2007-01-21
Switched Attenuator
I've been promising myself to build or buy one of these for years! Tacking together pads on a scrap of PCB gets pretty tiring, and measuring the power gain of amplifiers isn't much fun without one. It was *long* overdue.
I simply used my Resistive Pad Calculator to compute 50 Ohm Pi-pads for 1, 2, 3, 5, 10, and 20 dB losses. It gives results to 3 figures, and the nearest E12 preferred value.
I settled for 5% resistors, and built the unit direct on PCB by cutting the laminate on the board with a box-cutter style knife. In this way I formed the bypass shorts and mechanically supported the slide switches. The series resistor was placed between the switch terminals and the shut resistors soldered from the terminals to the ground plane. Two BNC connectors completed the device.
It has a range of 0-81 dB, but I doubt 81 dB is realistic at higher frequencies without shielding between the three 20 dB stages.
It worked first time, and some basic sanity checks show its performance to low VHF to be very acceptable. At last I can measure amplifier transducer gain without pain! The image above shows a simple BC547-based amplifier using a commercial 4:1 transformer in the collector. The results agree with my previous measurements pretty well, and it took me only 5 minutes to make them, not nearly an hour.
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