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I am looking for RF power meter to measure the output power of RF based PCB board which is estimated to give an output power of 45dBm at 868MHz.
While searching for meter I have come across specification "Dynamic Range" which is about 70dB and the power meter comes along with the power sensor which says that the measurement range of power sensors I have seen is around -60 to +20 dBm. Can anyone help me whether these specifications are suitable for my application ?

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  • \$\begingroup\$ Having a max input of +20 dBm makes it unsuitable for measuring +45 dBm without using an attenuator that is capable of handling up to possibly 50 watts (even though you are generating 31 watts). \$\endgroup\$ – Andy aka Aug 25 '17 at 12:01
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    \$\begingroup\$ Just as a sanity check, 45dBm is around 30 W of RF output. That's a fairly serious output power and in most parts of the world well into the licence required to operate it category. You're sure that number is correct? \$\endgroup\$ – Andrew Aug 25 '17 at 12:10
  • \$\begingroup\$ As far as I know, the open 868MHz band is restricted to 25mW/14dBm in most of the world. A 45dBm transmitter would effectively jam all other use of the band in a large geographic area... \$\endgroup\$ – Lundin Aug 25 '17 at 12:35
  • \$\begingroup\$ Actually RF module I am using gives output power of 14dBm. To increase the power I am using a power amplifier which gives a output power of 45dBm \$\endgroup\$ – Mounika Aug 25 '17 at 13:08
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    \$\begingroup\$ And you are supplying the RF amplifier with at least 50 W of power? Be aware that such a configuration is probably illegal to operate without a license. \$\endgroup\$ – Andrew Aug 25 '17 at 13:10
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Yes, what you need is an RF attenuator, you need to bring that +45 dBm down less than +20 dBm so that is at least 25 dB attenuation.

Usually RF attenuators come in 40, 20, 10, 5, 3 etc dB attenuation. You can put two in series and the attenuation adds up.

Here are some 30 dB attenuators with N connectors:

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Here's an adjustable one:

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Note that +45 dBm (in 50 ohms I assume) is around 30 Watt which is a lot !

You might need an attenuator looking like this for that:

enter image description here

Do study the datasheet of the attenuator before buying as you need it to have the proper connector, power rating, characteristic impedance (usually 50 ohms) and of course frequency range.

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  • \$\begingroup\$ can you tell me what does the dynamic range mean \$\endgroup\$ – Mounika Aug 25 '17 at 13:14
  • \$\begingroup\$ dynamic range is the most the signal can change by before things will start to go wrong. \$\endgroup\$ – Andrew Aug 25 '17 at 13:18
  • \$\begingroup\$ The dynamic range of a power meter is the difference between the lowest and highest powerlevel it can measure accurately. So a DR of 70 dB combined with a maximum input power of +20 dBm means that power meter can measure accurately between (+20 dBm - 70 dB =) -50 dBm and +20 dBm. With an attenuator you can shift this up, a 20 dB att. would change that -50 dB to +20 dBm to -30 dBm to +40 dBm. Note how the difference remains 70 dB because the DR is up to the power meter. \$\endgroup\$ – Bimpelrekkie Aug 25 '17 at 14:22
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The Dynamic Range is the ratio of the maximum to minimum power a sensor can measure, to some specification.

For measuring the power output of an amplifier, it's not particularly useful. If you want to measure the range of powers that a switchable attenuator can produce, then it's a key specification.

With your requirement to measure +45dBm, and the meter's +20dBm max input, you will also need to find a suitable fixed attenuator to drop the power to some level within the dynamic range of the meter, so to anything from +20dBm down to (so the manufacturer claims) -60dBm, but at the low end, you will find speed and accuracy compromised, so you would be best to stick to the top end, say 0dBm to +20dBm, for best readings.

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