AD606

REV. B

–8–

Adjustment Procedure

The slope and intercept adjustments interact; this can be mini-

mized by reducing the resistance of R1 and R2, chosen here to

minimize power drain. Calibration can be achieved in several

ways: The simplest is to apply an RF input at the desired oper-

ating frequency which is amplitude modulated at a relatively

low frequency (say 1 kHz to 10 kHz) to a known modulation

index. Thus, one might choose a ratio of 2 between the maxi-

mum and minimum levels of the RF amplitude, corresponding

to a 6 dB (strictly, 6.02 dB) change in input level. The average

RF level should be set to about –35 dBm (the midpoint of the

AD606’s range). R2 is then adjusted so that the 6 dB input

change results in the desired output voltage change, for ex-

ample, 226 mV at 37.5 mV/dB.

A better choice would be a 4:1 ratio (12.04 dB), to spread the

residual error out over a larger segment of the whole transfer

function. If a pulsed RF generator is available, the decibel incre-

ment might be enlarged to 20 dB or more. Using just a fixed-

level RF generator, the procedure is more time consuming, but

is carried out in just the same way: manually change the level by

the corresponding voltage.

Having adjusted the slope, the intercept may now be simply ad-

justed using a known input level. A value of –35 dBm (397.6 mV

rms, or 400 mV to within 0.05 dB) is recommended, and if the

A Low Cost Audio Through RF Power Meter

Figure 7 shows a simple power meter that uses the AD606 and

an ICL7136 3-1/2 digit DMM IC driving an LCD readout. The

circuit operates from a single +5 V supply and provides direct

readout in dBm, with a resolution of 0.1 dBm.

In contrast to the limited dynamic range of the diode and

thermistor-styled sensors used in power meters, the AD606 can

measure signals from below –80 dBm to over +10 dBm. An

optional 50

Ω termination is included in the figure; this could

form the lower arm of an external attenuator to accommodate

larger signal levels. By the simple expedient of using a 13 dB

attenuator, the LCD reading now becomes dBV (decibels above

1 V rms). This requires a series resistor of 174

Ω, presenting an

input resistance of 224

Ω. Alternatively, the input resistance can

be raised to 600

Ω using 464 Ω and 133 Ω. It is important to

note that the AD606 inputs must be ac coupled. To extend the

low frequency range, use larger coupling capacitors and an

external loop filter, as outlined earlier.

The nominal 0.5 V to 3.5 V output of the AD606 (for a –75 dBm

to +5 dBm input) must be scaled and level shifted to fit within

the +1 V to +4.5 V common-mode range of the ICL7136 for

the +5 V supply used. This is achieved by the passive resistor

network of R1, R2, and R3 in conjunction with the bias net-

works of R4 through R7, which provide the ICL7136 with its

reference voltage, and R9 through R11, which set the intercept.

The ICL7136 measures the differential voltage between INHI

and INLO, which ranges from –75 mV to +5 mV for a

–75 dBm to +5 dBm input.

To calibrate the power meter, first adjust R6 for 100 mV be-

tween REF HI and REF LO. This sets the initial slope. Then

adjust R10 to set INLO 80 mV higher than INHI. This sets the

initial intercept. The slope and intercept may now be adjusted

using a calibrated signal generator as outlined in the previous

section.

To extend the low frequency limit of the system to audio fre-

quencies, simply change C1, C2, and C3 to 4.7

µF.

The limiter output of the AD606 may be used to drive the high-

impedance input of a frequency counter.

51.1

C1*

100pF

dBm

INPUT

NC

200

+5V

AD606JN

NC

0.1 F

OPTIONAL

DRIVE TO

FREQUENCY

COUNTER

C3*

150pF

dBV

INPUT

174

C2*

100pF

NC

+5V

FOR AUDIO MEASUREMENTS CHANGE

C1, C2, AND C3 TO 4.7 F; POSITIVE POLARITY

CONNECT TO PINS 1, 16

*

NC = NO CONNECT

+5V

2.513V NOM

R8

100k

R9

5k

R10

100k

C4

1 F

+5V

R2

54.9k

R1

1M

R3

54.9k

2.433V NOM

80mV

FOR

0dBm

SIGNAL

INPUT

R5

4.32k

R6

500

R7

162

R4

4.99k

+5V

100mV

+5V

0.1 F

DISPLAY

–75.0

32

35

31

36

40

39

38

34

33

180k

50pF

0.1 F

1.8M

0.1 F

0.047 F

V–

INHI

INLO

REF HI

COMM

REF LO

ICL7136CPL

Figure 7. A Low Cost RF Power Meter