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AD604

Rev. F | Page 20 of 32

The 50 Ω termination resistor, in parallel with the 50 Ω source

resistance of the signal generator, forms an effective resistance of

25  as seen by the input of the preamplifier, creating 4.07 V of

rms noise at a bandwidth of 40 MHz. The noise floor of this

channel is consequently 6.5 µV rms, the rms sum of these two

main noise sources. The minimum detectable signal (MDS) for

this circuit is +6.5 µV rms (−90.7 dBm). Generally, the measured

signal should be about a factor of three larger than the noise

floor, in this case 19.5 µV rms. Note that the 25 µV rms signal

that this AGC circuit can correct for is just slightly above the

MDS. Of course, the sensitivity of the input can be improved by

band-limiting the signal; if the noise bandwidth is reduced by a

factor of four to 10 MHz, the noise floor of the AGC circuit with a

50 Ω termination resistor drops to +3.25 µV rms (−96.7 dBm).

Further noise improvement can be achieved by an input matching

network or by transformer coupling of the input signal.

VGN (V)

–30

–20

–10

GAIN (dB)

f =1MHz

0.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9

00540-045

Figure 45. Cascaded Gain vs. VGN (Based on Figure 44)

VGN (V)

–4

–3

–1

–2

GAIN ERROR (dB)

f =1MHz

0.2 0.7 1.2 1.7 2.2 2.7

00540-046

Figure 46. Cascaded Gain Error vs. VGN (Based on Figure 44)

The descriptions of the detector circuitry functions, comprising

a squarer, a low-pass filter, and an integrator, follow. At this

point, it is necessary to make some assumptions about the input

signal. The following explanation of the detector circuitry presumes

an amplitude modulated RF carrier where the modulating signal is

at a much lower frequency than the RF signal. The AD835

multiplier functions as the detector by squaring the output signal

presented to it by the AD604. A low-pass filter following the

squaring operation removes the RF signal component at twice

the incoming signal frequency, while passing the low frequency

AM information. The following integrator with a time constant of

2 ms set by R8 and C11 integrates the error signal presented by

the low-pass filter and changes VG until the error signal is equal

to V

SET

For example, if the signal presented to the detector is V1 = A ×

cos(ωt) as indicated in Figure 44, the output of the squarer is

−(V1)

/1 V. The reason for all the minus signs in the detection

circuitry is the necessity of providing negative feedback in the

control loop; actually, if V

SET

becomes greater than 0 V, the

control loop provides positive feedback. Squaring A × cos(ωt)

results in two terms, one at dc and one at 2ω; the following low-

pass filter passes only the −(A)

/2 dc term. This dc voltage is

now forced equal to the voltage, V

SET

, by the control loop. The

squarer, together with the low-pass filter, functions as a mean-

square detector. As should be evident by controlling the value of

SET

, the amplitude of the voltage V1 can be set at the input of

the AD835; if V

SET

equals −80 mV, the AGC output signal

amplitude is ±400 mV.

Figure 47 shows the control voltage, VGN, vs. the input power at

frequencies of 1 MHz (solid line) and 10 MHz (dashed line) at

an output regulated level of 2 dBm (800 mV p-p). The AGC

threshold is evident at a P

of about −79 dBm; the highest input

power that can still be accommodated is about +3 dBm. At this

level, the output starts being distorted because of clipping in the

preamplifier.

4.5

4.0

0.5

3.5

3.0

1.0

2.5

2.0

1.5

CONTROL VOLTAGE (V)

1MHz

10MHz

(dBm)

–80 –70 –60 –50 –40 –30 –20 –10 0 10

00540-047

Figure 47. Control Voltage vs. Input Power of the Circuit in Figure 44

As previously mentioned, the second preamplifier can be used

to extend the range of the AGC circuit in Figure 44. Figure 48

shows the modifications that must be made to Figure 46 to achieve

96 dB of gain and dynamic range. Because of the extremely high

gain, the bandwidth must be limited to reject some of the noise.

Furthermore, limiting the bandwidth helps suppress high-

frequency oscillations. The added components act as a low-pass

filter and dc block (C5 decouples the 2.5 V common-mode

output of the first DSX). The ferrite bead has an impedance of

about 5 Ω at 1 MHz, 30 Ω at 10 MHz, and 70 Ω at 100 MHz.

The bead, combined with R2 and C6, forms a 1 MHz low-pass

filter.