Figure 1 Basic components of a transceiver front end.

Modern communication systems need new filters, diplexers and multiplexers to meet electrical performance, size and manufacturing cost requirements, which are difficult to achieve using classical topologies. Diplexers are widely used in communication systems for reducing mass and volume of the required hardware. They separate different bands of a signal into different ports and enable the use of the same antenna for different frequency bands, resulting in more compact systems. Alternatively, a diplexer combines two different signals with different spectral components into one common port. A typical architecture of a transceiver RF front-end is shown in Figure 1.


The diplexer is one of the key components in the overall system, as shown in the figure within the dotted line. The transmitter and receiver operate in different frequency bands and are duplexed to the antenna by the transmit-receive diplexer. The diplexer consists of a power divider and two channel filters that can have very stringent specifications. The transmit filter must reject out-of-band noise generated in the power amplifier, so it must have a high level of stop band attenuation, particularly in the receive band. It should have a low pass band insertion loss in order to maximize the DC to RF efficiency of the power amplifier. On the other hand, the receive filter should have high attenuation in the transmit band in order to protect the front-end low noise amplifier (LNA) from the large transmit signals, which would otherwise saturate the receiver or produce high levels of distortion. The most important parameter in the diplexer design is the isolation between the receive and transmit channels.

Waveguide filters are an alternative solution and have been used until now, because of their good characteristics in terms of insertion loss at the expense of weight, volume and cost. In the near future, waveguides in satellite equipment will be replaced by planar structures. Therefore, a planar solution would be a better approach to achieve the desired specifications of insertion loss, return loss and rejection. Parallel coupled line (PCL) transmission lines can be used to construct many types of filters. A PCL approach, using the standard classical techniques, is unsuitable for the diplexer application as it does not achieve the desired narrow bandwidth. Alternatively, a multi-section bandpass coupled line filter, having a narrow bandwidth, can be employed, as required for the diplexer applications. However, limitations of this approach yield large size and uncontrollable transmission zeros resulting in poor selectivity.1 Hairpin topology is considered to be another approach for placing the transmission zeros (TZ), but the desired rejection is still not achieved. A modified hairpin topology has recently been proposed2 using the cross-coupling effect, but it increases the circuit complexity and needs tight fabrication tolerances.

This article presents the design and development of a microstrip diplexer using the dual behavior resonator (DBR) approach. To achieve the desired specifications, two filters at the Rx and Tx frequency bands are optimized. A notch effect on the filter performance is carried out and further implemented in the receive filter. To achieve the desired specifications, the notch, along with the concept of an oversized stub, has been implemented to have better rejection characteristics in both receive-transmit bands. Further, a suitable T-combiner has been designed to integrate the Rx and Tx filters for the diplexer arrangement.

Ku-band Diplexer Specifications

Figure 2 Receive band specification.

The main objective of this work is to demonstrate a compact diplexer design for onboard communication satellites. The precise specification for the receive band, as described in Figure 2, shows a high rejection for the transmit frequencies while preserving the receive frequencies. The same is true for the transmit frequencies, to be passed with low loss, eliminating the receive frequencies. In the present article, the transmit frequency is 12.5 GHz and the receive frequency is 14.5 GHz, suitable for the onboard Ku-band transponder. The specified criteria of insertion loss and flatness are better than 3 and 1 dB, respectively. The isolation requirements between the two bands are greater than 20 dB. The main challenge here is to get a better rejection while restricting the number of resonators to be used so as to reduce both the level of insertion loss and the structure size. This specification can be met with the DBR approach, using an oversized stub, which allows increasing the rejection level without altering the TZ frequency.3

DBR Approach

The dual behavior resonator (DBR) is a basic resonator that presents a dual frequency behavior in the pass band and stop band regions. It allows independent control of the rejected bands, through the addition of the transmission zeros and a centered bandpass. Dual behavior resonators are achieved by associating two different parallel open-ended stubs. Each stub brings its own TZ with respect to the fundamental resonant condition and a bandpass is created between them.4 It allows an independent control of each frequency band of interest, that is the center frequency bandpass and the attenuated bands.5 The resonator, in the case of a DBR, is composed of two stubs (low and high frequency stubs), in which only one of them can be modified, which consequently improves the rejection either near the low frequency transmission zero or the high frequency transmission zero. With the number of available parameters and its initial behavior, a dual behavior resonator allows independent control of the following:6

  • one pole in the operating bandwidth
  • one transmission zero in the lower attenuated band
  • one transmission zero in the upper attenuated band

The concept of the oversized stub improves the rejection either at the lower frequency end or at the higher frequency end as both cannot be modified simultaneously.

Design Methodology

Figure 3 Basic building blocks of a diplexer.

The diplexer design starts with the design of the channel filters (bandpass filters), which must achieve the required pass band performances before they are connected to the power divider. Usually, two methods are followed to obtain the final device. In the first one, the bandpass filters are re-optimized within the diplexer environment, which includes the effect of the power divider discontinuities as well as the interaction between the filters. In the second method, the original filter designs are maintained and an optimum matching circuit is incorporated into the power divider junction to obtain the required matching in the pass bands. In the present communication, the second approach is adopted, reducing the minimum number of variables for the circuit optimization. The design of the diplexer can be divided into three parts: Tx filter, Rx filter and T-junction. The overall basic building blocks are shown in Figure 3.

Transmitter Filter Design

Figure 4 The transmitter channel filter: (a) layout and (b) simulated performance.

The channel filter for the transmitter has been designed for 12.2 to 12.8 GHz. The circuit layout is shown in Figure 4 with its electromagnetic (EM) simulated response. The EM simulation has been carried out using Linmic and ADS tools.8 The simulated response shows a rejection level greater than 50 dB at the receiver frequency with an insertion loss less than 1 dB. The minimum strip width and gap is kept to approximately 0.1 mm. The notch in the filter is introduced in the transmission line as shown in Figure 4. The effect of the notch has been studied using the EM circuit simulator. The notch implemented filter called Mod in Figure 5 shows a rejection improvement of more than 15 dB at the receive band. There is no change in the other major characteristics like insertion loss and return loss of the filter. This is attributed to the field’s interaction in the notch, leading to coupled line behavior by introducing two nearby poles at the receive band.

Figure 5 Comparison of the proposed (mod) technology with the standard (typ) topology.

Receiver Filter Design

Figure 6 Layout (a) and current conduction behavior (b) of the receive filter.

The receiver filter is designed and optimized at 14.5 GHz. The layout and the current plot of the EM simulated structure are shown in Figure 6. The effect of the notch has also been studied in this configuration. Introducing a smaller notch in the transmission line has no significant effect on RF performance. Extending the notch length changes the RF performance considerably, as shown in Figure 7. The notch width in the present study is kept constant (0.12 mm) and the simulation study is carried out by taking two different notch lengths. The study shows that the notch introduces poles at the transmit frequency bands, which clearly shows an improvement of approximately 20 dB. This rejection enhancement overall improves the diplexer performance, compared to the standard topology suggested by Quendo, et al.7 Since the notch considerably affects the center frequency, the notch in the receive filter is not implemented as it affects the layout dimensions, which need re-simulation and layout reorientation. This concept is not implemented in the present Rx filter development.

Figure 7 Study of the notch on receive filter performance.

Power Divider

Figure 8 T-combiner topology.

Figure 9 Final layout of the diplexer.

The power divider is an essential part of the diplexer and a T-combiner is chosen to join both the transmitter and receiver filters. The lossless T-divider shown in Figure 8, is constructed using 100 lines at the output, but the discontinuity due to the lower width of the 100 leads to considerable losses at the output. Therefore, to achieve improved isolation characteristics between the output ports, the line width has been maintained to have impedances of 70.7 ohms and phases are adjusted accordingly to get the desired response. Transmitter and receiver filters are joined together using the T-combiner and further optimization has been carried out as the second approach discussed earlier in the Design Methodology section. The layout of the overall circuit is shown in Figure 9.

Fabrication Aspects

The proposed filter was implemented on a 10 mil thick alumina substrate having a dielectric constant εr = 9.9. The filter is realized on the microstrip substrate and attached using conductive epoxy. Further connectors are attached using ribbon bonds. The overall size of the diplexer is 1 x 1 inch. The line lengths at the input and output ports are extended by 2 mm to facilitate connector placement. The measured results show a pass band insertion loss of less than 3 dB and an attenuation of more than 10 dB in the transmit band. The results show a slight frequency drift with higher insertion loss at the receive frequency band. This is attributed to the fabrication tolerances along with permittivity dispersion. The drift in the permittivity affects the T-combiner matching, which degrades the overall circuit performance. The comparison of the simulated and measured performance is shown in Figure 10.

Figure 10 Comparison of simulated and measured performance of the diplexer fabricated on an aluminum substrate.

Figure 11 Fabricated diplexer on a silicon substrate (a) and its simulated and measured isolation characteristics (b).

The diplexer was also fabricated on a high resistivity silicon substrate (> 8 k Ω-cm). After being subjected to standard thin film substrate cleaning cycles, the high resistivity silicon substrates (25.4 x 25.4 x 0.6 mm) were sputtered with a thin layer of TiW (200 to 300 Å) followed by a 8000 Å gold film on both sides of the substrates. This combination of under layers was electroplated with gold to the required thickness of 4.5 µm ± 3 percent and the circuits were patterned using standard optical lithography and a subtractive etching process. The patterned substrate was attached to a gold metalized kovar carrier plate, using a silver-based conductive epoxy. The carrier plate was mounted in the test jig and RF connectors were connected by gold ribbon 20 mils wide and 1 mil thick, using parallel gap welding. The fabricated silicon diplexer has been measured with a PNA vector network analyzer (8261A), showing isolation characteristics better than 24 dB, as shown in Figure 11.

Conclusion

The planar realization of a diplexer is shown on alumina and silicon substrates using microstrip technology. The effect of a notch has been studied and implemented for high rejection, along with the concept of the oversized stub. Fabrication of the diplexer on the silicon substrate has been detailed. This planar diplexer shows a minimal effect of the cover height and can be easily implemented in the payload of telecommunication satellites, where size and cost are at a premium. Furthermore, a micro-machined patch antenna, integrated with the proposed diplexer on silicon, will enhance the overall system performance.

References

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