Page 356 - Complete Wireless Design

Support Circuit Design

Support Circuit Design 355

square-law detector. The envelope detector detects the amplitude of the enve-
lope of the RF signal, outputting a DC voltage that is equal to this value. The
square-law detector detects the power of signal, outputting a voltage that is
equivalent to this power. Both types of detectors use the same basic circuit
arrangement shown in Fig. 8.23, except for the minor modifications mentioned
above. But since a diode will act as a square-law device at low input powers
( 60 to 30 dBm), and operate as a relatively accurate envelope-to-voltage
converter at input powers greater than 15 dBm, it can be seen that both
detectors can not only use almost the same circuit, but also the very same
diode. Their type of operation will depend only on their input power.
Since the envelope detector iteration can operate with a much lower load
than the square-law version and still maintain much of its sensitivity, the
envelope detector is the most commonly accepted detector in AGC circuits
(because of the lower internal resistance of the detector diode caused by the
much higher RF input levels, the diode is now in almost full conduction).
Both kinds of detectors, especially the envelope type, will respond very quick-
ly to an RF signal’s increase in amplitude, but will usually have a much slower
discharge time because of the requirement that C discharge through the large
value of R . The charge-up is only through the RF source resistance of 50 ohms
2
in series with the diode’s on resistance, which can be relatively low during the
IF signal’s peaks. Decreasing the RC time constant of C and R can alleviate this
2
problem but, as discussed above, lowering R also decreases detector sensitivity.
2
AGC amplifiers and integrators. DC amplifiers are normally needed to increase
the AGC level of the DC signal that is fed into the VGA’s gain adjust port.
Normal RF amplifiers may also be required on the AGC’s IF end to increase
the signal into the detector. Raising the AGC’s DC level can be accomplished
with standard DC amplifiers, or with the circuit as shown in Fig. 8.24, a sin-
gle-supply op-amp set to function as an integrator. In this circuit, a smaller
C or R , or a faster change in V , will speed up the change in V into the
INT 1 IN OUT
VGA’s gain adjust port. When the voltage at the inverting input is more posi-
tive than the voltage at the noninverting input, the output voltage will ramp
down; if the voltage at the inverting input is more negative than at the non-
inverting input, the output will ramp up. The speed of the ramp-up/down will
depend on the RC time constant of the RC components. A large-value resistor
(R ) of approximately 2.2 megohms or higher is usually placed in parallel
BLEED
with the integrating capacitor, since all op-amps have a small input bias cur-
rent that will quickly charge up this small-value capacitor: The resistor sim-
ply bleeds the current away. However, the resistor must be significantly higher
in value than the input resistor R , or gain can be lowered excessively. Figure
1
8.21 demonstrates the integrator in a standard AGC circuit.

Variable-gain amplifiers. The variable-gain amplifiers in the IF strip can be of
either the variable-attenuator type or the variable-bias type. Design of the appro-
priate attenuator and amplifier are explained in other sections of this book.

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