Measuring an RF signal

[udirn1]

Hi All!

I need to measure a 1MHZ RF signal with an RMS value of up to 100V.
This can be done easily using scope, But I need a much "easier" and idiot proof method using tools that anyone has like DVM.
I though about building a full bridge rectifier but I will be happy to hear your input with already "bilt in" devices that one could purchase and doesnt cost a lot......
Regards
Udi

 

[eyec]

in the US, Radio Shack used to sell hand held RF meters

Steven C
Senior Member
ThirdPartyInspections.com

 

[VEBill]

If you need to measure the RMS value, then you either measure the RMS value properly (a la Fluke TrueRMS, except at 1 MHz), or you measure the peak and make some assumptions.

A good scope will include calculations that provide a digital read-out. They can be combined with ATE (a PC) to make it a go-noo test.

 

[ScottyUK]

There are instruments available specifically for this purpose, e.g. the HP400E or 400EL. Having an analogue scale they look a little old fashioned but they're perfectly good instruments. You can pick up a used one at a decent price if you shop around a bit, quoted accuracy is [±]1% from 40Hz to 2MHz.


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If we learn from our mistakes I'm getting a great education!

 

[biff44]

By your tone,I assume you want an "affordable" way of doing this.

I would have a probe that had a built in 100:1 voltage divider. Then I would use a series shottky detector diode and a shunt capacitor (like a 0.1 uF). You would turn your 100 V rms signal into a 1 V rms signal, and then detect it (turn it into DC voltage) with the diode and capacitor.

The trick in implementing this is that you need a good connection to two points in the circuit--the hot lead AND the ground return. The ground return has to be intimately associated with the hot lead (i.e. part of the shield on a connector, or cable, or the ground plane on a PCB that is right next to the signal). If you connect to a ground that is too far away (say >3" away) you run the risk of not getting an accurate reading.

And, of course, with 100V rms signal, you do NOT want to be using two hands to make the measurement! You want to connect the ground wire with the RF power turned off, THEN turn the RF power on, and then make the measurement. You can get electrocuted with RF power just the same as if you walked up to a fuse panel and grabbed the mains!

Of course, you can just use a 100:1 voltage divider probe, and then run it into an RMS voltmeter (assuming you find one that can respond to 1 MHz signals).

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