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 Wattmeter calibration 
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Post Wattmeter calibration
Hi,

I'm in process of assembling OZ2CPU watt-meter. It will be my main RF measuring device (50ohm impedance) and I'm looking for anyone in or around Leicester who has accurate RF test equipment that I could calibrate this meter against. Having one RF measuring device calibrated would allow me to set levels in my homebrew TRX for best performance and would help me to build RF equipment that will be accurate enough to make amateur RF tests within +/- 1db. For example - I have built W7ZOI spectrum analyzer, where I used POS-300 VCO as first local oscillator. I relay on datasheet which says that this VCO has output level of 10dBm. Using ADE-1 mixer I designed 3dB attenuator to bring VCO level down to +7dBm required for diode mixer. This should be right, if POS-300 output was really +10dBm. If it wasn't then mixer is not working in optimal LO level. This is why I would like to calibrate my RF meter against known and accurate source to be sure, and to know what is going on in my RF circuits. If anyone can help, I would really appreciate any help.

Best Regards
Marcin M0HKD


Fri Nov 21, 2014 9:24 am
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 Wattmeter calibration 
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Joined: Wed Sep 09, 2009 2:04 pm
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Post Re: Wattmeter calibration
+/- 1dB is going to be pretty much unattainable with components available and a realistic budget. I've got a £1300 spectrum analyser and even it isn't specced as accurate as that, its spec is <1.5dB

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Fri Nov 21, 2014 1:55 pm
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 Wattmeter calibration 
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Post Re: Wattmeter calibration
Thanks for your replay.

This meter is based on AD8307, and author states that without calibration it can measure +-1db, with calibration +-0.5dB. I can say, that I dont really believe in that sort of accuracy but after comparing with known source +-1dB can be achieved. That is why I look for someone with good RF source to compare my meter. I do like to know, where I am.

Regards,
M.


Fri Nov 21, 2014 2:10 pm
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 Wattmeter calibration 
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Post Re: Wattmeter calibration
I use an oscilloscope and an accurate load and measure the voltage, a bit of ohms law it seems to produce pretty accurate results. I have used that method to set up my mixers on my old home brew gear, the same as what you are proposing.

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Fri Nov 21, 2014 2:25 pm
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 Wattmeter calibration 
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Joined: Mon Apr 08, 2013 5:29 pm
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Location: Leicester IO92KP
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Post Re: Wattmeter calibration
Hi Paul,

Yes, reading datasheets and some math can make good results. Unfortunately I have a background in chemistry and I do like to know exactly what I'm measuring. That is why I need to calibrate one of my meters to be able to compare other test gear against it. Hope this make sense.

Regards,
M.


Fri Nov 21, 2014 3:32 pm
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