image; in this example the ghost’s time
delay is 2.63 µs.
This basic concept of reflections ap-plies to both analog TV channels and
digitally modulated signals.
Let’s look at another example, this
one with two water-damaged taps separated by 100 feet of coax. Assume that
difference between the two is + 30 dBmV
– + 14 dBmV = 16 dB. We can also say
that the reflection is – 16 dBc relative to
the incident signal. The – 16 dBc reflection will have a time delay relative to the
incident signal, defined by the roundtrip
propagation time between the 23 dB and
20 dB taps. The cable span between the
“Reflection time delays on the
order of less than a symbol
period to several symbol a
periods are of interest.“
the span of coax has 1 dB of loss, and
the water damage has caused both taps’
return loss to degrade from an out-of-the box 15 to 18 dB to 7 dB. For the
sake of discussion, the first tap is a 23
dB 4-port, and the second tap is a 20 dB
4-port. Further assume that an incident
signal whose amplitude is + 31 dBmV
leaves the output connector of the 23
dB tap.
When that incident signal reaches the
20 dB tap’s input connector, its amplitude will be + 31 dBmV – 1 dB cable loss
= + 30 dBmV. Most of the incident signal will continue downstream beyond
the 20 dB tap, but some of that signal
will be reflected by the 7 dB return
loss impedance mismatch of the water-damaged 20 dB tap. The amplitude of
the reflection will be + 30 dBmV – 7 dB
return loss = + 23 dBmV. That first reflection will travel back towards the 23
dB tap, where its amplitude will be + 23
dBmV – 1 dB = + 22 dBmV. Here most
of the reflection will continue upstream
beyond the 23 dB tap, but the 7 dB
return loss of the also-water-damaged
23 dB tap will result in the first reflection being re-reflected at an amplitude
of + 22 dBmV – 7 dB return loss = + 15
dBmV. This second reflection will travel
toward the 20 dB tap, reaching it at + 14
dBmV. And so on.
What will we see at the input connector of the 20 dB tap?
The signals of primary concern are
the + 30 dBmV incident signal and the
+ 14 dBmV reflection. The amplitude
two taps is the previously mentioned
100 feet. Since it takes RF signals 1.17
nanosecond (ns) to travel through 1 foot
of coax that has 87 percent velocity of
propagation, the reflection’s roundtrip
time is (100 feet + 100 feet) x 1.17 ns
= 234 ns. This tells us that the – 16 dBc
reflection’s time delay relative to the + 30
dBmV incident signal at the input to the
20 dB tap is 234 ns.
For what it’s worth, a – 16 dBc 234 ns
reflection is sufficient to be a problem for
256-QAM (quadrature amplitude modulation) operation.
So how does the term micro-reflection
fit in all of this? Earlier I said that reflection time delays on the order of less
than a symbol period to several symbol periods are of interest with digitally
modulated signals. A downstream DOCSIS 256-QAM signal has a symbol rate
of 5.360537 megasymbols per second
(Msym/sec), so the period of each symbol is 1/5,360,537 = 0.000000186548
second or about 187 ns.
In this case, reflections with a time
delay of somewhat less than 187 ns to
a few microseconds can be described as
micro-reflections.
Ron Hranac is a technical leader, broadband
network engineering, for Cisco Systems and
senior technology editor for Communications
Technology. Reach him at rhanac@aol.com.
