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Measuring with the RF Explorer

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Dannyboy:
Hi, complete RF beginner here. I am using the RF Explorer http://micro.arocholl.com/ to check the power level on these 433Mhz radios: http://www.nordfield.com/wireless-serial-adapter

The datasheet for the radios say that the radios has a power level of 50mW but when I measure the signal with the RF Explorer I can only measure around -48dbm which equals to only 0.000016mW.
I am using the 6db RF Power Limiter and have compensated for this in the RF Explorer.

Is it normal that I cannot measure the exact power level or is my RF Explorer broken?

ElectroIrradiator:

--- Quote from: Dannyboy on July 12, 2013, 06:26:31 pm ---Is it normal that I cannot measure the exact power level or is my RF Explorer broken?

--- End quote ---

You may have fried it. According to the specifications the absolute maximum input power is +5 dBm, and you used a 6 dB attenuator. That puts the limit at 11 dBm, which is barely more than 10 mW. So the input may be toast, as you poured 50mW into it.

'Absolute maximum' specifications frequently mean 'you don' really want to do this (but the widget may survive)'. Going four times over ought to give grief.

Dannyboy:
Damn it..I was afraid of that. What kind of power limiter (is it called an attenuator?) do I need to use if I want to measure 50mW? and how about if I want to measure 500mW?

Is it worth getting the RF Explorer repaired or is it cheaper just to buy a new (at $129)?

ElectroIrradiator:
No idea if it is worth trying to repair it, but the first question I can help with.

First, dBm is power measured on a dB scale, compared to one mW.

So 1mW is 10*log10(1/1) = 0 dBm.
50 mW is 10 * log10(50/1) = 17 dBm.
500 mW is then 27 dBm.
5 mW, the limit, is 7 dBm.

If you want to play it reasonably safe, then I wouldn't put more than 1 mW into it, 0 dBm. That makes it easy to calculate how many dB of attenuation you need at the least, 17 dB for 50 mW, 27 for 500mW. 20 dB and 30 dB attenuators are standard, and gives a nice margin of safety of a further 3 dB.

Note that many RF coaxial attenuators, particularly the small ones with SMA connectors, have strict power limits. Some may be damaged by as little as your 500 mW. However there are specialist types, which can take quite a lot of power. Additionally high power models may be unidirectional in the sense, that you must connect the input power to a particular connector.

Fraser:
A golden rule of direct connection RF power measurement is to always insert adequate RF attenuation to protect the test equipment. As has been stated, it looks like you fried the Explorer's front end. It is a dedicated commercial radio module that will need to be replaced. Put it down to experience and be glad that it was not a US$30K Rohde & Schwarz analyser that you fried !  They have a $5K fixed repair fee  :scared:  A lesson well learned I feel sure.

Ariel is a great guy and I feel certain that he will assist you in repairing your RF Explorer. The damaged components are very cheap to replace.
Ariel also sells a limiter unit that contains a 6dB attenuator and will limit a +30dBm input signal to just +4dBm, so protecting the Explorer . Its $45 but worth every penny and may be used to protect other RF equipment as well.

http://www.seeedstudio.com/depot/power-limiter-p-1444.html?cPath=63_64

Email Ariel via the RF Explorer web site and ask what the repair options are. You can also just buy one of the RF expansion modules that is sold on seeed studio for the Explorer. The blown front end is then obsolete and you use the expansion board instead. It may be an easier and more economic solution. You will NOT have damaged anything beyond the little Rf front end module so expansion boards will still work fine.

I have two RF Explorers. One is a 2.4G model plus RFEMWSUB1G expansion module, and the other is a WSUB1G model plus WSUB3G exoansion module  :)

The RF Explorer is a very versatile little unit with very easy frequency coverage expansion.

http://www.seeedstudio.com/depot/rfemwsub1g-module-p-909.html?cPath=63_64

Or upgrade to the top model by adding the 3G expansion board. This module already contains integral protection up to +30dBm

http://www.seeedstudio.com/depot/rfem-wsub3g-module-p-1264.html?cPath=63_64


Fraser

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