[Elecraft] Accuracy of Wattmeter W2 under 2 Watt

[Ron D'Eau Claire]

Hajo:

A power meter based on the AD8307, such as the PM2 uses, can be very
accurate and sensitive but requires calibration. The AD3807 takes RF in and
produces a dc output proportional to the logarithm of the input signal
amplitude. 

I have a homebrew version based on a design by W7ZOI that works FB down to
below -90 dbM. That's the native sensitivity of the AD8307.  You are quite
right that shielding is needed otherwise any local source of RF can upset
the readings. I presume the PM2 has an input attenuator but any RF getting
to the AD8307 chip can cause errors. If you have a steady source of RF
nearby (i.e. a broadcast station) it can introduce a fixed error in addition
to the jigger you saw from the RS232 bus. That will depend upon how they
have terminated the input to the AD8307.   

I suggested a solution that is not dependent upon specialized test equipment
most Hams don't have handy or depending upon the manufacturer's calibration
data. If that is not an issue, by all means the PM2 is suitable.

73 and have fun! 

Ron AC7AC


-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Hajo
Dezelski
Sent: Monday, October 26, 2015 11:47 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Accuracy of Wattmeter W2 under 2 Watt

Hello,

Thanks for your suggestions. It seems that the owners of the W2 use it for
higher power settings than qrpp. So it is natural that nobody could answer
my questions.

So I will continue to use my Pm2 http://www.foxdelta.com/products/pm2.htm .
But in a private discussion I was pointed to a solution of the irritating
numbers shown on the display. I have no cabinet and the cable from the
RS232 was too long and not shielded. So there was some hf going direct into
the circuits. So I will build a box and take some ferrits and hopefully will
be happy.

Thanks for the advice and your time.

73 de Hajo DL1SDZ

Gruss
Hajo

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Cela est bien dit, ...,mais il fault cultiver notre jardin.