Gain vs. VAGC Voltage
The overall transimpedance at 47MHz is related to the
voltage at VAGC by the relation:
The gain at 870MHz is 4dB greater (70dBΩ at V
VAGC
=
0.175V) because of the uptilt, although the amount of
uptilt can be modified as described above.
Between 0 and 0.175V the gain is constant, and above
1.5V it falls off relatively quickly. Operation above
V
VAGC
= 1.6V should be avoided to obtain adequate
linearity performance.
The high-impedance VAGC input should be driven by a
source (op amp, DAC, etc.) capable of sinking up to
200µA.
Feed-Forward AGC
With a feed-forward circuit like that of the EV kit, the
MAX3660 provides a constant (±1dB) output of
19dBmV/channel at 47MHz and 23dBmV/channel at
870MHz, for optical input levels ranging between
-8dBm and +2dBm at OMI = 4.2%.
Feedback AGC
The VAGC voltage can also be controlled from a power
detector, such as the MAX2014 or MAX9933, for feed-
back AGC.
It is important to note that the Gain (ZT) vs. VAGC char-
acteristic includes hysteresis at the two points where
the input stage switches gain (350mV and 700mV),
which can cause problems such as limit-cycle oscilla-
tion with continuous analog feedback implementations.
The feedback circuit should be designed to avoid oscil-
lation or dithering.
Uptilt
The integrated uptilt results in equal input levels pro-
ducing an output voltage that is 4dB greater at 870MHz
compared to 47MHz, eliminating the loss normally
associated with an external passive tilt network. The
amount of uptilt can be varied by adjusting the triplexer
lead length, or by adding small inductors in series with
the anode and cathode, to compensate for photodi-
odes/triplexers that differ significantly from Figure 1.
Equivalent Input Noise
The typical equivalent input referred noise (EIN) of the
MAX3660 with a photodiode connected at the input is
5.5pA/Hz
1/2
, yielding 48dB or better CNR under normal
BPON/GPON conditions. Without a photodiode con-
nected, the typical EIN is 4.5pA/Hz
1/2
.
RF Output
The RF output should be connected to the MAX3660
using AC-coupling capacitors and a balun transformer
to achieve the desired noise and linearity performance.
Without the capacitors, shorting OUT+ and OUT-
together, or shorting OUT+ or OUT- to ground, can
draw sufficient current to damage the output stage.
EV Kit Circuit
The MAX3660 EV kit circuit shown in Figure 4 was used
to collect the data in the
Typical Operating
Characteristics
figures. When connected to a photodi-
ode-equipped triplexer, the EV kit circuit provides a
complete receiver, including photodiode bias, feed-for-
ward AGC, and output transformer.
Jumper JU1 controls the MUTE input, JU3 sets the
amount of hysteresis, and JU2 controls the input of the
op amp driving the VAGC input. Install JU2 to enable
feed-forward VAGC, or alternatively, the gain can be
controlled by TP6 with JU2 removed.
0...175 1 4VV V
