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Filter technology delivers new levels of performance in surge protection and RF performance
By Patrick Heeren, Marketing Services Manager, PolyPhaser Corp.
PolyPhaser, a Smiths Interconnect company, developed the first lightning protectors based on filter technology engineered for RF and microwave applications more than twenty years ago. From this foundation in RF engineering technology, the company continues to develop products and services that combine the highest useful levels of surge protection with excellent RF performance and system level cost effectiveness. PolyPhaser technology is an appropriate alternative to quarter wave stubs and gas discharge tubes, offering the lowest achievable levels of let-through voltage and energy throughput in a wide band product (for example 0.8 to 2.5GHz) at 1.1:1 VSWR and 0.1 dB insertion loss. These products provide both primary and secondary lightning protection, Bias T, signal conditioning, and weatherproof installation—usually in one device.
DC-Block applications: the alternative to quarter wave stubs Two different protection approaches can be utilised for applications where dc power is not supplied to the radio equipment. These are: ‘dc-blocked’ (PolyPhaser SX™Series) or ‘dc-short’ (quarter wave stub). The SX™series has many advantages over quarter wave stubs relative to RF performance, surge attenuation characteristics, and surge suppression capabilities.
Quarter wave stub surge and RF performance Quarter wave stub devices, high Q narrow band filter coaxial protectors, are based on well known band-pass/band-reject principals. By using a coaxial T fitting, and calculating the length of a quarter wave coaxial section from the horizontal centre conductor to the ground base of the T, a band pass filter can be formed at a given frequency. Since most of the energy in a lightning strike is from dc to 1MHz, it would fall on the lower frequency reject side of the band pass filter and be conducted to ground.
However if the equipment input is also dc shorted, the quarter wave stub will allow significant divided dc and low frequency energy to flow toward the equipment input. The quarter wave stub will continue dividing energy between the protector’s ground and the equipment input for the entire duration of the strike or series of strikes. Quarter wave devices are tuned, through their design, to relatively narrow operating frequency bands (approximately +/- 10% of the centre operating frequency). The surge voltage let through and the energy let through of a quarter wave stub, as tested to the 6kV/3kA wave shape, are 6.875V and 7.36µJ.
There are quarter wave based designs that claim to operate over an extremely wide frequency band. These designs employ a gas discharge tube in the T fitting and will allow for a dc component to pass to the Tower Top Electronics. Such a design relies purely on the gas discharge tube to crowbar the voltage to ground allowing voltages in excess of a 700Vpk to reach the critical equipment.
SX™Series surge and RF performance PolyPhaser’s patented SX™series designs are characterised by their broadband performance (+/- 50% of centre operating frequency) over a wide spectrum of frequencies. The operating bandwidth of a single device covers frequencies from 700 to 2500MHz. These low Q cavity-based designs are tuned LC networks and operate on band pass / band reject principals.
In contrast to the quarter wave stub’s design concept which capitalises on a dc short principal, the SX™series employs a patented dc blocking principal. The dielectric voltage breakdown on the centre pin of a dc blocking mechanism is in excess of 30kV for a surge condition. Utilising the dc blocking capacitive component in the centre pin and inductive current discharge circuit to shield/ground ensures not only broadband RF performance but also very good surge suppression characteristics.
The attenuation of lightning frequencies by the SX™series reaches levels of –98dB or better. The surge voltage let through and the energy let through of an SX™series device, again as tested to the 6kV/3kA wave shape, are 195.313mV and 6.26pJ. This means that the dc blocking SX™series lets 1,000,000 times less energy through than a quarter wave stub. Further, SX™devices do not allow any dc components to be passed to the radio equipment.
DC Pass applications: the alternative to gas discharge tubes In the case of applications that need dc power to be supplied to the radio equipment – such as Tower Top Electronics and global positioning systems - there is a requirement for dc continuity through the protection device. This prohibits either dc block and dc short circuitry from being used in the protection design. Two different technological approaches, the gas discharge tube and Polyphaser’s patented coaxial protectors, which are appropriate in such applications, are discussed below.
Straight gas discharge tube A gas discharge tube coaxial type protector, without dc blocking, has dc continuity from surge side connector centre pin to equipment side connector centre pin. The fast rise time lightning pulse can achieve very high magnitude voltages across the tube before the gas can ionise and become conductive. The gas discharge tube is a voltage activated device. The faster the voltage waveform rise time (dV/dt), the higher the protection voltage and energy let through. Since there is no dc blocking mechanism, this high voltage pulse is applied directly to the equipment input before the gas tube becomes active.
If the radio equipment input is via a ferrite circulator/isolator, the incoming pulse is converted to current in the ferrite’s resistive load, creating a magnetic field that can re-align the critically adjusted field in the circulator, change the magnet’s flux density, and damage the resistive load. If the incoming voltage pulse appears across a coupling loop (as in most filters and combiners), it sees a low resistance short and is almost entirely converted to current.
A dc shorted equipment input loop directly shorts the gas tube. The gas tube may never see enough voltage to ‘turn-on’ resulting in damage to the sensitive equipment. Gas discharge devices are rated for their breakdown voltage in dc values, - 90Vdc, 150Vdc, 250Vdc, etc. Lightning generated surges have rise times ranging from nanoseconds to microseconds. Therefore the surge performance of a gas tube based protection device should be evaluated based on this dynamic condition, not on the dc rating of a component employed in its design.
Tower Top Electronics, including semiconductor-based amplifiers, power supplies, and diplexers, require different methods and technology of protection than that of a gas discharge tube. This is because the solid-state components employed in Tower Top Electronics, will respond to the incoming fast dV/dt voltage surge and not allow the gas tube to “turn-on”. This will unfortunately result in serious damage.
GX™Series RF pass and DC pass A new series of PolyPhaser’s patented coaxial protectors uses a dc blocking technique on the RF path while isolating and protecting/conditioning the dc circuitry. The dc component supplied through the protector to Tower Top Electronics is de-coupled from the RF path, passes through a dc protection circuit and is re-coupled to the coaxial cable center conductor. This innovative design combines RF and dc protection functions with a Bias-T in one unit.
GX™devices are broadband units covering multiple operating frequencies in one design. The dc blocking mechanism ensures very high levels of surge suppression, and the dc protection modules cover a wide range of operating voltages of both positive and negative polarities. GX™series devices are fully weather-proofed, enabling them to be used in tower mounted amplifiers, tower mounted electronics and global positioning systems.
Integrated approach PolyPhaser currently focuses on six product platforms:
1) RF pass dc blocked coax protectors – the SX™family which offers >1000 times lower throughput energy than a quarter wave stub;
2) RF pass, dc pass platform – GX™family, which offers up to 100 times less let-through voltage than simple gas tubes.
The other four ingredients are:
3) RF pass, dc pass IF platform – the MDS™family, which covers intermediate frequency (dc to 2.5GHz) and telemetry at 75 Ohms;
4) Data and dc power platform – IX family;
5) OEM focused integrated system solutions – IS² service;
6) Grounding and entry panel solutions – which forms the foundation for surge protection.
The company’s new data and dc platform focuses on the integration of Ethernet and IP interfaces into Tower Top Electronics, as well as new backhaul and access point architectures.
For further information contact: Patrick Heeren on phone: +1 775 783 0223, email: pheeren@polyphaser.com, web: http://www.polyphaser.com/
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