27 January 2012

Feedthrough capacitors in XXI century

Since I am planning to go QRO with my 4m CW TX (5W RF out), I thought it would be a good precautional idea to shield the main oscillator. So, if it is going to be boxed, I need a way to let allowed electrons in and out:
  • power supply
  • RF out (more a square than a sine wave)
  • frequency control (capacitor or varicap)
Remembering projects seen in the past two decades, I imagined feedthrough capacitors would be ideal interfaces for my signals. But, which values amongst those that I could easily locate? 1000pF, 150pF, 22pF.

The DC line does not mind being decoupled to ground with 1nF, so that one would do. As an alternative, I have been suggested to simply drill a hole through the shield and decouple +V to ground with 100nF. Plain simple and equally effective.

The RF out would be totally shorted to ground by a 22pF. And 1000pF imposes an unacceptably high capacitive load to my oscillator. Alternatives are:
  • use a passthrough RF connector, like SMA
  • use a small piece of RG174 coax with the braid soldered to the shield/ground
  • make the 22 or 150pF part of an output filter
Given that my circuit needs a square wave signal rather than a sine wave, I will probably use the short piece of coax.

Last but not least, the frequency control. Not sure whether this will be a variable capacitor or a varicap. In the former case I could install the capacitor inside the shield and let out only the component shaft. The varicap control voltage instead is DC, so the same rule as DC line applies (standard decoupling).

(My) Conclusion. Feedthrough capacitors are a useful tool that is becoming harder and harder to source. Practical alternatives are available to the hobbyst with a little imagination (or guidance from The Wise Ones), so don't give up!