Electronics > RF, Microwave, Ham Radio

How to measure DC voltage in microwave circuit?

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robert1111:
How is the DC voltage offset measured in a microwave circuit?  Is a super expensive oscilloscope needed?  Thanks.

TheUnnamedNewbie:
Some kind of bias-T comes to mind? Or even without any bias-T, most 'DC' tools are completely insensitive to anything past a few MHz. But I'm know for throwing a UXR at every problem so that works too, I guess.

radiolistener:

--- Quote from: robert1111 on July 09, 2021, 09:07:18 am ---How is the DC voltage offset measured in a microwave circuit?  Is a super expensive oscilloscope needed?  Thanks.

--- End quote ---

No, you can use any oscilloscope for that.

Since you're needs just DC component, oscilloscope bandwidth doesn't matter.

Also you can use DMM, and even cheap one like Chinese DT830 :)
The possible issue with DMM is that it's probe impedance on a microwave frequency can affect your microwave circuit mode. If it's not an issue for your microwave circuit, then you can use usual DMM in DC mode.

Just switch your DMM to DC Voltage mode and connect it's probes. It will show you DC offset, because microwave frequency will be cutoff with a low input bandwidth of the DMM.

But be careful, if your microwave circuit has a high amplitude/power output you can damage your DMM or a power amplifier of your microwave circuit. You can use it for low power RF output. For a high power signal you can use RF inductor to feed your DMM with DC component. But don't forget to attach proper load for your signal source and use proper soldering for inductor to avoid impedance issues - don't use wires, solder one pin of the inductor into transmission line directly.

For example I'm using my Brymen BM867S in DC Voltage mode to measure DC offset at 10-400 MHz. And it works ok.

PS: just tested my BM867 DMM and it works ok from 20 Hz, but the lowest frequency limit will depends on DMM type.

TheUnnamedNewbie:

--- Quote from: radiolistener on July 09, 2021, 10:33:57 am ---
--- Quote from: robert1111 on July 09, 2021, 09:07:18 am ---How is the DC voltage offset measured in a microwave circuit?  Is a super expensive oscilloscope needed?  Thanks.

--- End quote ---

No, you can use any oscilloscope for that.

Since you're needs just DC component, oscilloscope bandwidth doesn't matter.

Also you can use DMM, and even cheap one like Chinese DT830 :)
The possible issue with DMM is that it's probe impedance on a microwave frequency can affect your microwave circuit mode. If it's not an issue for your microwave circuit, then you can use usual DMM in DC mode.

Just switch your DMM to DC Voltage mode and connect it's probes. It will show you DC offset, because microwave frequency will be cutoff with a low input bandwidth of the DMM.

But be careful, if your microwave circuit has a high amplitude/power output you can damage your DMM or a power amplifier of your microwave circuit. You can use it for low power RF output. For a high power signal you can use RF inductor to feed your DMM with DC component.

For example I'm using my Brymen BM867S in DC Voltage mode to measure DC offset at 10-400 MHz. And it works ok.

PS: just tested my BM867 DMM and it works ok from 20 Hz, but the lowest frequency limit will depends on DMM type.

--- End quote ---

You do need to watch out here though. Just poking it with a multimeter will change the impedance, and some devices (eg, the output of a PA) can have a DC-level that depends on the impedance presented.

radiolistener:

--- Quote from: TheUnnamedNewbie on July 09, 2021, 10:56:09 am ---You do need to watch out here though. Just poking it with a multimeter will change the impedance, and some devices (eg, the output of a PA) can have a DC-level that depends on the impedance presented.

--- End quote ---

Yes, for a such circuit you can load it with a proper dummy load and solder some inductor at transmission line to get DC voltage through it for DMM.

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