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Author Topic: Check out this remote reading digital RF wattmeter kit for a PC!  (Read 5849 times)
AD0AR
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Posts: 26




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« on: September 04, 2013, 07:11:29 AM »

I came across this and wonder if anyone else has ever tried building this piece of kit.
  It has no display as it uses windows based software to provide a on screen wattmeter. 
I'm a little confused with the specs as it says it can read mw to 50 watts with 40 db of attenuation, but then also indicates that the attenuator can go as high as 50db, which would indicate to me that
it could read beyond 50W and reach 100W???

http://electronics-diy.com/store.php?sel=kits&sub=rf_power_meter
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G3RZP
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Posts: 4962




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« Reply #1 on: September 04, 2013, 07:37:00 AM »

A "50dBm attenuator"? Measures to 2 watts, but 50 watts with a 40dB attenuator. Why does it need a 40dB attenuator for 50 watts, rather than 14dB? Probably because of the software. It doesn't say anywhere what the accuracy is either, and there's a lot of difference between resolution and accuracy.
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DJ1YFK
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Posts: 193


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« Reply #2 on: September 04, 2013, 09:24:34 AM »

I came across this and wonder if anyone else has ever tried building this piece of kit.
  It has no display as it uses windows based software to provide a on screen wattmeter. 
I'm a little confused with the specs as it says it can read mw to 50 watts with 40 db of attenuation, but then also indicates that the attenuator can go as high as 50db, which would indicate to me that
it could read beyond 50W and reach 100W???

http://electronics-diy.com/store.php?sel=kits&sub=rf_power_meter

Some confusing statements there, indeed.

First of all, note that this does not include a coupler of any kind, it just measures the power into its 50 Ohm input, where the power is dissipated. Looking at those two parallel 100 Ohm resistors at the input, and the input connections, I doubt the input match and therefore the accuracy will be very good in the VHF/UHF range. The "500 MHz" statement is probably just what the AD8307 datasheet says.

Then, it says: "Measures RF Power from 1nW to 2W (50W with 40dB attenuator) / -66.6dBm to 33dBm"... well, 1 nW is -60 dBm, and 50 W minus 40 dB is about 7 dBm, way below the specified maximum input power. RZP's assumption that it's probably limited by the value you can set in the software sounds right, then.

Another thing that raised my interest is the display shown: 17.4 dBm, 54.95 mW (that's correct) and 2.058 V... that's about 84 mW at 50 Ohms if my math is correct. No idea, maybe it is not supposed to be the RF voltage, or it doesn't have a 50 Ohm reference...at 75 Ohms it would be pretty close :-)

At 60 USD, not exactly a bargain, considering those things, and that one could build one for less than half that _and_ have some fun with the development :-)

YMMV.
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G3RZP
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Posts: 4962




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« Reply #3 on: September 04, 2013, 11:06:26 AM »

I hadn't noticed the display error. To read 54.95mW, it needs to resolve to three decimal places or to 0.00079 of a dB! Plus it needs accuracy of 0.0001mW......

So it is, as with so many digital displays, absolutely pointless having 2 decimal points displayed. I doubt the accuracy is such that only the first digit is needed in reality: follow it by the correct number of zeros depending on range.

YFK's comment on the input resistors and the $ value is very true.
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KE4JOY
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Posts: 1384




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« Reply #4 on: September 04, 2013, 12:09:02 PM »

Looks to me to be only good for testing the output of handheld transceivers.  Huh
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