
ATTENUATION: The reduction
of RF power through a device, usually measured in decibels (dB), and given
mathematically by
Attenuation (dB) =
10 log (Pout/Pin)

BANDPASS FILTER: A filter
that passes a specific band of frequencies and rejects frequencies both above
and below the passband.

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.

BANDWIDTH: The difference
between the lower and upper edges of a passband, F1 and F2 respectively.

BESSEL FUNCTION: A
mathematical function which yields a constant time delay. See linear phase
filter.

BUTTERWORTH FUNCTION: A
mathematical function which yields a maximally flat amplitude response.

CAUER FUNCTION: See Elliptic
function.

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)

CHEBYCHEV FUNCTION: A
mathematical function that produces a curve with predetermined ripples and
yields the sharpest possible monotonic attenuation slope beyond the cutoff.

CROSSOVER FREQUENCY: In
multiplexers, the common frequency at which the amplitude levels are equal for
two adjacent channels.

CROSSOVER LOSS: The
insertion loss that occurs at the crossover frequency. It is generally only of
concern in contiguous band multiplexers.

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 midband insertion loss of the filter. In
waveguide, the frequency at which energy will no longer propagate through the
guide.

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)

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.

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.

DISTORTION: The nonuniform,
undesirable, modification of a signal. These modifications can be related to
amplitude, phase, delay or pulse response.

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.

FLATNESS: The absolute
limits of the amplitude variation through the passband of a filter. It
includes ripple due to VSWR as well a monotonic rolloff due to finite element
Q.

GAUSIAN FUNCTION: A
mathematical function which yields a time domain response that passes a step
function with zero overshoot and no ringing.

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

HIGHPASS FILTER: A filter
that passes frequencies above a certain cutoff and rejects all lower
frequencies.

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.

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.

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.

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.

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.

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.

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

LOWPASS FILTER: A filter
that passes frequencies below a certain cutoff and rejects all higher
frequencies.

MONOTONIC: The
characteristic of a filter response which exhibits steady, increasing
attenuation versus frequency, with no slope reversals.

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.

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.

OVERSHOOT: The amount by
which the initial output response of a step or pulse function exceeds its
final, steadystate value.

PASSBAND: The frequency
range a filter is required to pass with low attenuation, usually specified in
terms of relative attenuation or absolute insertion loss.

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 nonconstant group delay
response.

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.

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)

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.

RIPPLE: The wavelike
variations in the amplitude response of a filter's passband due to VSWR
mismatch.

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.

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.

STOPBAND: The frequency
range(s) outside of the filter's passband where high levels of attenuation are
required.

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

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[(VSWR1)/(VSWR+1)]