testing

  • 2.8.2013: The board was assembled and connected to power and terminated in 50 ohms. A modification was added to the board to insert a 1uH inductor between the varactor cathode and the VCO input pin. Scans were made around 1100-1300 MHz with an RTL-SDR. Initial scans revealed no carriers, so the board was connected to an Alinco DJ-X11 in frequency counter mode through a 40dB attenuator. The frequency counter indicated the frequency to be in the range of 700-800 MHz, far from the simulated result.
  • 2.9.2013: After calculation, it was clear that the simulation was incorrect, and the series LC circuit was tuned for the 700-800 MHz range. The inductor and series capacitor were modified by trial and error to the final values of 5.6pF, and 2.2nH. The final tuning range is 1215-1350 MHz.

PCB/Construction Photos