Active HF Probe

This is a straight clone of Lloyd Butler VK5BR's detector used in his excellent heterodyne sweep generator project. I have plans to build something similar myself shortly, but I had a more immediate need for a simple HF detector for a few casual measurements around the bench.

Circuit Diagram

The circuit is quite cleanly designed, V1 is biased to 5 mA and its gain is controlled via its emitter degeneration in a very conventional manner. Its input impedance isn't especially high, about 450 ύ - 3 kύ depending on the gain setting. This is OK for most purposes. A FET input would be an easy way to achieve a much higher input impedance, however it would leave the circuit more vulnerable to stray signal pickup.

The follower V2 offers a low output impedance drive to the doubler pump and works quite well.

I used a BC547 for V1, a BC548 for V2, and two random diodes from the junkbox that seem to work fine. The circuit covers from MF to a bit beyond HF despite not using particularly good transistors or layout. I didn't use precisely the same values all over, especially the 10 nF capacitors I used everywhere. I also added a 10 nF cap across the rails under the board, right near the transistors, and a 22 uF at the end to ensure good decoupling.

Picture of Device

It has proved itself to be such a useful little circuit I may rebuild it with UHF transistors and a suitable layout for operation well into the VHF region.


I use it mostly as a buffered RF probe:

The detector is ideal for bridge nulling. Using your signal generator, a few resistors, and a small bifilar transformer you can quickly detect and/or tune resonances in antennas and all kinds of circuits. You can measure Q and unknown inductors, including their resistance directly with a simple resonance bridge.

If you happen to have a swept generator you can use it as Lloyd intended and produce plots of filters, or any other frequency dependant circuit.

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