Category Archives: impedance

Characterizing A Cheap 500MHz Counter Module

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An exciting development over the last few years has been the arrival of extremely cheap instrumentation modules easily bought online and usually shipped from China. Some of them have extremely impressive paper specifications for their price, and it was one of these that caught the eye of [Carol Milazzo, KP4MD]. A frequency counter for under $14 on your favourite online retailer, and with a claimed range of 500 MHz. That could be a useful instrument in its own right, and with a range that significantly exceeds the capabilities of much more expensive bench test equipment from not so long ago. …read more

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What’s Special about Fifty Ohms?

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If you’ve worked with radios or other high-frequency circuits, you’ve probably noticed the prevalence of 50 ohm coax. Sure, you sometimes see 75 ohm coax, but overwhelmingly, RF circuits work at 50 ohms.

[Microwaves 101] has an interesting article about how this became the ubiquitous match. Apparently in the 1930s, radio transmitters were pushing towards higher power levels. You generally think that thicker wires have less loss. For coax cable carrying RF though, it’s a bit more complicated.

First, RF signals exhibit the skin effect–they don’t travel in the center of the conductor. Second, the dielectric material (that is, the …read more

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Simple Vacuum Tube Preamp Results in a Beautiful Build

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We have no intention of wading into the vacuum tube versus silicon debates audiophiles seem to thrive on. But we know a quality build when we see it, and this gorgeous tube preamp certainly looks like it sounds good.

The amp is an attempt by builder [Timothy Cose] to give a little something back to the online community of  vacuum tube aficionados that guided him in his journey into the world of electrons under glass. Dubbed a “Muchedumbre” – Spanish for “crowd” or “mob”; we admit we don’t get the reference – the circuit is intended as a zero-gain preamp …read more

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