How to measure DC voltage in microwave circuit?

[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]

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

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]

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

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.

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]

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.

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.

[robert1111]

Thank you for the replies.  Will a DMM pose a low impedance at 3Ghz?  Will a DMM produce an excessive load on a low powered microwave circuit?  Thank you.

[radiolistener]

Will a DMM pose a low impedance at 3Ghz?  Will a DMM produce an excessive load on a low powered microwave circuit?  Thank you.

3 GHz is ultra high frequency and such circuit is very sensitive, DMM cannot be used directly because attaching DMM probes will break normal operation of the circuit. It needs to be connected through some SMD inductor to get DC value and don't affect normal operation. SMD inductor should be soldered directly to the signal line (without wires). But it will be not so easy task for 3 GHz, because even a little solder drop or the size of inductor leads will affect the normal operation.

[RoV]

I would use a standard DMM or scope with a small 1 k resistor (e.g. 1/8 W) in series with the probe. The resistor side that touches the RF circuit should be cut as short as possible, while the other side doesn't care.
1 k won't change the RF performance of the circuit and won't affect the DC measurement. It will allow also AC measurements up to several MHz bandwidth.
Even better, you can use a small SMD resistor and short wires: but, since the ceramic substrate is very fragile, you'll have to solder the resistor on a small PCB patch.

[coppercone2]

first of all, how are you probing your microwave circuit? What setup do you have right now. you can use a whole bunch of different tools to look at it with different kinds of equipment.

[Joel_Dunsmore]

If you are also testing things like gain and VSWR (S21 and S11) then many VNAs include built-in bias tees that allow you to drive or sense the DC on the RF mainline.  For some lower cost or high-power options, the DC bias tees are left out (low cost VNAs don't have them because they are expense, and high-power options don't have them due to their loss -which reduces the available drive power- as well as the fact that they limit the power handling through the main-arm of the test port coupler.

[TimFox]

Even with HF circuits (say, 10 MHz), old-school “VTVMs” (e.g. RCA WV98C “Senior Voltohmyst”) placed a large (typ. 1 megohm) resistor close to the probe tip to reduce loading on a circuit node with an AC component when measuring the DC component.  The resistor was part of the 11 megohm input resistance, with 10 megohms in the range switching.

[coppercone2]

well don't forget for VHF with stuff most people have, probing it with an active probe, with the attenuator, with a digital low pass filter does give you a good DC reading. But they not common at microwave frequencies. I would assume its a little drifty and noisy compared to the other methods.

[TimFox]

A good bias tee is the preferred method at GHz frequencies, analogous to the old VTVM probe.  You need to minimize the (mainly capacitive) loading on the node, but the DC voltage can be read by a high-resistance voltmeter after a simple passive filter.  A d’Arsonval microammeter would probably ignore HF ripple, but DVMs can react badly to the AC component of the voltage, so low-pass filtration is required.

[bdunham7]

You didn't mention the DC voltage or RF power levels involved, nor if you need to externally probe an existing device or if you can do some circuit construction.  One very inexpensive option is:

https://www.minicircuits.com/WebStore/dashboard.html?model=PBTC-3GW%2B

Or if you need a more precise, wideband SMA version, you can spend 10X+:

https://www.pasternack.com/0.1-mhz-18-ghz-sma-bias-tee-750-ma-24-volts-dc-pe1bt1002-p.aspx

What you don't need is an oscilloscope.  The DC voltage measurement device can be almost anything.  Isolating the DC part without loading your microwave circuit is a very specific and tricky task.

[coppercone2]

the scopes have digital filters though, and cutoff (20MHz cutoff for instance), that can make probing easier

[TimFox]

If there be a problem with the AC component on top of the DC at the oscilloscope input that you need to measure, a digital filter will have no effect.  A passive filter before digitization is required.  There are lots of problems that active circuitry is heir to that would not affect an old-fashioned d'Arsonval meter.

[eb4fbz]

I would use a standard DMM or scope with a small 1 k resistor (e.g. 1/8 W) in series with the probe. The resistor side that touches the RF circuit should be cut as short as possible, while the other side doesn't care.
1 k won't change the RF performance of the circuit and won't affect the DC measurement. It will allow also AC measurements up to several MHz bandwidth.
Even better, you can use a small SMD resistor and short wires: but, since the ceramic substrate is very fragile, you'll have to solder the resistor on a small PCB patch.

I agree 

[TimFox]

If a larger value than 1 k is used with a normal DMM, a simple arithmetic correction is needed for the DC voltage.  Most DMMs have 10 megohm input resistance (check the manual).