Note: This work was presented by Dr. Snyder at the European Microwave Conference, October, 1998, The Netherlands . It can be read in its entirety as a series of articles on our website's "Technical Corners".
Given maximum values for available volume and unloaded Q, and with maximum
insertion loss, minimum stopband attenuation, maximum VSWR as specifications:
how does one best utilize the arsenal of available reactive elements to
satisfy the specifications? We will herein define an “optimum” filter
network as one most closely approxi-mating the specifications with the
mini-mum usage of volume and number of resonant elements. A “quasi-optimum”
filter is defined as one having some of the characteristics of the optimum
network. In the last few years, several develop-ments have
contributed to moving “quasi-optimum” a bit closer to “optimum”.
These include contributions in synthesis and implementation. Among
the most important are:
1. Synthesis of circuit canonic blocks which are then cascaded
(e.g. Cascade Triplet, Cascade Quad, Chained, etc.)
2. Synthesis of cross-coupled filters (both symmetric and
asymmetric types), and the implementation of the syntheses into convenient
software packages.
3. Combination of lumped, evanescent and distributed
elements in the same network, using the differing characteristics of each
as appropriate and necessary.
4. Electromagnetic simulation as applied to the extraction
of equivalent circuits to enable fast, repetitive and accurate simulation
of networks.
Part 2: CROSS-COUPLING
Implementation of generalized cross-coupled filters (see Fig. 7- a
symmetric cross-coupled lowpass prototype filter with inverters) requires
the use of both positive impedance (inductive) and negative impedance (capacitive)
couplings. The former are used for placing real-axis transmission
zeros for delay equalization while the latter are used for placing real-frequency
zeros, used for additional selectivity. Inductive and capacitive
irises are common in the literature and, in many cases, can be used for
the aforementioned positive and negative couplings. The iris couplings
usually take the form of a simple opening between two segments of the filter.
One example is a direct opening between the input portion of the filter
and the output. This opening in essence provides a shorter path between
the two terminating ports than is represented by the full traverse of all
the filter elements. Some energy “leaks” from the input directly
to the output. If the “leaking” energy is coupled in an inductive
manner, the coupling is such as to reduce the net effect on attenuation
of the remaining filter elements and simultaneously to reduce the total
group delay variation due to the interference generated at the output termination
between the leaking energy and the remainder which fully traverses the
filter. In other cases, the “leaky” (or cross) couplings will be placed
well within the filter, between any pair of resonant sections. In some
cases, however, the values of computed coupling are such as to make impractical
a simple opening between two parallel portions of the filter. The use of
resonated evanescent mode sections allows implementation of both positive
and negative couplings by employing the phase shift and impedance characteristics
of the bandpass element represented by a short resonated section of evanescent
mode waveguide.
Figure 8 illustrates the pi and tee models for ideal inverters, Figures 9 and 10 the use of odd and even mode half circuits, as used in the synthesis of symmetrical cross-coupled structures, in the state-of-the-art synthesis program FILPRO. Cross-coupling can also be used effectively in lumped element circuits. A good example of this is illustrated in Fig. 11, in which the result of applying asymmetric couplings is a quasi-elliptic response, comparing quite favorably to Chebychev for loss/slope, or for element count and ultimate attenuation floor to an elliptic design.
The above text is part of a series of articles on our website
where examples of the above are provided . Those examples show that
filter networks are approaching optimality, but that advances in technology
will still provide plenty of room for improved performance in the future.
In this latter regard, it is suggested that embedding active elements into
passive structures can further reduce size and enhance performance. In
the future, such active elements might include quantum-dot based resonators.
P/N 80501AD1
This is a high power GPS application, bandpass/bandstop diplexer, fully
militarized. The channel responses are quasi-elliptic, using crossover
coupling to implement real frequency transmission zeros for close-in rejection
while still achieving maximum passband width.
PERSONNEL NEWS
You may not be aware, but we have some famous young musicians in our midst! Jovo Galic's sons, Marko and David, both part-time employees with us, have made it big on the international scene! These young musicians, who play in several bands between them, performed publicly with the band "Chaos Theory" this past summer at the SKC Club in Belgrade, and are currently recording a CD of both original and other songs.
Marko, future professional studio guitarist/composer, is studying classical guitar at Kean College and does composing as well as plays guitar and keyboard for their bands. His music is a blend of classical and rock. He and his brother's band, "Carpathia", is similar to the '70s groups like Rush and Deep Purple. David calls their music "Progressive metal" with complicated time signatures. Though he sees his future in international business, David enjoys playing bass guitar and writing lyrics.
The boys are currently busy looking for a singer for their group.
They will be having a concert to promote their CD upon completion and we
will be sure to announce the details! We wish them every success
and congratulations to their proud parents.
Multiplexers - 1 MHz to 50 GHz
Contiguous
Non - Contiguous
Switched
Subsystems - 1 MHz to 20 GHz
Combinations, including:
Filters
Circulators
Amplifiers and Switches