AD8307-EB datasheet(16/20 Pages) AD

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AD8307

–16–

REV. A

W to 1 kW 50

Power Meter

The front-end adaptation shown in Figure 39 provides the mea-

surement of power being delivered from a transmitter final am-

plifier to an antenna. The range has been set to cover the power

range –30 dBm (7.07 mV rms, or 1

µW) to +60 dBm (223 V rms,

or 1 kW). A nominal voltage attenuation ratio of 158:1 (44 dB) is

used; thus the intercept is moved from –84 dBm to –40 dBm and

the AD8307, scaled 0.25 V/decade of power, will now read 1.5 V

for a power level of 100 mW, 2.0 V at 10 W and 2.5 V at 1 kW.

The general expression is:

P (dBm) = 40 (VOUT – 1)

The required attenuation could be implemented using a capaci-

tive divider, providing a very low input capacitance, but it is

difficult to ensure accurate values of small capacitors. A better

approach is to use a resistive divider, taking the required precau-

tions to minimize spurious coupling into the AD8307 by placing

it in a shielded box, with the input resistor passing through a

hole in this box, as indicated in the figure. The coupling capaci-

tors shown here are suitable for f

≥ 10 MHz. A capacitor may be

added across the input pins of the AD8307 to reduce the re-

sponse to spurious HF signals which, as already noted, extends

to over 1 GHz.

The mismatch caused by the loading of this resistor will be

trivial; only 0.05% of the power delivered to the load will be

absorbed by the measurement system, a maximum of 500 mW

at 1 kW. The post-demodulation filtering and slope-calibration

arrangements should be chosen from other applications described

here, to meet the particular system requirements. The 1 nF

capacitor lowers the risk of HF signals entering the AD8307 via

the load.

INP VPS ENB INT

INM COM OFS OUT

AD8307

ANTENNA

604

100k

1/2W

51pF

VR1

INT

3dB

51pF

0.1 F

OUTPUT

LEAD-

THROUGH

CAPACITORS,

1nF

INPUT

FROM P.A.

1 W TO

1kW

1nF

NC = NO CONNECT

+5V

VOUT

Figure 39. 1

µW to 1 kW 50-Ω Power Meter

Measurement System with 120 dB Dynamic Range

The dynamic range of the AD8307 can be extended further—

from 90 dB to over 120 dB—by the addition of an X-AMP™

such as the AD603. This type of variable gain amplifier exhibits

a very exact exponential gain control characteristic, which is

another way of stating that the gain varies by a constant number

of decibels for a given change in the control voltage. For the

AD603, this scaling factor is 40 dB/V, or 25 mV/dB. It will be

apparent that this property of a linear-in-dB response is charac-

teristic of log amps; indeed, the AD8307 exhibits the same

scaling factor.

The AD603 has a very low input referred noise: 1.3 nV/

√Hz at

its 100-

Ω input, or 0.9 nV/√Hz when matched to 50 Ω, equivalent

to 0.4

µV rms, or –115 dBm, in a 200 kHz bandwidth. It is also

capable of handling inputs in excess of 1.4 V rms, or +16 dBm.

It is thus able to cope with a dynamic range of over 130 dB in

this particular bandwidth.

Now, if the gain control voltage for the X-AMP is derived from

the output of the AD8307, the effect will be to raise the gain of

this front-end stage when the signal is small and lower it when it

is large, but without altering the fundamental logarithmic nature

of the response. This gain range is 40 dB, which, combined with

the 90 dB range of the AD8307, again corresponds to a 130 dB

range.

VPOS

GNEG

FDBK

GPOS

VNEG

COMM

VINP

VOUT

AD603

INP VPS ENB INT

INM COM OFS OUT

AD8307

0.65V

0.1 F

VN , –5V

0.15V TO 1.15V

OUTPUT

10mV/dB

20k

BANDPASS

FILTER*

VR1

INT

8dB

464

4.7

1nF

80.6k

0.3V

2.3V

150pF

750nH

INPUT

–105dBm

+15dBm

28k

187k

330

100k

VP , +5V

* E.G., MURATA SFE10.7MS2G-A

NC = NO CONNECT

Figure 40. 120 dB Measurement System

Figure 40 shows how these two parts can work together to pro-

vide state-of-the-art IF measurements in applications such as

spectrum/ network analyzers and other high dynamic range

instrumentation. To understand the operation, note first that

the AD8307 is used to generate an output of about 0.3 V to

2.3 V. This 2 V span is divided by 2 in R5/R6/R7 to provide the

1 V span needed by the AD603 to vary its gain by 40 dB. Note

that an increase in the positive voltage applied at GNEG (Pin 2)

lowers the gain. This feedback network is tapped to provide a

convenient 10 mV/dB scaling at the output node, which may be

buffered if necessary.

The center of the voltage range fed back to the AD603 is 650 mV

and the

±20 dB gain range is centered by R1/R2. Note that the

intercept calibration of this system benefits from the use of a

well regulated 5 V supply. To absorb the insertion loss of the

filter and center the full dynamic range, the intercept is adjusted

by varying the maximum gain of the AD603, using VR1.

Figure 41 shows the AD8307 output over the range –120 dBm

to +20 dBm and the deviation from an ideal logarithmic re-

sponse. The dotted line shows the increase in the noise floor

that results when the filter is omitted; the decibel difference is

about 10log10(50/0.2) or 24 dB, assuming a 50 MHz band-

width from the AD603. An L-C filter may be used in place of

the ceramic filter used in this example.

X-AMP is a trademark of Analog Devices, Inc.

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