[HomeBrew] linear scale analog watt meter

[John Marshall]

Nice project, Steve!  I just finished (or as near finished as my 
projects get) an SWR/Wattmeter based partly on KI6WX's Tandem Match.  I 
already had a cross-needle meter to use, so I just built the coupler 
and diode compensation circuit and left out the analog SWR computer.  
The squaring circuit might be a nice addition.  Anyway, here are a 
couple of observations based on that experience:

First, you should be able to improve the detector compensation accuracy 
at low power levels by optimizing the values of the detector load 
resistors (R5, R10) and the compensating diode load resistors (R6, 
R11).  John Grebenkamper, KI6WX, explains this very well in his article 
"Calibrating Diode Detectors" in QEX for August, 1990.  The short 
version is that the DC current in the compensating diode should equal 
the peak current in the detector diode.  Since the detector diode only 
conducts for a small part of the cycle, its peak current will be much 
higher than if it were driven by the same DC voltage, and thus its 
forward drop will also be higher.  R6 and R11 want to be around 10 - 20 
percent of R5 and R10 to improve compensation.

Second, matched transistors.  It might be cheaper to use a single 
matched quad than two matched pairs.  For example, Mouser part number 
526-NTE2321 is a replacement for MPQ3904, a matched quad of 2N3904's in 
a 14 pin DIP.  Price is $3.80 each, not cheap but better than two duals 
at $5 each.

Third, for those who might want to power this device from a 12 volt 
supply at home (or 12 volt battery in the field), a 7660 or similar 
charge pump circuit might be used to get the negative supply.

Finally, I'm wondering if there might be a way to do the squaring 
circuit with rail to rail op amps and avoid the negative supply 
altogether.  Hmm, it's past beer time now so I'd better not think about 
that until tomorrow.

73,
John, KU4AF
Pittsboro, NC



On Jun 8, 2005, at 4:52 PM, Steven Weber wrote:

> ** Please do NOT cross-post messages when posting to HOMEBREW **
>
> I built a BLT with air variable caps and the biggest -2 core Diz 
> sells. To
> make it complete, I added a SWR bridge and a nice 4", 500 uA meter.
>
> I didn't want to pull apart the meter to redo the scale for watts, so 
> added
> a bunch of stickers to point to the .5, 1, 2, 3 and 4 watt points. 
> Looks
> like crap. Got to thinking it would be nice to have a linear scale, 
> but to
> do that the voltage from the SWR bridge needs to be squared.
>
> I looked through an old National Linear applications book and found a
> fairly simple circuit which would square an input voltage and convert 
> it to
> a current to drive a meter. Just what I needed. So, I built it and 
> works
> FB. 100 on the meter now equals 1 watt, 200 to 2 watts, ect.
>
> I put the circuit up on my web page, direct link is
> <http://www.qsl.net/kd1jv/Linear_analog_SWR_meter.HTM>
>
> The only down side to the circuit is it needs to be run on three 9V
> batteries, as it needs a regulated -15 volt supply. It also needs a 
> couple
> of dual transistors, which only seem to be available in SMT today. I
> happened to have a LM3086 transistor array which worked fine, but 
> don't see
> those listed in the catalog anymore.
>
> 72,
> Steve, KD1JV
> "Melt Solder"
> White Mountains of New Hampshire
> http://www.qsl.net/kd1jv/
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