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Ben Kahtan, Mini-Circuits
The 75Ω equipment often used in CATV applications occupies a small enclave of a largely 50Ω world. Although dedicated 75Ω equipment does exist, its availability and its applicability to other test needs are limited; most general-purpose test and measurement equipment is designed for 50Ω use. Because of this, it can be difficult to create test setups entirely out of dedicated 75Ω test equipment.
Although it is possible to interface 50Ω and 75Ω equipment using minimum-loss resistive matching pads, this approach can cause issues due to the test setup’s insertion loss—good resistive matching pads achieve close to the theoretical minimum loss of 5.7 dB. The need for a matching pad on both ends of the 50Ω equipment leads to large insertion losses—for each 50Ω component inserted into the 75Ω path with resistive L-pads, this is about 12 dB (not including the loss of the component itself). The high losses resulting from resistive impedance matching can severely impact the dynamic range of some setups, limiting accurate measurement.
For example, the precision of insertion phase measurements may be severely degraded by high system losses. Similarly, measurements of return loss and high-isolation measurements require a test setup with high directivity and low losses. Better measurement precision can be achieved by decreasing measurement bandwidth, but this is at the expense of increased test time—slowing down production test lines and increasing test costs.
In this application note, we describe a method of interfacing Mini-Circuits 50Ω switch boxes with 75Ω CATV test setups, using a combination of minimum-loss resistive matching pads and matching transformers. This method reduces signal loss to an acceptable level and is easily applicable to other Mini-Circuits test solutions including ZT- and ZTM-series test systems, port extenders, programmable step attenuators, amplifiers, and more.
Figures 1 through 3 show three hypothetical 75Ω test setups which use two SP4T switches to route signals to various DUT ports. This setup can be realized in practice using Mini-Circuits RC-2SP4T-A18 USB/Ethernet controlled switch boxes. In Figure 1, the 50Ω SP4T switches are inserted using minimum-loss resistive matching pads. The loss for this setup, in addition to the insertion loss of the switches, is approximately 24 dB, which is potentially problematic for the reasons explained above. We can substantially decrease this loss—by over 22 dB, a factor of about 160—by replacing the minimum-loss resistive matching pads with low-loss impedance matching transformers, as shown in Figure 2. Each replacement of a matching pad by a transformer saves up to 5.7 dB of insertion loss at mid-band, and about 4.7 dB at the band ends (depending on choice of transformer model).
In general, minimum-loss resistive matching pads exhibit higher return loss and a flatter response over a wide bandwidth, so in certain situations it may be beneficial to use these and sacrifice insertion loss in exchange for better return loss and flatness.
Figure 3 shows a combination of matching transformers and minimum-loss resistive matching pads, with approximately half the insertion loss of the resistive-only setup in Figure 1, but also with higher return loss around the DUT and better flatness than the transformer-only setup in Figure 2.
If your CATV test setup requires minimal insertion loss to maximize dynamic range, we recommend using only transformers. On the other hand, if return loss is critical, or flatness variations can’t be completely calibrated out, then a combination of matching transformers and minimum-loss resistive matching pads may be used.
Figure 4 shows the simulated insertion loss contributed by the matching components in each of the three setups above. The pink graph shows the insertion loss of four matching transformers, the blue graph shows the combined transformer/pad approach (two transformers and two pads), and the red graph shows the insertion loss of four resistive matching pads.
Mini-Circuits offers both matching transformers and minimum-loss resistive matching pads that can be used to this effect. They can be combined with Mini-Circuits’ test and signal routing solutions to meet the diverse needs of a CATV test lab.
The TC1.5-1G2+ is a 1:1.5 unbalanced to unbalanced matching transformer specified from 0.5 to 2200 MHz. It has a low typical mid-band loss of 0.3 dB and a 1 dB bandwidth of 2 to 1100 MHz, making it ideal for use in certain CATV test setups. It is available as a surface-mount component and on a connectorized evaluation board (TB-41), with 50Ω SMA and 75Ω BNC connectors. This model is used in the examples above, and is a good choice if low insertion loss is critical.
The Z7550-FFNM+, Z7550-FMSF+, and Z7550-NMNF+ are DC-passing coaxial matching transformers with 75Ω F-type to 50Ω N-type and SMA connectors. These have excellent wideband performance up to 2300 MHz, with low insertion loss (0.6 dB typical) and low VSWR (1.5:1 typical). These models are an excellent choice for the wide bandwidths required when testing DOCSIS 3.1-compliant equipment.
Mini-Circuits’ BMP-5075+ is a 50Ω to 75Ω resistive matching pad in a coaxial package with BNC connectors. It is specified from DC up to 2000 MHz, making it ideal for CATV test and measurement applications. Mini-Circuits also offers resistive matching pads with N-type connectors (BMP and UNMP series). The BMP and UNMP model families are available with a variety of connector configurations and genders; customized solutions may also be feasible on a case-by-case basis.
In addition to discrete impedance-matching components, Mini-Circuits can also provide purpose-built custom test equipment (e.g. signal routing equipment such as switch matrices) with built-in impedance matching components. For example, the ZT-209 unit in Figure 8 uses Mini-Circuits’ SP6T mechanical switches and mechanical transfer switches to route two 75Ω input ports to any pair of its ten 75Ω output ports. ZT-209 uses special impedance-matching transformers to achieve an excellent return loss (22 dB typ.) on its input ports with minimal insertion loss.
While all of these products can be used to properly insert 50Ω components into 75Ω signal chains, certain combinations of these may be advantageous over others for particular system needs. From the results presented here, it is clear that Mini-Circuits’ 50Ω test solutions can easily be used in 75Ω CATV production and test environments with the addition of a few inexpensive components and adapters.