Measuring Induced Voltage Wirelessly?

Hello folks,

I am trying to measure the induced voltage of a small coil when it is placed near a larger coil operating between 10 MHz and 20 MHz, but i'm having a problem as I know the oscilloscope cable(s) I have connected to the coil terminals are affecting the result.

1. Is there any method of rectifying this? I need accurate voltage measurments.

2. Could this experiment be performed wirelessly is any way?



Thanks for your time

Cheers

Cat

There are two methods I can think of:

1) terminate the coil with a detector diode. The diode turns RF voltage into DC voltage. then I would run very short leads to a digital voltmeter with an RF blocking cap in parallel and read the meter face when standing 5 feet away.

2) terminate the coil with a detector diode. Capacitively filter out the Rf, and send it to the tuning port of a VCO oscillator (a 900 MHz one for instance). You can use a small stub of wire as the antenna for the 900 MHz oscillator output. Then you set up a receiving antenna a few feet away and either see the RF output on a spectrum analyzer, or amplifier and feed it to a frequency counter. The 900 MHz VCO will tune to a frequency that you can measure, and later you can figure out what tuning voltage that frequency corresponded to.

Hi, thanks for the reply....

Could you explain what an "RF blocking cap in parallel" means.


Any additional ideas would be appreciated as well.


Cheers

Cat

Is there anything to add to this dilemma, any other methods that people have used and found successful?

Kind Regards


Cat :D

Hi,

First, an RF blocking capacitor is a shunt across two wires where you do not wish to pass any RF signal level through the wires, the capacitor makes a short circuit between the measuring wires from 10MHz and up in your case (if you choose the capacitors' reactance to be under a few Ohms or less at 10MHz in your case).

I think biff44 nicely tackled the problem but if your induced voltage is too small (say a few mV) then no diodes can detect it, you have to amplify the induced voltage by a wide band amplifier first then rectify it.
There are dedicated integrated circuits just for wide band detection, see mainly Linear Technology or Analog Devices.

For instance a wide band power detector is here:

http://www.linear.com/pc/downloadDoc...74,P2219,D2368

Or if you are happy with a log output, see this IC:

http://www.linear.com/pc/downloadDoc...3,P12432,D8877

I did not check analog.com for similar products, it is worth doing so too.

With such ICs you will have a reasonably accurate DC equivalent voltage from your induced 10-20MHz signals and it is always easier to handle/measure a DC voltage at a certain distance.

Regards
unkarc

Good point, these new log detector IC's are great for small voltages. BTW, analog devices does make some also.

By RF blocking cap in parallel, I mean that a 20 MHz signal is low frequency enough to be read as AC on some voltmeters, so you use a diode to turn the 20 MHz sine wave into a pulse train, then you use a shunt capacitor to filter out the RF content and just leave the DC voltage. The DC is easy to measure.