GS1522 Gennum Corporation, GS1522 Datasheet - Page 14

• Manufacturer Part Number: GS1522
• Description: Hd-linx (tm) THDTV Serial Digital Serializer
• Manufacturer: Gennum Corporation

10. JITTER DEMODULATION (DM)
The differential jitter demodulation (DM) signal is available
at the DM pin (86). This signal is the phase correction signal
of the PLL loop, which is amplified and buffered. If the input
jitter is modulated, the PLL tracks the jitter if it is within loop
bandwidth. To track the input jitter, the VCO has to be
adjusted by the phase detector via the charge pump. Thus,
the signal which controls the VCO contains the information
of the input jitter modulation. The jitter demodulation signal
is only valid if the input jitter is less than 0.5UIp-p. The DM
signal has a 10kΩ output impedance, which can be low
pass filtered with appropriate capacitors to eliminate high
frequency noise. DFT_V
GND to activate the DM signal.
The DM signal can be used as a diagnostic tool. Assume
there is an HDTV SDI source which contains excessive
noise during the horizontal blanking because of the
transient current flowing in the power supply. To discover
the source of the noise, probe around the source board with
a low frequency oscilloscope (Bandwidth < 20MHz) that is
triggered with an appropriately filtered DM signal. The true
cause of the modulation is synchronous and appears as a
stationary signal with respect to the DM signal.
Figure 22 shows an example of such a situation. An HDTV
SDI signal is modulated with a signal causing about 0.2UI
jitter (Channel 1). The GS1522 receives this signal and
locks to it. Figure 22 (Channel 2) shows the DM signal.
Notice the wave shape of the DM signal, which is
synchronous to the modulating signal. The DM signal can
also be used to compare the output jitter of the HDTV signal
source.
GENNUM CORPORATION
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
Fig. 21 Input Jitter Indicator (Typical at T
0.00
0.20
INPUT JITTER (UI)
EE
(88) should be connected to
0.40
0.60
A
= 25°C)
0.80
14
11. MUTE
The logic controls the mute block when the PLL_LOCK (15)
signal has a LOW logic state. When the mute signal is
asserted, the previous state of the output is latched.
12. CABLE DRIVER
The outputs of the GS1522 are complementary current
mode cable driver stages. The output swing and
impedance can be varied. Use Table 4 to select the R
resistor for the desired output voltage level. Linear
interpolation can be used to determine the specific value of
the resistor for a given output swing at the load impedance.
For linear interpolation, use either Figure 23 or the
information in Table 4. Find the admittance and then, by
inverting the admittance, a resistor value for the R
found.
The output can be used as dual 0.8V 75Ω cable drivers. It
can also be used as a differential transmission line driver. In
this case, the pull-up resistor should match the impedance
of the transmission line because the pull-up resistor acts as
the source impedance. To reduce the swing and save
power, use a higher value of R
HD-LINX™ products that can handle such low input swings.
NOTE: For reliability, the minimum R
less than 50Ω because of higher current density.
Fig. 22 Jitter Demodulation Signal
SET
SET
resistor. There are
resistor cannot be
SET
522 - 26 - 02
can be
SET