AD607 of current exactly balances the 4.5 µA discharge current. (It makes no difference what the actual value of VG is at that point, since the AGC filter is an integrator.)
pass filter does. This “input” is an INCREASED AMPLITUDE required at IFOP. The AGC loop thus does not level the output at IFOP.
Thus, at 20 mV/dB
Reasons for Using a Larger AGC Capacitor
4.5 µA × 93 ns 1 nF
1. In applications where gain modulation may be troublesome, raise the capacitor from 1 nF to 2.7 nF; the 80 dB slew time (at 20 mV/dB) is now close to 1 ms.
= 0.42 mV
This corresponds to 0.021 dB, and the ripple will modulate the gain by that amount over each cycle. The effect of such modulation on the signal is hard to quantify, but it roughly translates to a 2% amplitude modulation. Also, the gain ripple depends on the scale factor. For this example, at GREF = 1.23 V and a 16.4 mV/dB scale factor, the gain ripple increases to 0.025 dB. AGC Charge Time
When the gain is too high, the IF amplifier will be overdriven to produce a square wave output (roughly) of ± 560 mV. If perfectly square and time- and amplitude-symmetric, this would be sliced at the 300 mV level to generate a current of 76 µA/2, or 38 µA. After subtracting the 4.5 µA, we should have about 33 µA. In fact, the maximum ramp-up current is about 20 µA, because the waveform is not a crisp square wave (and as the loop approaches equilibrium it is more nearly sinusoidal). Thus, the ramp-up rate is 20/4.5 = 4.4 times faster than the discharge rate. In our example, a 1.6 V change will require about 1.5 ms using C = 1 nF. Applications Hints
Do not place a resistor from Pin 12 to Ground: The resistor converts the integrator—ideal for AGC—into a low-pass filter. An integrator needs no input to sustain a given output; a lowVPOS
3. In AM applications, the AGC loop must not track the modulation envelope. The objective should be that the gain should not vary by more than the amount required to introduce, say, 1% THD distortion at the lowest modulation frequency, say, 300 Hz. Note that in AM applications it is the modulation bandwidth that determines the required AGC filter capacitor, not the IF. 4. In some applications, even slower AGC may be desired than that required to prevent modulation tracking. AD607 EVALUATION BOARD
The AD607 evaluation board (Figures 46 and 47) consists of an AD607, ground plane, I/O connectors, and a 10.7 MHz bandpass filter. The RF and LO ports are terminated in 50 Ω to provide a broadband match to external signal generators to allow a choice of RF and LO input frequencies. The IF filter is at 10.7 MHz and has 330 Ω input and output terminations; the board is laid out to allow the user to substitute other filters for other IFs.
JUMPER R10 4.99kΩ
C11 10nF FDIN
2. As the IF is lowered, the capacitor must be increased accordingly if gain ripple is to be avoided. Thus, to achieve the same ripple at 455 kHz requires the 1 nF capacitor to be increased to 0.022 µF.
C1 0.1µF FDIN COM1
C13 0 R7 51.1Ω C14 0
LOIP C16 1nF
I C2 0.1µF
JUMPER R4 0
AD607 EVALUATION BOARD (AS RECEIVED) VPOS
VPOS R15 50kΩ
FDIN R14 51.1Ω
MOD FOR LARGE MAGNITUDE AC COUPLED INPUT
MOD FOR DC COUPLED INPUT
Figure 46. Evaluation Board