DC Feeders are Eye Openers

The DC Feeder is often placed in the same category as a Bias Tee because they are used in the same or similar applications, but there are fundamental design differences. In some applications replacing a Bias Tee with the right DC Feeder can contribute to improvements in overall system performance.

Bias Tee

Figure 1: Bias Tee Circuit

Bias Tees are commonly used to introduce DC current/voltage to an AC (RF) signal while isolating one side of the RF path from the added DC content. Most electronic component and systems designers understand that the Bias tee architecture (Figure 1) is a 3-port device that consists of an inductive DC feed port, connected to a 2-port RF transmission line path. The circuit implements a blocking capacitor at one of the RF ports to prevent the passage of the injected DC content in that direction.

Bias Tee applications include powering antenna amplifiers, biasing transistors, supplying Power over Ethernet (PoE), and more. In the data communications world, Bias Tees are commonly used with Modulator Driver Amplifiers for biasing photodiodes in Optical Modulators.

DC Feeder

Figure 2: DC Feeder Circuit

Similar to a Bias Tee, a DC Feeder is also a 3-port device that is used to insert a DC bias current/voltage onto an RF transmission line path. As shown in Figure 2, what separates the DC Feeder from the Bias Tee is the omission of the DC blocking capacitor within the RF path. Therefore, the DC Feeder does not provide any isolation to the injected DC content at either RF port. 

Like a Bias Tee, DC Feeders are used for biasing Amplifiers, Lasers, Optical Modulators, and other devices that may already contain a DC-blocking capacitor. In data communications systems, implementing a DC Feeder in lieu of a Bias Tee can help to improve system performance.

Eye Diagrams

Figure 3:  112 Gbps PAM4 Eye Diagram

Eye Diagrams or Eye Patterns (Figure 3), are a means for evaluating the signal integrity within high-speed digital communications systems, subsystems or components. Simply stated, an Eye Diagram is created by transmitting a random stream of symbols through a system (or device) under test. The resulting Eye Pattern is a persistence plot of those transmitted symbols, which allows for the evaluation of what effects a system has had on the data.

Eye Diagrams provide valuable insight into system jitter, mismatches, losses, channel noise, dispersion, intersymbol interference (ISI), and more. The more open or “clean” an Eye Diagram is, the more likely the receiver will properly discern between those original symbols while reducing the probability of errors.

DC Feeders Can Help Open Eyes

A DC Feeder is intended to replace a Bias Tee in cases where it is being used with an AC-coupled device such as an AC-coupled Amplifier. Since the Amplifier contains a DC blocking capacitor at the output, it does not require the DC protection that is provided by the capacitor inside a Bias Tee.

Introducing a DC Feeder after the Amplifier eliminates the unnecessary second/series capacitor, which can help to preserve the original low-frequency cutoff (Figure 4) of the Amplifier while also improving the overall insertion loss. Overall RF bandwidth and subsystem losses both directly affect resulting Eye Diagrams.

Figure 4: Low-Frequency Cutoff - DC Feeder versus Bias Tee

Below is an example where an AC-coupled Amplifier is driving into a Bias Tee (Figure 5a) and separately, into a DC Feeder (Figure 5b). In both examples, the input signal is the 112Gbps PAM4 Eye Diagram data as shown in Figure 3.

Figure 5a: The Bias Tee introduces an additional capacitor, potentially degrading an Eye Diagram

Figure 5b: The DC Feeder eliminates two series capacitors for optimized Eye Diagrams

Comparing the eye diagrams in Figures 5a and 5b demonstrates that the additional capacitor introduced to the system by the Bias Tee (Figure 5a) does in fact degrade the Eye Diagram when compared to using a DC Feeder (Figure 5b). This is important for applications where Biased Amplifiers are used to drive devices such as Optical Modulators. At a glance, the degradation may appear to be subtle, but with high-speed data communications, every BIT counts!

Keep Your Eyes Wide Open™

HYPERLABS products are designed to maintain those clean Eye Diagrams that digital systems designers work so hard to maintain. Our DC Feeder products incorporate the highest quality conical inductor networks to ensure the flattest, resonance-free insertion loss responses (Figure 6) and extremely flat group delay responses (Figure 7).

Figure 6: S21 Insertion Loss & S11 Return Loss (Part No. HL9349 110 GHz DC Feeder)

Figure 7: S21 Group Delay (Part No. HL9349 110 GHz DC Feeder)

HYPERLABS offers several DC Feeder products, covering different frequency ranges to match an application. If a desired frequency range is not available, contact HYPERLABS or your local Sales Representative to request product information for the desired bandwidth and connector options.

The HL9349 110 GHz DC Feeder (as shown in Figure 8) and the HL9348 90 GHz DC Feeder, both have 1.0 mm connectors on the RF path with the option of either DC feed-through pins or an SMA connector on the DC Port.

Figure 8: Part No.  HL9349-U-JPC

As with many other HYPERLABS products, the DC Feeders are available with an Unmatched option (“-U”) or as a Matched Pairs option (“-M”), which is specifically optimized for Amplitude and Phase matching between both units within a given pair.

At HYPERLABS, we understand what it takes to help KEEP YOUR EYES WIDE OPEN™.