Comments for "Multimeter RF Power Add-On"

13th October 2008 11:34

Alan Yates wrote...


Hello again, love you tractor project BTW.

Yes you could build the sensing "head" remote from the multimeter. The resistors and layout are only good to VHF anyway so a small pigtail of coax doesn't make a lot of difference to the accuracy. I like having it fixed to the multimeter actually, I've grown accustom to using it like that (same with the return-loss bridge) but I have taken the split head/display approach in my latest log power meter (based on an AD8307). I haven't finished calibrating the unit yet, but it will soon be up on the site. Its head is a small diecast box, all DC signals passing through feed-thru capacitors.

Another alternative is to use a diode probe and measure the voltage across a known-impedance (ie a 50 Ohm resistor, etc) in the circuit. I generally use the CRO for that, but beyond its bandwidth (20 MHz unfortunately), I use the diode probe. I've never written up the diode probe because it is so trivial, just your typical 1N5711 diode, 4M7 resistor to make it read "RMS" with the input impedance of the multimeter, and a few capacitors. Physically it is made on a narrow strip of PCB soldered to a brass tube which implements the handle. Some wires carry the DC back to banana plugs for the multimeter, there is also a DC grounding alligator clip. I carefully calibrated its "non-linear" section at DC (similarly to the power meter) but that doesn't really hold that well at RF because of the diode capacitance and carrier life-time, it is pretty close at MF and even at HF for casual use. (The picture in the article shows a 1M resistor, I replaced this with 4M7 recently because I was sick of multiplying by 0.707 - and getting the wrong answer because of the input impedance of the meter, so I decided to use it to implement the ~0.7 voltage divider. You can also do this with the terminated power jig so it reads roughly in RMS volts).

As for RF upsetting the multimeters, well if I built the power measuring jig again I'd probably add some chokes and caps to remove the last of any RF that might make it into the meter. Unfortunately it seems many cheap multimeters are easily upset by common-mode RF on their inputs. Your idea of the remote head makes it easier to choke off any RF on the DC lines.



13th October 2008 07:57

Arfv - K7HKL wrote...


Looking at your multimeter RF Power load and detector, it occurred to me that there is a possibility for putting the load box and detector in the probe end and just sending DC through a cable to the DVM. Would not this help with VHF and possibly higher frequency accuracy? Of course the load resistors would have to be hefty enough to avoid excess heat in a hand-held dummy-load RF probe.



24th June 2008 13:19

Alan Yates wrote...


The code isn't anything special, it just dumps out the Peak and RMS voltages for various dBm values from limits that are hard-coded. It doesn't automatically compensate for the diode drop, but at large levels (about +10 dBm and above) the calibrated device is almost perfectly:

Watts = (Vmeasured + 0.1)^2/100

The 0.1 volt drop factor is fairly insignificant above +30 dBm.

I recently had it calibrated against a precision generator at a calibrate-a-thon the local Homebrew Group ran. It is quite accurate to at least 200 MHz which is surprising considering the resistors I used. The good news is that the DC calibration was accurate to within 2% at HF down to -10 dBm, and quite respectable down to -20 dBm. Even if all you have is a bench PSU the results should be OK.

Its limits are about -20 dBm where the output is only about 6 mV, and it can transiently survive +43 dBm which is limited by the resistors' power handling at which point the output voltage will be about 47 volts.

Anyway, I've converted the program into a PHP script and put it up in the calculators here. I guess I should add the option to change the Zo and pick the range and granularity... The source code is available here.

How did you linearise the 555 ramp? I was thinking of a JFET or PNP as a current source in the cap charging circuit, that should do the trick. The varicap itself is still a problem of course.

24th June 2008 05:06

Doug-KM6OR wrote ...

Hi Alan. A buddy of mine just built a simple power meter like the above circuit but on the back of a 100 ua meter movement. When I saw that you made a program that corrolated DC voltages, peak and RMS AC on a chart I was sure hoping that I might be able to get that from you. He is at a point that he only has a variable voltage bench power supply for calibrating. I am also still working on my version of your sweep generator and made a simple mod to the 555 sawtooth gen for a linear ramp. CU Soon, 73's, Doug..

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