Understand the basics of coaxial adapters to better use these very useful components

[Introduction]Users of Electronic instruments and equipment are familiar with coaxial connections as long as they involve high-frequency electrical signal transmission or reception, because they are used in too many places. So much so that this type of connection is taken for granted—until it is necessary to connect multiple instruments together or extend coaxial cables. At this point, designers or other device users may use a variety of adapters; but before doing so, they need to fully understand the usefulness and characteristics of each adapter they may use.

There is a reason why there are so many types of adapters. “Tee” adapters connect one source to multiple instruments, while “sleeve” adapters are used to extend coaxial cable connections. There are also DC blocks, bias tees, impedance pads, surge protectors, and terminations – all of which are commonly used, but sometimes people don’t fully understand them. The correct use of these adapters requires some basic knowledge of transmission lines, which is also required when choosing an adapter.

This article first gives a brief introduction to the transmission line. It then introduces the various types of coaxial adapters, describes how they work, and shows how to best use them. The example devices used were from Amphenol RF, Times Microwave Systems, a division of Amphenol, and Crystek Corporation.

What is a transmission line?

A transmission line is a line that connects a signal source to a load using coaxial cables, flat wires, microstrip lines, or other forms. The characteristic impedance of a transmission line is determined by the physical size of the conductors, the spacing, and the dielectric material used to isolate the conductors. The most common characteristic impedance for coaxial cable is 50 ohms (Ω) for general RF installations or 75 Ω for video applications.

In order to ensure that the power supply transmits power to the load with maximum efficiency, the impedance of the signal source, the characteristic impedance of the transmission line and the impedance of the load should be matched. If the impedances are not matched, then some energy will be reflected back from the mismatched junction. For example, if the load impedance is different from the signal source and transmission line impedance, energy will reflect from the load back to the signal source (Figure 1).

Understand the basics of coaxial adapters to better use these very useful components

Incident and reflected waves are superimposed along the transmission path to form standing waves whose amplitudes vary periodically over the physical length of the path. Standing waves can cause measurement errors and can cause component damage. Source, transmission line, and load impedance matching prevents standing waves, thus helping to ensure the most efficient transmission from source to load.

Because of impedance matching requirements, it is important to use the right adapter; however, designers quickly discover that there are many types of adapters that often do more than make a basic connection.

Tee adapter

Consider a basic instrumentation system consisting of a single signal source, oscilloscope, and spectrum analyzer (Figure 2).

Understand the basics of coaxial adapters to better use these very useful components

The source has an output impedance of 50 Ω and is designed to input a 50 Ω load. If you connect an oscilloscope and a spectrum analyzer with a tee adapter, both with their input terminals set to 50 Ω, they will be loaded at 25 Ω from the source, thus attenuating their output and creating a standing wave on the cable. The secret to solving this problem is to set the instrument in the middle of the coaxial connection to the high impedance input terminal and the instrument at the far end of the coaxial connection to the 50 Ω input terminal, as shown above. At this point from the source point of view, its load will be 50 Ω and everything will run smoothly.

The Amphenol RF 112461 (Figure 3) is a BNC tee with one BNC plug, two BNC jacks, and a 4 gigahertz (GHz) bandwidth. In the configuration shown in the example above, it can be used to complete the instrument connection with a bandwidth below 4 GHz.

Understand the basics of coaxial adapters to better use these very useful components

The type of tee selected depends on the connector used on the instrument and depends on the bandwidth of the individual instrument. In general, coaxial adapters like tees cannot be used for bandwidths beyond 40 GHz because signal loss in the adapter can be an issue at these frequencies. Below is a list of common instrumentation coaxial connectors for which adapters are generally available and their salient properties (Table 1).

Understand the basics of coaxial adapters to better use these very useful components

Table 1: Common coaxial connector families for which adapters can be used. Above 40GHz, the adapter is too lossy to use. (Table source: Digi-Key Electronics)

Connector Series Adapter

Since there are multiple connector types, it needs to be able to convert from one type to another. Consider that an SMA cable must be mated from the input BNC connector of the oscilloscope or spectrum analyzer. For this purpose, the Amphenol RF 242103 provides a BNC plug to connect to the instrument and an SMA jack to install an SMA cable (Figure 4).

Understand the basics of coaxial adapters to better use these very useful components

Equipment users should keep in mind that whenever an adapter is used, the interconnect bandwidth is reduced to the lower of the two connector families. Take the BNC to SMA adapter as an example, the bandwidth is 4 GHz, depending on the connector type is BNC.

There are also adapters that provide impedance conversion from 50 Ω to 75 Ω and vice versa.

Sleeve and Bulkhead Adapters

Extending cables or routing cables through panels requires a straight-through (sleeve) or bulkhead adapter. These adapters are available for the connector types shown in Table 1. For example, the Amphenol RF 132170 Bulkhead Adapter has two SMA jacks, and cables using SMA plugs can be connected to either side of a bulkhead or panel (Figure 5).

Understand the basics of coaxial adapters to better use these very useful components

Sleeve connectors can be configured as jack-to-jack, or plug-to-plug, and less often, plug-to-jack.

terminal

50 Ω terminals are required when connecting multiple high impedance input instruments in series from a 50 Ω signal source (Figure 6).

Understand the basics of coaxial adapters to better use these very useful components

The Amphenol RF 202120 50 Ω Terminator is an example of a coaxial contact configured as a BNC jack (Figure 7).

Understand the basics of coaxial adapters to better use these very useful components

BNC jacks accept coaxial cables directly. In addition, there are terminals in the form of BNC plugs, which can be mated with BNC jacks. These terminals are useful when the instrument needs to be terminated directly on the instrument front panel. While most oscilloscopes offer both high impedance and 50 Ω inputs, the 50 Ω range input has a voltage limit, typically 5 volts. The oscilloscope’s 50 Ω input also has a 0.5 watt power limit. The 202120 is rated at 1 watt and can handle more than 7 volts.

Terminals are also available for other impedances. For example, 75 Ω terminators are commonly used in TV and video applications. When performing network analyzer calibrations, zero Ω or shorting terminals are used.

DC block

A DC blocker is a coaxial adapter that blocks DC signals and allows RF signals to pass through. It uses capacitors to block direct current and is used to protect sensitive RF components from direct current. There are three types of DC blockers:

1. Internal DC blocker, which uses a capacitor in series with the inner or center conductor of a coaxial cable

2. External DC blocker, which uses a capacitor in series with the shield conductor of the coaxial cable

3. Inner/outer DC blocker, capacitors are connected in series with both inner and outer conductors at the same time

All types of DC blockers are specified with a specific characteristic impedance, usually 50 or 75 Ω. The Crystek Corporation CBLK-300-3 is a 50 Ω inner conductor DC blocker that passes signals at frequencies from 300 kilohertz (kHz) to 3GHz, while blocking DC voltages up to 16 volts, over its operating frequency range has low insertion and return loss (Figure 8).

Understand the basics of coaxial adapters to better use these very useful components

offset tee

The offset tee is associated with the DC block. It is a three-port adapter where DC power is applied to one port. The second port combines the DC bias with the incident RF signal from the isolated RF port (Figure 9).

Understand the basics of coaxial adapters to better use these very useful components

The bias tee can be used to provide DC power to remote electronics, power equipment such as an antenna-mounted low noise amplifier (LNA), and also provide a DC-free port to connect an RF receiver. When a DC bias is applied through a series Inductor, it blocks RF signals from being applied to the DC source device. Like a DC block, a pure RF port isolates the DC input with a series capacitor. Combination ports pass both RF and DC components.

The Crystek Corporation BTEE-01-50-6000 is an offset tee that uses an SMA jack with an RF bandwidth of 50 megahertz (MHz) to 6 GHz. The RF port accepts RF signals with a maximum power of 2 watts. The maximum DC input to the DC port is 16 volts. The insertion loss of this biased tee is typically 0.5 decibels (dB) at 2 GHz. In operation, the RF + DC port is connected to the LNA and antenna. The DC power supply is connected to the DC port and the receiver is connected to the RF port.

In-line filter

Another useful coaxial adapter is an in-line filter. Low-pass, high-pass, and band-pass filters are available on BNC or SMA connector types to control the frequency spectrum of the signal carried on the cable. For example, to measure the effective number of bits of an analog-to-digital converter (ADC), a low-pass filter should be inserted between the signal generator and the ADC. The filter attenuates the generator’s harmonic levels, greatly improving measurement accuracy. This allows the use of lower cost signal generators.

A good example of this is Crystek’s CLPFL-0100, a 7th order, 100 MHz low-pass filter with a 100 MHz cutoff frequency (Figure 10).

Understand the basics of coaxial adapters to better use these very useful components

For a 100MHz input signal, the second harmonic of the device will be attenuated by 30dB, and its higher harmonics will be attenuated by more than 60dB. If the signal generator in the example above had a harmonic level specification of -66dB, the filter would reduce it below -96dB.

surge protector

Surge protectors, sometimes called lightning arresters, are used to protect sensitive electronics from transient surges such as lightning. This can be accomplished with spark gaps, gas discharge tubes, or diodes, which discharge the surge to ground before electrical breakdown occurs before the surge damages the equipment being protected.

Amphenol Time Microwave Systems LP-GTR-NFF is an N-connector in-line surge protector that uses a replaceable gas discharge tube. The discharge tube will break down at DC voltages above ±90V/20A and can handle surges up to 50W. It is in-line, with bandwidths from DC to 3GHz, and insertion loss of 0.1dB (up to 1GHz) and 0.2dB (up to 3GHz) (Figure 11).

Understand the basics of coaxial adapters to better use these very useful components

Surge protectors are typically mounted on an L-bracket with a large, low-inductance conductor for electrical and mechanical connection to a low-impedance ground. It is important to note that the quality of the ground connection can affect the performance of the surge protector.

In-line attenuator

Attenuators reduce the power level of a signal without distorting the signal waveform. Coaxial in-line version attenuators provide fixed attenuation and are available in a variety of plug and jack configurations for use with numerous connector types.

The Crystek Corporation CATTEN-03R0-BNC is a 3dB, 50 Ω, BNC attenuator with a bandwidth of 0 to 1GHz and a power rating of 2W (Figure 12). It is one of 13 attenuator models in the company’s product line, with attenuation capabilities ranging from 1 to 20dB.

Understand the basics of coaxial adapters to better use these very useful components

In-line attenuators are obviously used to reduce the power level of a signal, but less obviously they can also be used to provide impedance isolation in series devices, as well as reduce impedance mismatch and unwanted reflections.

Consider inserting a matched 3dB attenuator in front of the mismatched load impedance. The attenuator input signal is reduced by 3dB by the attenuator when propagating to an unmatched load. Assuming the mismatch is an open circuit, the entire signal will reflect at the load and bounce back through the attenuator, suffering another 3dB loss at the attenuator input. So the return loss at the input of the attenuator is improved by 6dB. Therefore the mismatch observed at the input of the attenuator is improved by twice the attenuator value – in this case, the total reduction is 6dB.

A disadvantage of this technique is that the amplitude of the passing signal is reduced by 3dB, which must be compensated elsewhere in the network. Crystek CATTEN-03R0-BNC performs well in this application.

Epilogue

When connecting instruments or other equipment with coaxial adapters, designers and other equipment users need to understand the basics of transmission lines. Once this knowledge is gained, users can better utilize these very useful components for a wider range of uses, including changing connector type and characteristic impedance, signal branching, filtering, surge protection, signal attenuation, and DC control and isolation.

Source: Internet

The Links:   QM100HY-2H LQ150X1LG91