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At high frequencies (>100MHz), "analogue multiplexer" branding gives way to "rf-switch" branding for switches which are controlled by logic levels.

But if computer control is irrelevant mechanical switches require less engineer time and are possibly cheaper.

What are the accepted guidelines for mechanical switches? I've never seen a mechanical switch data sheet with a frequency response. Still, presumably, at different frequencies, it does start behaving like a capacitor/inductor, and there is loss, hostile signal coupling etc.

Is there any accepted guidance on what frequencies this matters?

At DC and audio frequency I've only ever seen people worry about the quality of the contact.

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    \$\begingroup\$ They aren't regular electromechanical relays that you use to switch power currents. They are called RF relays. \$\endgroup\$
    – DKNguyen
    Commented Feb 16, 2023 at 16:43
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    \$\begingroup\$ There exists purpose-made mechanical RF switches that will operate from DC to 26 GHz, and they can be controlled from computers too. Take a look at this Hewlett-Packard 8765C Coaxial Switch, it used to be the workhorse at microwave test laboratories. These are precision RF coaxial devices, and they usually have better impedance matching and lower insertion loss than many semiconductor switches. \$\endgroup\$ Commented Feb 17, 2023 at 2:47

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But if computer control is irrelevant mechanical switches require less engineer time and are possibly cheaper.

Mechanical, manually-actuated RF switches are quite expensive, because they are precision devices, and because there is not a big market for them (quite the opposite). The market is small because it's hard to use them properly. Usually you don't want to be routing RF to the front panel! So most designs seeking manual RF switching don't use such switched.

Instead, any old switch is used to control an RF relay. That relay is then the electromechanical switch you sought. Relays suitable for switching RF signals are specified as to their usable bandwidth, and often come with S-parameter plots to further characterize their behavior across frequencies.

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  • \$\begingroup\$ Up to ~1 GHz manual RF switches are very cheap because of ham radio/CB/scanner market demand. \$\endgroup\$
    – user71659
    Commented Feb 17, 2023 at 21:33
  • \$\begingroup\$ @user71659 That’s good to know! \$\endgroup\$ Commented Feb 18, 2023 at 23:11
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    \$\begingroup\$ Putting in "ham radio rf relays" into Google gave me a wealth of examples and testing that the ham community has done, the HF3 from Axicom looks very good. Thank you @user71659 for the magic words to put into google :) \$\endgroup\$ Commented Feb 19, 2023 at 12:39
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I've never seen a mechanical switch data sheet with a frequency response. Still, presumably, at different frequencies, it does start behaving like a capacitor/inductor, and there is loss, hostile signal coupling etc.

Before a mechanical switch's self-inductance and self-capacitance is of critical interest, its inclusion in a circuit will create a transmission line (due to the wires that connect to it).

The effect of it creating a transmission line is well-known to anyone dealing with high frequencies and, as said above, is due to the switch's wire connections rather than the inductance and capacitance of the switch element itself (a more negligible effect).

Hence, in all but a few cases, knowing the parasitic inductance and capacitance of the switching contact is missing the bigger problem of the switch wires becoming a transmission line.

At DC and audio frequency I've only ever seen people worry about the quality of the contact.

There are "mechanical" switches that can be operated by magnets (called reed switches) and, some of these are designed to operate well into the 100s of MHz however, implementing a design that uses a reed switch still calls for expertise to avoid transmission line problems.

Here's an example of a reed-relay that works up to several GHz whilst maintaining a well-defined characteristic impedance of 50 Ω or 75 &ohm: -

enter image description here

Image from Pickering Electronics.

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