CX25870/871
3.0 PC Board Considerations
Flicker-Free Video Encoder with Ultrascale Technology
3.6 Applications Information
3.6.6 Filtering Radio Frequency Modulator Connection
The CX25870/871 internal upsampling filter alleviates external filtering
requirements by moving significant sampling alias components above 19 MHz
and reducing the sinx/x aperture loss up to the filter’s passband cutoff of
5.75 MHz. While typical chrominance subcarrier decoders can handle the
CX25870/871 output signals without analog filtering, the higher frequency alias
products pose some EMI concerns and may create troublesome images when
introduced to a radio frequency (RF) modulator. When the video is presented to
an RF modulator, it should be free of energy in the region of the aural subcarrier
(4.5 MHz for NTSC, 5.5–6.5 MHz for PAL). Hence some additional frequency
traps may be necessary when the video signal contains fundamental or harmonic
energy (as from unfiltered character generators) in that region. Where better
frequency response flatness is required, some peaking in the analog filter is
appropriate to compensate for residual digital filter losses with sufficient margin
to tolerate 10 percent reactive components.
A three-pole elliptic filter (one inductor, three capacitors) with a 6.75 MHz
passband can provide at least 45 dB attenuation (including sinx/x loss) of
frequency components above 20 MHz and provide some flexibility for mild
peaking or special traps. An inductor value with a self-resonant frequency above
80 MHz is chosen so that its intrinsic capacitance contributes less than 10 percent
of the total effective circuit value. The inductor itself may induce 1 percent
(0.1 dB) loss. Any additional ferrites introduced for EMI control should have less
than 5 Ω impedance below 5 MHz to minimize additional losses. The capacitor to
ground at the CX25870/871 output pin is compensating for the parasitic
capacitance of the chip plus any protection diodes and lumped circuit traces
(about 22 pF + 5 pF/diode). Some filter peaking can be accomplished by splitting
the 75 Ω source impedance across the reactive PI filter network. However, this
will also introduce some chrominance-luminance delay distortion in the range of
10–20 ns for a maximum of 0.5 dB boost at the subcarrier frequency.
The filter network feeding an RF modulator may include the aforementioned
trap, which could take two forms depending on the depth of attenuation and type
of resonator device employed.
The trap circuitry can interact with the low-pass filter, compromising
frequency response flatness. A simple PNP buffer can preserve the benefits of an
oversampling encoder when simultaneous Composite Video Baseband Signals
(CVBS) are required for driving external cables. In addition, an active video
buffer, serves to isolate the RF modulator signal amplitude from anomalies in the
external termination. This buffer can be implemented with a transistor array or
video amplify IC which provides a gain of two (before series termination),
capable of driving 740 µA into the 75 Ω destination, and is biased within its
input/output compliance range. When simultaneous Y/C (s-video) outputs are not
required, a second CVBS signal can be created (with a 600 mV sync to tip offset)
by tying these pins together with a single termination resistor (typically 75 Ω) and
driving the low-pass filter circuit.
The RF modulator typically has a high input impedance (about 1 k Ω 30
percent) and loose tolerance. Consequently, the amplitude variation at the
modulator input will be greater, especially when the trap is properly terminated at
the modulator input for maximum effect. Some modulators, video or aural
fidelity, degrade dramatically when overdriven, so the value of the effective
termination (nominally 37.5 Ω) may need to be adjusted downward to maintain
sufficient linearity (or depth of modulation margin) in the RF signal.
100381B
Conexant
3-29
CONEXANT [ CONEXANT SYSTEMS, INC ]
