Using rotary switches for HF??

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vkj

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Rotary switches are extremely convenient to use for input attenuator switching. AN old Elektor ac-voltmeter design uses a rotary sw. mounted on the pcb itself and the input resistors and caps mounted around the pins (on the board). SImilarly old timers will remember old TV and radios used rotary switches for channel switches (point to point wiring).

I am tempted to use a rot. sw for range switcihing for a DIY DSO freq upto 20MHz (hopefully). Most designs use reed relays and/or separate amps for each range. Would a rotary switch work or would it be not worth the trouble? Any tips on using them at these frequencies?

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vkj
 

A good rotary switch can be used up to 30...50 MHz. The problem is that due to internal capacitances, isolation grows poorer with frequency. Using reed switches allows to improve the isolation as each reed switch can be located in a screening tube which lowers the capacitive coupling. In RF switched attenuators, the isolation problem is solved by using double contacts which also reduces the coupling. You can use two mechanically coupled rotary switches to reduce the coupling. One contact before and one after the switched device (attenuator, filter, etc.)
 
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    vkj

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Thanks. Are there any special precautions that one can take (such as ground plane below the switch) that will minimize this effect? I read some where that its best to solder the comps directly on the switch terminals. Is this better than soldering the switch directly on the board?

Thanks,

vkj
 


All depends upon the RF frequency- the higher the frequency, the poorer the isolation due to capacitive coupling. In RF and microwave rotary attenuators molded bodies are used to screen the resistor attenuating blocks from the neighbors. Such attenuators, e.g. 0-10 dB or 0 -40 dB can be used up to 2...4 GHz with rotary switches.
A good example to follow are HP (Agilent), and Rohde & Schwarz rotary attenuators. Some older models allowed to be opened and closed without any damage, to see the know-how.
 

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