Thomas Martin, DF7TV, Stuttgart, Germany.

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PM8307 -- A Simple RF-Power Meter Using The AD8307

The idea and circuit for this power meter go back to a publication of Wes Hayward[1]. Having the surface mount version of Analog Devices AD8307 I changed some parts (capacitors) to SMD which better fit to the SO-8 case. Apart from that the circuit used is very similar to the one published. The gain of the noninverting amplifier is set to G=2 to get an output signal of about 50mV/dB which may be displayed by an external DVM or scope. Using an internal 50uA instrument I used a 100k multiplier resistor to translate a range from -80dBm to +20dBm into a displayed value of 0 to 50uA. The AD8307 will fade out at low signals of about -75dBm and its maximum input is +16dBm. A test showed that this instrument is usable with very good results in the range -70dBm (0.1nW) to +16dBm (40mW).


PM8307, front

PM8307, back

PM8307, dead bug construction


Test of the PM8307 with a hp 8640B Generator

A visit at DK8SI, Udo allowed me to test the power meter using his hp 8640B generator. The first table shows the measured output voltage (in mV) for the power level given in dBm at the input of the power meter; frequency has been 10MHz during this test. The third column shows value read from the internal 50uA-instrument. The last two columns are related to a linear regression - a "best-fit" line which has been calculated using measured points from -70dBm to 13dBm. The last column clearly shows that this line (according to equation (1) below the graphic) is a good approximation which may be used in some PM8307---A/D-converter---Processor setup. V_OUT from the second column of the table is used in equation (1) to calculate the values in column four.

Level P_IN/dBmOutput V_OUT/mVCurrent Display/uACalculated Level/dBm according Equ. (1)Difference "Calculated - Actual Level"/dBm
-90 414 4 (-76.952) (13.048)
-80 469 4.5 (-75.855) (4.145)
-70 772 7.5 -69.813 0.187
-60 1268 12.2 -59.921 0.079
-50 1759 17.0 -50.130 -0.13
-40 2264 22.0 -40.059 -0.059
-30 2770 26.5 -29.969 0.031
-20 3260 31.5 -20.197 -0.197
-10 3756 36.2 -10.305 -0.305
0 4280 41.5 0.14376 0.14376
10 4781 46 10.135 0.135
13 4930 47.5 13.106 0.106
16 5073 48.5 15.958 -0.042

The graphic shows the data of the first table. A straight line was established using linear regression for the measured values from -70 dBm to 13 dBm (...well my Casio pocket calculator fx-5500LA did it for me). The correlation coefficient r, which describes the exactness of a linear dependance, was found to be r = 0.99998. r ranges from -1 to +1; the value +1 represents an exact linear dependance. Yes -- both the measured curve (from - 90dBm to 16dBm) and the calculated best-fit line are drawn even so you can't see them separatly within the linear region because the good fit of the linear regression. The graphic is made available as a PDF-file pm8307_test1b.pdf. Equations which describe this line are given below the graphic and may become useful in later applications. You may observe the fading out of the AD8307 at low power levels as the curve shows a S-shape. To not kill my AD8307 during the measurements the input power has been limited to a maximum of 16dBm.

PM8397, test results, graphic

Wow! the calculated slope is very, very close to the 50mV/dB I had assumed during my measurement of a filter curve before I could do this test. For input power levels in between -70dBm and 16dBm the measured curve is very close to this straight line. When time allows a second test will be made with a close look to the range below -70dBm. Nonlinearity in this range could be compensated using an A/D converter and processor so that useful measurements may be possible in the range below -70dBm.


The second table shows a wideband test. Power level used here was -20dBm for all frequencies ranging from 625kHz to 320MHz and again the output voltage is given in mV.
Frequency/MHzOutput/mV
0.625 3257
1.25 3256
2.5 3261
5 3259
10 3260
20 3264
40 3266
80 3237
160 3187
320 3093

So regarding the 0.625 to 40 MHz range we got a deviation of less than +/-10mV (or abt. +/-0.2dBm) if the output voltage at 10MHz is taken as a reference.


References:
[1] Hayward, Wes, W7ZOI, "Simple RF-Power Measurement", QST, June 2001.


Modified: September 2007.
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