LMH7324_0710 Datasheet, PDF (17/20 Pages) National Semiconductor (TI) – Quad 700 ps High Speed Comparator with RSPECL Outputs

normally specified at 20% and 80% of the signal amplitude
(60% difference). Assuming that the edges at 50% amplitude
are coming up and down like a sawtooth it is possible to cal-
culate the maximum toggle rate but this number is too opti-
mistic. In practice the edges are not linear while the pulse
shape is more or less a sinewave.
FIGURE 18. Bit Rates
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Need for Terminated Transmission Lines
During the 1980’s and 90’s, National fabricated the 100K ECL
logic family. The rise and fall time specifications were 0.75 ns,
which were considered very fast. If sufficient care has not
been given in designing the transmission lines and choosing
the correct terminations, then errors in digital circuits are in-
troduced. To be helpful to designers that use ECL with “old”
PCB-techniques, the 10K ECL family was introduced with rise
and fall time specifications of 2 ns. This is much slower and
easier to use. The RSPECL output signals of the LMH7324
have transition times that extend the fastest ECL family. A
careful PCB design is needed using RF techniques for trans-
mission and termination.
Transmission lines can be formed in several ways. The most
commonly used types are the coaxial cable and the twisted
pair telephony cable. (See Figure 19.)
FIGURE 19. Cable Types
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These cables have a characteristic impedance determined by
their geometric parameters. Widely used impedances for the
coaxial cable are 50Ω and 75Ω. Twisted pair cables have
impedances of about 120Ω to 150Ω.
Other types of transmission lines are the strip line and the
microstrip line. These last types are used on PCB boards.
They have the characteristic impedance dictated by the phys-
ical dimensions of a track placed over a metal ground plane.
(See Figure 20.)
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FIGURE 20. PCB Lines
Differential Microstrip Line
The transmission line which is ideally suited for complemen-
tary signals is the differential microstrip line. This is a double
microstrip line with a narrow space in between. This means
both lines have strong coupling and this determines the char-
acteristic impedance. The fact that they are routed above a
copper plane does not affect differential impedance, only CM-
capacitance is added. Each of the structures above has its
own geometric parameters, so for each structure there is a
different formula to calculate the right impedance. For calcu-
lations on these transmission lines visit the National website
or order RAPIDESIGNER. At the end of the transmission line
there must be a termination having the same impedance as
that of the transmission line itself. It does not matter what
impedance the line has, if the load has the same value no
reflections will occur. When designing a PCB board with
transmission lines on it, space becomes an important item
especially on high density boards. With a single microstrip
line, line width is fixed for a given impedance and for a specific
board material. Other line widths will result in different
impedances.
Advantages of Differential Microstrip Lines
Impedances of transmission lines are always dictated by their
geometric parameters. This is also true for differential mi-
crostrip lines. Using this type of transmission line, the distance
of the track determines the resulting impedance. So, if the
PCB manufacturer can produce reliable boards with low track
spacing the track width for a given impedance is also small.
The wider the spacing, the wider tracks are needed for a spe-
cific impedance. For example two tracks of 0.2 mm width and
0.1 mm spacing have the same impedance as two tracks of
0.8 mm width and 0.4 mm spacing. With high-end PCB pro-
cesses, it is possible to design very narrow differential mi-
crostrip transmission lines. It is desirable to use these to
create optimal connections to the receiving part or the termi-
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