EMRFD Message Archive 523
Message Date From Subject 523 2007-03-15 14:03:41 la3pna Problems with power meter. Hello group.
This is my first post in this group.
I build the W7ZOI power meter with AD8307 from an shematic in an
swedish article about Manhattan style building. I don't get any
reading on the meter, and with an osciloscope I don't see any change
in the output from the AD8307 when changing the input. I do have
-10dBm out from my old signal generator, and on my osciloscope this
show an 0.2V p-p RF out so i guess this should be good. I do have
signal in to the AD8307 so i guess this migth be dead? The powersuply
is delivering 5V and 9V regulated. Any thougts on what migth be wrong?
I just bougth the KANGA spectrum analyzer boards and the book EMRFD,
International shipping takes some time, so still waiting.
73 de Thomas LA3PNA
524 2007-03-15 15:24:57 Wes Hayward Re: Problems with power meter. Hi Thomas,
It is hard to say what the problem might be with your power meter.
I just put the circuit for the original power meter in the "Files"
section of this site so you can see what was in the paper. There
might have been an error in that SM posting. The original paper is
listed for download from the TIS part of the ARRL web site.
However, it is a "members only" file. The paper is on the EMRFD CD.
Be careful with unknown signal sources. Signals that are too large
can easily damage the AD8307. Also, this is a project where short
lead length is required.
You should see a residual reading from the power meter when it has dc
power applied. This is natural and is just the result of circuit
noise. It is a "feature" to tell the user that the circuit is
As an aside, I have some new information on my web site about the
525 2007-03-15 16:52:46 Thomas S Knutsen Re: Problems with power meter. Hi Wes.
Thanks for the answer. I found the error, there was an short in an
wearoboard pice holding the LM358.
After changing the AD8307 I did got it working.
The swedish article is here:
So now I got the meter working at the bench here.
The meter is not an 0-1mA but an 200µA full scale reading, so I migth
have to fiddle a bit with the shunt to get it corect.
Thanks for the help, This meter should come in to use often here.
73 de Thomas LA3PNA
Wes Hayward wrote:
> Hi Thomas,
> It is hard to say what the problem might be with your power meter.
> I just put the circuit for the original power meter in the "Files"
> section of this site so you can see what was in the paper. There
> might have been an error in that SM posting. The original paper is
> listed for download from the TIS part of the ARRL web site.
> However, it is a "members only" file. The paper is on the EMRFD CD.
> Be careful with unknown signal sources. Signals that are too large
> can easily damage the AD8307. Also, this is a project where short
> lead length is required.
> You should see a residual reading from the power meter when it has dc
> power applied. This is natural and is just the result of circuit
> noise. It is a "feature" to tell the user that the circuit is
> As an aside, I have some new information on my web site about the
526 2007-03-16 10:15:01 Wes Hayward Re: Problems with power meter. Hi Thomas,
Great! Glad that you found the problem. Now comes the fun part of
using it. I find that I use mine all the time.
I used the 1 mA meter movement because it was cheap and readily
available from Radio Shack. I didn't want to specify a more
expensive meter when it was not needed. However, if you have a good
meter with a high resolution scale, it will work just fine. Don't
shunt the meter. Rather, for a 200 uA movement, just increase the
value of R6 by a factor of 5. So 33K should work just fine.
You note that this will take the signal at R6 to 6.8 volts at full
scale. With a 9 volt battery, it is then impossible to excessively
overdrive the meter, which could be destructive. That was
intentional when we did the design.
If you have not yet packaged the meter, it might be interesting to
consider larger batteries, for they will last longer. AA cells would
be good. The 9 volt battery has to be replaced often if you use it
a lot. One friend built his meter with the ability to use an
external power supply. However, that means that he had to use a
very good feedthrough capacitor to guarantee that external RF did not
sneak in. This becomes a non-issue with a built in battery.
Clearly, you can run the design with lower voltage and that might be
a good thing to do. I think the AD8307 chip will run nicely with a
3.3 volt supply. A LM317L regulator or one of the many other parts
that are available for 3.3 volts would then let you run with a lower
voltage. You could then drop the gain of the op-amp to unity,
change the value of R6 and still have all of the original features.
Anyway, have fun. Again, glad to hear that you got the problem
527 2007-03-16 10:23:54 Wes Hayward Re: Problems with power meter. Hi again Thomas, et al,
I just looked at the web site that you referenced. SM0JZT did a
wonderful job with it. I would recommend this site to all of the
readers. Great photos. I liked the comparis
528 2007-03-16 10:24:01 Jim Miller Re: Problems with power meter. if the rest of the circuit will run on 3.3v a cr123 is good for 1500maH,
never leaks and has a 10 year shelf life.
the 8307 only consumes 7.5ma so by itself would be good for 200 operational
shop around on the net for cr123 instead of getting them at the grocery
store. the off brand ones work fine and are about $1 each. digikey has a
nice battery holder for them for about a $1.
530 2007-03-17 05:17:11 email@example.com... Re: Problems with power meter. I just looked at the web site that you referenced. SM0JZT
Jeeeesh!!!!!! it's all Greek to me :-)
(Still lurking with fascination)
531 2007-03-17 10:22:01 bkopski Re: Problems with power meter. 534 2007-03-17 19:31:00 kerrypwr Re: Problems with power meter. I built my power meter as suggested by Bob;
I built two heads; one is the power head & the other is an RLB with an
8307 as the detector;
I call it my Poor Man's Scalar Network Analyser. :)
535 2007-03-17 23:23:16 Gary Johnson Re: Problems with power meter. Now THAT'S a nice instrument! Very nice work. I have to chime in and say that this 8307-
based meter is one of the most useful instruments on my bench. I even built one for work!
Mine uses a digital panel meter scaled to dBm, uses micropower parts, and runs on 4 AAA
batteries seemingly forever. And it's tiny. Here's link to a writeup