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Filters & Multiplexers

Lumped Element

Miniature   Connectorized or Surface Mount

Lumped Element technology is commonly used in applications where small size is required, especially at lower frequencies where transmission line devices might be excessively large.

Both narrowband and broadband designs are available in highpass, lowpass, bandpass, bandstop and multiplexer form. Various filter prototypes are used combining capacitive or inductive coupling to produce asymmetric or symmetric responses.

Depending upon customer requirements, prototypes are chosen which provide transmission zeros for improved selectivity filters or linear phase characteristics for filters requiring flat group delay performance.

The filters are realized with discrete capacitors and inductors. High Q porcelain chip-capacitors and air-spaced inductive coils are soldered to an RF substrate, the top surface of which is etched to produce interconnections, bonding pads and additional reactive elements. Small trimming capacitors are provided to allow for final alignment in production and the substrate is bonded into a metal housing. The overall unloaded Q of the structure is in the range of 150 to 400 and depends on absolute frequency and resonator size relative to the frequency of operation.

Sage Labs pays particular attention to the selection of components and the design of the housing so that parasitic resonances and waveguide modes are suppressed to ensure broad spurious free stopbands.

At lower operating frequencies, the reactances required increase with a corresponding increase in physical size. This effect is less noticeable for the fixed value capacitors, as high dielectric constant material is used in their manufacture, but the size of an air-spaced inductor increases as the frequency decreases, for a given Q value. To achieve small size at lower frequencies miniature toroidal inductors are used with only a slight degradation in overall electrical performance.

For high power lumped element filters, commonly used in lower frequency communications, use is made of high voltage capacitors and larger air cored inductors.

Some devices employ Sage Labs unique proprietary techniques to compensate for changes in filter performance over temperature. Stabilities of better than 5 ppm/°C are readily achieved over the temperature range -55°C to +125°C. Filters are manufactured in ultra small packages and are available with most coaxial connector types. Devices are also available with RF pins or feed throughs for surface or PCB mount.

TEM Machined Structures

High Q, Low Loss   High Power

TEM Machined Filters are used in applications where closely spaced signals need to be separated. A high Q structure is very important for narrowband applications, particularly where high powers are involved. Low passband loss, VSWR and high attenuation are characteristics of Sage Labs machined filters. Sage Labs has also extensively investigated the intermodulation performance of its high power filters and developed special designs to meet these requirements.

Sage Labs' machined TEM filters are typically combline or interdigital structures, comprising short circuit stub resonators, which are inductively coupled. Depending upon the frequency and bandwidth, either circular or rectangular resonators are used. The resonators are capacitively loaded and are typically 45° long at their resonant frequency. This leads to a very broad upper stopband with high levels of attenuation.

Combline filters can be designed to have bandwidths of 1% to 50% of the center frequency. Center frequencies in the range of 100MHz to 20GHz are possible with stopbands extending to 5 times f_o. Wider stopbands are achievable with appropriate resonator loading.

By introducing inductive or capacitive cross-coupling between the appropriate non-adjacent nodes it is possible to produce attenuation poles in the rejection skirts or equalize the group delay response.

Sage Labs' combline filters are generally machined from aluminum to achieve low weight and are generally silver plated to give the lowest possible passband loss.

Multiplexers can be formed by coupling bandpass filters together at common transformer junctions. Both contiguous and non-contiguous types are available. Diplexers, for example, are often used in applications such as Tx/Rx communications systems, where a common antenna is shared.

Doubly Multiplexed Filters.
By recombining multiplexed bandpass filters, devices with multiple passbands may be realized. This technique is very useful to achieve broadband bandstop characteristics.

Interdigital.
In some applications requiring wider bandwidths and where flat group delay is important, Sage uses interdigital structures. The electrical and mechanical characteristics are similar to combline but the resonators are longer and are alternately inverted. The longer resonators result in a lower upper stopband frequency.

Bandstop.
High Q bandstop filters with broad passbands can be realized from short circuited stubs coupled to a manifold and spaced a quarter wavelength apart.

Suspended Substrate Stripline

Printed Technology Low Volume and Weight High Q, Low Loss

Suspended Substrate Stripline (SSS) is a printed circuit technology that can be used for both broadband and narrowband filters in highpass, lowpass, bandpass, bandstop and multiplexer form. It is also used in many of Sage Labs' subsystem products, such as Switched Multiplexers, Frequency Activity Detectors and Frequency Synthesizers.

The wide range of realizable impedance values makes this medium particularly suitable for highpass and lowpass filters that can be cascaded together to form broadband Bandpass Filters and Multiplexers. It can also be used for printed combline and halfwave parallel coupled filters in narrowband applications. In this case proprietary temperature compensation techniques are used to ensure frequency stability. Suspended Substrate Stripline devices offer low parts count, small volume, light weight and are ideally suited to volume production.

Generalized Chebychev filter prototype designs result in highly selective band edges with low passband loss and high stopband attenuation. As Suspended Substrate is a printed technology it exhibits very repeatable performance and devices can be made with very tight amplitude and phase tracking.

The versatility of the printed circuit technology enables designs to be implemented in very complex mechanical configurations. In addition, this medium allows for the easy integration of other components and devices onto the circuit board. Its low profile and small footprint results in very compact devices with low volume.

The Suspended Substrate technique yields a higher Q than similar stripline techniques. This is because most of the electric field is supported in air and results in lower loss filters with sharper band edges and temperature characteristics virtually independent of dielectric material variations. Sage Labs uses unique, proprietary techniques for temperature compensation and can offer temperature stabilities of ±0.1% over -55°C to +125°C.

A Suspended Substrate filter consists of a circuit photolithographically etched on a thin copper clad PTFE based substrate which is suspended in air between two silver plated ground planes. The substrate is secured between the top and bottom halves of the housing with internal walls included to add further support to the circuit. Plated through holes in the area of the internal support walls provide an effective means of grounding the top and bottom halves of the housing. These plated through holes also provide for channel to channel isolation and suppress RF leakage.

The filter housing may be fitted with a variety of RF connectors or may be provided with 50ohm feedthroughs for direct connection to MICs. The complete assembly contains the minimum of machined parts and is very rugged in its construction making it highly reliable and well suited to the harsh extremes encountered in military environments.