AD607 데이터 시트보기 (PDF) - Analog Devices
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AD607 Datasheet PDF : 24 Pages
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AD607
As noted earlier, the gain in dB is reduced linearly with the volt-
age VG on the GAIN pin. Figure 43 shows how the mixer and
IF strip gains vary with VG when GREF is connected to VMID
(1.5 V) and a supply voltage of 3 V is used. Figure 44 shows
how these vary when GREF is connected to a 1.23 V reference.
90dB
80dB
70dB
60dB
50dB
40dB
30dB
20dB
10dB
0dB
0
(67.5dB)
IF GAIN
(21.5dB)
MIXER GAIN
(7.5dB)
(1.5dB)
0.4V
1V
1.8V 2V 2.2V
NORMAL OPERATING RANGE
Vg
Figure 43. Gain Distribution for GREF = 1.5 V
90dB
80dB
70dB
60dB
50dB
40dB
30dB
20dB
10dB
0dB
0
(67.5dB)
IF GAIN
(21.5dB)
MIXER GAIN
(7.5dB)
(1.5dB)
0.328V
1V
1.64V
NORMAL OPERATING RANGE
2V
Vg
Figure 44. Gain Distribution for GREF = 1.23 V
Using the Internal AGC Detector
The AD607 includes a detector cell at the output of the IF am-
plifier that allows it to provide its own AGC and output-leveling
function in receiver applications where DSP support is not
needed. It is only necessary to connect a filter capacitor between
the GAIN pin and ground to invoke this feature. The voltage
appearing on this pin may then be used as an RSSI output, with
the scaling discussed earlier; note particularly that the voltage on
GREF continues to determine this scaling.
Figure 45 shows a simplified schematic of the detector. Transis-
tor Q2 remains cut off by a 300 mV bias (when VP = 3 V; in gen-
eral, ≈ VP/10) until the positive tip of the IF waveform causes
it to briefly conduct, charging the AGC filter capacitor CAGC in
a positive direction. The voltage across this capacitor is VG.
˜ 30µA
LAST IF STAGE
IFOP
1.5V
IF OUTPUT
77µA
4.5µA
Q1 Q2
IC2
ZERO
1.5V + 316mV
GAIN
CAGC
(EXT)
4.5µA
(INT)
COMM
TO INTERNAL
GAIN CONTROL
AVERAGE OF IC2 IS
FORCED TO 4.5µA BY
INTEGRATION IN CAGC
Figure 45. Simplified Schematic of AGC Detector
Acting against this is an internally generated 4.5 µA pull-down
current, which operates to within a few millivolts of ground. As
VG, the voltage at the GAIN/RSSI pin, rises, the gain falls, so re-
ducing the amplitude of the IF output and reducing the ampli-
tude of the current spike in Q2; eventually a point is reached
where its average collector current is balanced by the pull-down
current, and the charging ceases. It will be apparent that the
loop filter is essentially a perfect integrator.
This simple system can be used because the input impedance of
the gain-control system, also internally tied to the GAIN/RSSI
pin, is several megohms, and its bias current is small. The volt-
age VG may be used as an RSSI output; however, if it is to be
heavily loaded, a buffer amplifier must be used.
Note that, unlike a post-demodulation AGC detector (via DSP),
this scheme responds to signal plus noise. Thus, when operating
at high gains, the AGC loop will “see” a substantial output at
the IFOP node, even though a filter may be added by the user
between the pins IFOP and DMIP. This will trick the loop into
lowering the gain until the composite output signal (IF plus
noise) reaches the reference level and satisfies the average-
current requirement. In these circumstances, the wanted signal
will be smaller than expected. Thus, the internal AGC system
will result in a slight compression of the demodulated output for
very small signal levels.
AGC Discharge Time
The discharge current is approximately 4.5 µA; thus, to restore
gain in the event of a rapid drop-out requires a time of
T = C × VG/4.5 µA. Using a 1 nF capacitor, and noting that an
80 dB gain change corresponds to 1.6 V, the discharge time is
355 µs. Note, however, that when GREF is tied to a different
value, the scaling changes. For GREF = 1.23 V, the scale factor
is 16.4 mV/dB, 80 dB corresponds to a 1.312 V change, and the
discharge time decreases to 290 µs.
VG could also be expressed in dB: with a scaling of 20 mV/dB, it
works out to T = C × P × 44,000, where P is the change in input
power, expressed in dB. Thus, using C = 1 nF, checking the
time needed for 80 dB we get T = 355 µs. For the case where
the scaling is 16.4 mV/dB, T = C × P × 36,000.
The AD607’s AGC detector delivers only one brief charging
pulse per cycle of the IF. At a 10.7 MHz IF, for example, this is
every 93 ns. When the AGC system is in equilibrium, this pulse
–18–
REV. 0
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