Keithley 7038, Инструкция по эксплуатации

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Operation
3-3
3.5 Switching considerations
Signals passing through the Model 7038 Multiplexer Card
are subject to various effects that can influence their charac-
teristics. The following paragraphs discuss some of these ef-
fects and ways to minimize them.
3.5.1 Card characteristics
Insertion loss
Insertion loss indicates signal power lost while passing
through the card. This loss occurs in the various signal path
components through the card (connectors, PC board traces,
and relays). The amount of power lost will, of course, depend
on the particular insertion loss specification as well as the ap-
plied power. For example, with an insertion loss of 1dB, and
a 10W source signal applied to the card, about 2W will be
dissipated in the card, and approximately 8W will appear at
the load. Note that, as with most transmission lines, Model
7038 insertion loss values increase with increasing
frequency.
Crosstalk
Crosstalk figures indicate the amount of signal leakage be-
tween channels or switches on the card. With similar power
levels applied to the various channels, crosstalk will be of lit-
tle consequence. With widely different power levels, howev-
er, crosstalk may produce undesired results. For example,
assume that 10W is applied to channel 1, and 1mW is applied
to channel 2. Assuming a -60dB crosstalk figure, the unwant-
ed signal coupled from channel 1 to channel 2 will be only
20dB below the desired channel 2 signal.
VSWR
The term VSWR (Voltage Standing Wave Ratio) is defined as
the ratio of the maximum and minimum voltages along a
transmission path, and it indicates the degree of impedance
mismatch. In a perfectly matched system, the VSWR is 1:1,
while open and shorted paths have infinite VSWR values.
Since VSWR is related to both the return loss and reflection
coefficient, VSWR figures indicate the degree of signal loss
and reflection; the lower the VSWR figure, the less signal at-
tenuation that occurs along the transmission path.
Isolation resistance
The isolation resistance is simply the equivalent resistance
between two given connecting points on the card and is of
importance primarily for DC and low-frequency AC signals
switched by the card. The effects of this characteristic
depend on the particular isolation specification. Center-to-
shield isolation resistance, for example, may cause loading
problems with signals having high source resistance.
Channel-to-channel isolation resistance may result in
leakage currents generated in one channel caused by a
voltage source connected to another channel.
3.5.2 Cable characteristics
Characteristic impedance (Z
O
)
Characteristic impedance is the value of cable impedance
obtained by an RF measurement at either end. For example,
a cable with a 75
Ω
characteristic impedance is per unit
length. (The characteristic impedance of a cable depends on
the relative diameters of the inner and outer conductors, as
well as on the dielectric constant of the insulating material.)
Since the Model 7038 is designed to work with 75
Ω
cables,
you must use 75
Ω
cables to assure proper impedance match-
ing (RG-179 recommended). Mismatching anywhere along
the transmission path will increase VSWR and signal reflec-
tions, decrease return loss, and consequently result in signal
attenuation.
Cable attenuation
Cable attenuation is analogous to insertion loss of the card it-
self in that it defines the degree of attenuation of the signal as
it passes through the cable. Cable attenuation factors are gen-
erally given in dB per 100 ft. and generally increase with ris-
ing frequency. For optimum performance, choose a high-
quality 75
Ω
cable with a minimal insertion loss.
Cable termination
Proper cable termination is imperative to ensure maximum
signal transfer and to minimize VSWR and signal reflec-
tions. In the case of Model 7038 operation, both source and
load impedances should be as close to 75
Ω
as possible to as-
sure optimum matching and thus maximum power transfer.
For example, if a 75
Ω
cable is terminated with 150
Ω
, the re-
flection coefficient is 0.33, the VSWR increases to 2, and the
return loss is reduced to less than -10dB.
Cable connectors and adapters
Although miniature SMB connectors are optimized for best
performance, some small impedance mismatch at connect-
ing points is virtually inevitable. For that reason, it is consid-
ered good practice to minimize the number of connectors
used in a transmission path, especially at higher frequencies.
Where possible, avoid using adapters to convert between
various connector types.
L
C
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