How to Reduce Noise When Measuring 20MHz Frequency Using Relay

Depending on the signal voltage, either analog switches (with suitable overvoltage protection) - or reed relays?

eg. Quad switch or multiplexer:



RF Reed relays:

(All available from Mouser etc.)

Define too much noise and what is acceptable?

Possible problems;
1. missing flyback snubber on coil ( diode resistor capacitor) V=LdI dt so choose dt = L/R when turning off coil)
2. not using low impedance twisted pair. ( depends on pF/m loading effects)
3. not using shielded pair STP
4. Ground loop noise , shield grounded in multiple locations.
5. Not using differential inputs with high CMRR cable.
6. Not using a high BW differential to single ended amplifier.

I assume when you say 20 MHz limit that you do not mean a 20 MHz high speed clock which has at least a 300 MHz BW.

If you introduce ground inductance or ESL to coax gnd. then to your Keysite DSO gnd , you will get massive ringing on step pulses. This is NOT from Relays but from impedance mismatch and ESL from not using above methods to single wire ground leads shorter than 2cm.

Which method to use depends on your signals and layout in addition to low speed signal MUX's suggested above.

If I guessed correct that 20MHz is your clock and that clock has an impedance of 22 to 50 Ohms, it will be greatly affected by your probe wire load capacitance. Therefore I suggest isolating that with a 4:1 voltage divider to match your wire impedance 50 to 220 ohms? e.g. 450 > 150 ohms shunt = Zo = 112 ohms , 25% Vin. for twisted magnet wire.

Tony

R&D and Test Engineer since 1975.

General-purpose mechanical relays and solid-state relays (SSRs) are not designed for high-frequency signals. You can use Reed relays instead. A better solution would be to use high-frequency analog switches or RF multiplexers.