GS1522 Gennum Corporation, GS1522 Datasheet - Page 16

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

12. RETURN LOSS
In an application where the GS1522 directly drives a cable,
it is possible to achieve an output return loss (ORL) of about
17dB to 1.485GHz. PCB layout is very important. Use the
EB1522 as a reference layout (see Figures 28 to 31). When
designing high frequency circuits, use very small ‘0608’
surface mount components with short distances between
the components. To reduce PCB parasitic capacitance,
provide openings in the ground plane. For best matching, a
12nH inductor in parallel with a 75Ω resistor and a 1.5pF
capacitor matches the 75Ω cable impedance. The inductor
and resistor cancel the parasitic capacitance while the
capacitor cancels the inductive effect of the bond wire. To
verify the performance of any layout, measure the return
loss by shorting the inductor with a piece of wire without the
GS1522 installed.
Unless the artwork is an exact copy of the recommended
Fig. 26 Compensated Output Return Loss at Logic HIGH
layout, verify every design for output return loss. Tweak the
layout until a return loss of 25dB is attained while the
GS1522 is not mounted and L1 is shorted. When the device
is mounted, use different inductors to match the parasitic
capacitance of the IC. When the correct inductor is used,
maximum return loss of 5MHz to 800MHz is achievable. To
increase the return loss 800MHz to 1.5GHz, use a shunt
capacitor of 0.5pF to 1.5pF. The larger inductor causes
slower rise/fall time. The larger shunt capacitor causes a
kink in the output waveform. Therefore, the waveform must
be verified to meet SMPTE 292M specifications.
There are two levels at the output depending upon the
output
state
(logic
HIGH
or
LOW).
When
taking
measurements, latch the outputs in both states. An
interpolation is necessary because the actual output node
voltages are different when a stream of data is passing as
compared to the static situation created when measuring
return loss. See the GS1508 Preliminary Data Sheet for
more information.
Fig. 27 Compensated Output Return Loss at Logic LOW
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