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ATTENUATION: The reduction
of RF power through a device, usually measured in decibels (dB), and given
mathematically by
Attenuation (dB) =
10 log (Pout/Pin)
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BANDPASS FILTER: A filter
that passes a specific band of frequencies and rejects frequencies both above
and below the passband.
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BANDREJECT FILTER: A filter
that rejects a specific band of frequencies and passes frequencies both above
and below the reject band. Also called a notch filter.
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BANDWIDTH: The difference
between the lower and upper edges of a passband, F1 and F2 respectively.
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BESSEL FUNCTION: A
mathematical function which yields a constant time delay. See linear phase
filter.
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BUTTERWORTH FUNCTION: A
mathematical function which yields a maximally flat amplitude response.
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CAUER FUNCTION: See Elliptic
function.
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CENTER FREQUENCY: Most
often, it is defined as the arithmetic mean of the lower and upper passband
edges, F1 and F2 respectively, and given mathematically by:
C.F.=(F1 + F2)/2
The geometric center frequency is defined as:
C.F.=SQR(F1 * F2)
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CHEBYCHEV FUNCTION: A
mathematical function that produces a curve with predetermined ripples and
yields the sharpest possible monotonic attenuation slope beyond the cutoff.
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CROSSOVER FREQUENCY: In
multiplexers, the common frequency at which the amplitude levels are equal for
two adjacent channels.
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CROSSOVER LOSS: The
insertion loss that occurs at the crossover frequency. It is generally only of
concern in contiguous band multiplexers.
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CUTTOFF FREQUENCY: The upper
passband edge of a lowpass filter or the lower passband edge of a highpass
filter, abbreviated Fc. It is usually specified in terms of a relative
attenuation level below the mid-band insertion loss of the filter. In
waveguide, the frequency at which energy will no longer propagate through the
guide.
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DECIBEL (dB): A unit used to
express the difference between two power levels, P1 and P2, or between two
voltage levels, E1 and E2, and given mathematically by:
dB = 10 LOG (P1/P2)
or dB = 20 LOG (E1/E2)
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DIPLEXER: The most basic
form of multiplexer, it contains two filters connected to a common junction. A
three port device, it is most commonly used to connect a transmitter and a
receiver to a common antenna. See multiplexer.
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DISSIPATION: The energy lost
in a filter due to finite element Q. It includes resistive, dielectric and
core losses. It does not include reflective losses due to VSWR.
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DISTORTION: The non-uniform,
undesirable, modification of a signal. These modifications can be related to
amplitude, phase, delay or pulse response.
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ELLIPTIC FUNCTION: A
mathematical function used to yield the sharpest possible amplitude response
for a given number of circuit elements. An elliptic function filter has an
equal ripple passband as well as a finite stopband ripple level. Also known as
a Cauer function.
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FLATNESS: The absolute
limits of the amplitude variation through the passband of a filter. It
includes ripple due to VSWR as well a monotonic roll-off due to finite element
Q.
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GAUSIAN FUNCTION: A
mathematical function which yields a time domain response that passes a step
function with zero overshoot and no ringing.
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GROUP DELAY: The amount of
time required for a signal to propagate though a device. It is the derivative
of the phase response of the device with respect to frequency, and is given
mathematically by:
Group Delay = (dŲ /
df) /360
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HIGHPASS FILTER: A filter
that passes frequencies above a certain cutoff and rejects all lower
frequencies.
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INPUT IMPEDANCE: The
impedance measured at the input port (S11) of a device when the output port of
the device is terminated with the proper load impedance.
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INSERTION LOSS: The
attenuation through a filter, usually measured in decibels. It is typically
measured at the center frequency of a bandpass filter or over a specified
passband in lowpass, highpass, and bandreject filters.
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INSERTION PHASE: The change
in the electrical length of a circuit as a result of the introduction of a
device into the circuit. The insertion phase caused by a device varies with
frequency and is a useful parameter when specifying phase matching between
filters.
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ISOLATION: In multiplexers,
the amount of attenuation measured at one port with a signal present at
another port and the common port terminated with its appropriate impedance.
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LINEAR PHASE FILTER: A
filter that exhibits a constant change in phase per unit of frequency. Best
approximated by a Bessel function filter. This type of filter typically has
very poor amplitude characteristics.
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LOAD IMPEDANCE: The input
impedance of the circuit that terminates the output of a device. The load and
the output impedance of the device must be matched in order to preserve the
passband VSWR, ripple, and insertion loss characteristics of the device.
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LOADED Q (QL): Ratio of the
center frequency to the 3 dB bandwidth of a bandpass filter, and given
mathematically by:
QL = C.F. / BW 3dB
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LOWPASS FILTER: A filter
that passes frequencies below a certain cutoff and rejects all higher
frequencies.
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MONOTONIC: The
characteristic of a filter response which exhibits steady, increasing
attenuation versus frequency, with no slope reversals.
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MULTIPLEXER: The combination
of two or more filters (channels) into a single package with one common input
(or output) and separate outputs (or inputs) for each of the individual
filters. An "n" channel multiplexer will have "n+1" ports.
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OUTPUT IMPEDANCE: The
impedance measured at the output port (S22) of a device when the input port of
the device is terminated with the proper source impedance.
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OVERSHOOT: The amount by
which the initial output response of a step or pulse function exceeds its
final, steady-state value.
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PASSBAND: The frequency
range a filter is required to pass with low attenuation, usually specified in
terms of relative attenuation or absolute insertion loss.
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PHASE LINEARITY: A measure
of the phase shift through a device relative to the phase shift through a
completely linear phase device. Typically, most filters do not exhibit good
phase linearity near the passband edges due to a non-constant group delay
response.
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RELATIVE ATTENUATION: The
reduction of RF power through a filter in the stopband, measured in decibels,
relative to the minimum insertion loss point in the filter's passband.
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RETURN LOSS: The ratio, in
dB, of the reflected power from a device to the incident power upon the
device. The greater the absolute value of return loss, the better the
impedance match between the device and its terminating impedances. (see VSWR)
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RINGING: The decaying
oscillation of the output signal of a device as a result of a transient signal
being applied to the input of the device.
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RIPPLE: The wavelike
variations in the amplitude response of a filter's passband due to VSWR
mismatch.
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SHAPE FACTOR: The ratio of
two relative bandwidths for bandpass or bandreject filters or the ratio of a
stopband frequency to a cutoff frequency for lowpass and highpass filters.
Usually, it is the comparison of a desired rejection bandwidth to that of the
filter's passband bandwidth.
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SOURCE IMPEDANCE: The output
impedance of the circuit that drives the input of a device. The source and the
input impedance of the device must be matched in order to preserve the
passband VSWR, ripple, and insertion loss characteristics of the device.
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STOPBAND: The frequency
range(s) outside of the filter's passband where high levels of attenuation are
required.
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UNLOADED Q (QU): The figure
of merit , or quality factor, of an inductor or a capacitor. It is the ratio
of the component's reactance at a give frequency to its equivalent series
resistance, and is given mathematically by:
QU = XL / R or QU =
XC / R
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VSWR (Voltage Standing Wave
Ratio): The ratio between the peaks and valleys of standing waves on a
transmission line, which directly relate to how well a device is matched to
the characteristic impedance of a system. It can be expressed in terms of
return loss (R.L.) by the equation:
R.L. =
20LOG[(VSWR-1)/(VSWR+1)]
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