Upstream CATV Amplifiers
be lost upon entering this mode. Current consumption
is reduced to 10µA (typ) in shutdown mode.
When used in conjunction with a 2:1 voltage-ratio trans-
former, the MAX3514/MAX3516 are internally resistively
matched to 75Ω. This internal resistor is across the
OUT+ and OUT- terminals.
To improve the match at the high end of the frequency
range (65MHz), a reactive match may be employed as
part of the ensuing diplex filter. A series inductor (typ
180nH) followed by a shunt capacitor (typ 33µF) can
be placed directly after the output transformer. This
match will also improve the gain flatness substantially.
As mentioned above, the matching components may be
incorporated into the diplex filter design. Optimize the
input impedance of the diplex filter to be 35 + j35 (typ) at
65MHz when using the specified output transformer.
The MAX3517 does not have an internal matching
resistor. This allows the device performance to be opti-
mized for various load impedances.
When 300Ω resistors are placed across the output ter-
minals of the device, performance identical to the
MAX3514 will result. If an impedance higher than 300Ω
is used, additional gain will result.
Note also that a 2:1 voltage-ratio output transformer is
When operating the device with arbitrary output resis-
tance and XFMR turns ratio, take care not to exceed the
allowable power dissipation (see the Absolute Maximum
To match the output of the MAX3514/MAX3516 to a 75Ω
load, a 2:1 voltage-ratio transformer is required. This
transformer must have adequate bandwidth to cover the
intended application. Note that most RF transformers
specify bandwidth with a 50Ω source on the primary and
a matching resistance on the secondary winding.
Operating in a 75Ω system will tend to shift the low-fre-
quency edge of the transformer bandwidth specification
up by a factor of 1.5 due to primary inductance. Keep
this in mind when specifying a transformer.
Bias to the output stage is provided through the center
tap on the transformer primary. This greatly diminishes
the on/off transients present at the output when switch-
ing between transmit and transmit-disable modes.
Commercially available transformers typically have
adequate balance between half-windings to achieve
substantial transient cancellation.
Finally, keep in mind that transformer core inductance
varies proportionally with temperature. If the application
requires low temperature extremes (less than 0°C),
adequate primary inductance must be present to sus-
tain low-frequency output capability as temperatures
drop. In general, this will not be a problem as modern
RF transformers have adequate bandwidth.
To achieve rated performance, the inputs of the
MAX3514/MAX3516/MAX3517 must be driven differen-
tially with an appropriate input level. The differential
input impedance is approximately 1.5kΩ. Most applica-
tions will require a differential low-pass filter preceding
the device. The filter design will dictate a terminating
impedance of a specified value. Place this load imped-
ance across the AC-coupled input pins (see the Typical
The MAX3514/MAX3517 have sufficient gain to pro-
duce an output level of +61dBmV (QPSK through a 2:1
transformer) when driven with a +34dBmV input signal.
The MAX3516 provides an additional 3dB of gain and
output level. When a lower input level is present, the
maximum output level will be reduced proportionally
and output linearity will increase. If an input level
greater than +34dBmV is used, the 3rd-order distortion
performance will degrade slightly.
If single-ended sources drive the MAX3514/MAX3516/
MAX3517, one of the input terminals must be capaci-
tively coupled to ground (IN+ or IN-). The value of this
capacitor must be large enough to look like a short cir-
cuit at the lowest frequency of interest. For operation
at 5MHz with a 75Ω source impedance, a value of
0.001µF will suffice.
A well-designed PC board is an essential part of an RF
circuit. For best performance, pay attention to power-
supply layout issues, as well the output circuit layout.
Output Circuit Layout
The differential implementation of the MAX3514/
MAX3516/MAX3517s’ output has the benefit of signifi-
cantly reducing even-order distortion, the most signifi-
cant of which is 2nd-harmonic distortion. The degree of
distortion cancellation depends on the amplitude and
phase balance of the overall circuit. It is important to
keep the trace lengths from the output pins equal.