by Harley Berman
In a transceiver design, it can be advantageous to use a single antenna for both transmitting and receiving signals instead of two separate antennas. The process of combining multiple signals onto the same communication channel is called multiplexing, and it can be done many different ways (time, spatial, code, frequency, etc). Frequency domain multiplexing, often called duplexing, requires placing some form of high isolation filter circuitry between the antenna and the Tx/Rx signal chains to only allow signals of the proper frequency to flow into their respective chains. For the sake of discussion, let’s say that you are tasked with designing a duplexer system that must discern between two narrowly spaced signals, one transmitted and one received, operating on the same antenna. Imagine the received signal is at -50 dBm (after antenna gain), and the transmitted signal is at +30 dBm. Therefore, very high isolation from the Tx to the Rx chain is required (at least 80 dB just to make the two signals equal power). Let’s say for the sake of the example that your design requires 80dB of isolation. Below are four methods I would consider as approaches to solve this problem:
- PBR and Bandpass Filter
The first duplexer design utilizes a very high isolation Marki PBR power combiner/splitter to split the incoming signal, followed by a bandpass filter to allow only the Rx signal to enter the Rx signal chain. The high isolation of the PBR relaxes the requirements on the bandpass filter as the filter no longer needs to supply the entire 80dB of suppression and can be realized much easier. This duplexer is surface-mountable and has the benefit of being simple and quick to prototype and implement into a final design due to the relaxed filter requirements. The cost of the high isolation is that the PBR also exhibits 6dB of insertion loss and has a maximum power handling spec. The insertion loss reduces the range of the transceiver and if the maximum power handling spec is exceeded, there may be permanent damage to the device. This configuration would be best suited for an application where rapid prototyping and board space is a concern and there is suitable dynamic range, so long as the input power is below the absolute maximum rating of the PBR.
- Highly modular design, simply swap the bandpass filter to satisfy a new frequency plan
- Less expensive to implement
- PBR is a high isolation power splitter so the BPF doesn’t need extremely high rejection
- BPF can be implemented with a simple lumped element bandpass filter design
- Physically small and surface mountable
- Output power of the PA may break the PBR if too high powered
- PBR has 6dB of loss and thus reduces range in both directions
- Lumped Element Filter and Impedance Transformation Network
During my undergraduate career at UC Santa Cruz, I worked on a project called SlugSat which is a 2U cubesat designed to carry two payloads into low earth orbit: a science experiment for the Santa Cruz Institute for Particle Physics (SCIPP) and an HF transponder for use in the ham radio community. In our system architecture, we receive a signal on the 10m band, down convert it to a 10.7MHz IF for power stabilization and signal processing, and then upconvert it to the 15m band for transmission. One of the contributions I made to satellite was the design of the antenna duplexer system which was implemented with this method. The matching network serves as a reactive splitter matching from PA to antenna at the Tx frequency and from antenna to pre-select bandpass filter at the Rx frequency. We used this method because it was cheap, we had the parts readily available to us, and it was very easy to prototype multiple duplexers in a day.
This is a great system to use at low RF frequencies (<1 GHz), where lumped components are nearly ideal. Higher frequencies would require larger and more expensive cavity filters.
- Can be very low cost
- Can use free online filter design tools to help design the whole system
- Rapid prototyping is possible due to availability of components and simplicity of design
- Difficult to implement at high frequencies
- Low Q lumped elements will not attenuate unwanted signals as sharply as other methods
- Difficult to achieve a very deep notch and requires large PCB
- Circulator and Bandpass Filter
Much like the previous duplexer system architectures, this duplexer follows the same basic structure: first split the incoming signal and then filter the high-powered Tx signal out of the Rx chain. This design however utilizes a circulator to split the signal. Circulators are three-port, non-reciprocal, ferromagnetic devices that allow for signal flow only in specified directions in its rated frequency range. The ferrite material circulates the signal flow within its operating band from one port to the adjacent port only, blocking signal flow in the opposite direction. Because of this, they are highly frequency selective and can be very bulky at lower frequencies. The isolation of a circulator is also typically low at around 25dB, therefore to achieve the rejection we need for our example a very deep notch filter is required. This design can be very simple to implement however custom circulators are often required which drives up cost and fabrication time.
- Simple to implement
- Commonly used
- Capable of handling very large powers with low loss
- Cannot provide high levels of isolation without the use of additional circuitry
- Frequency dependent devices would require custom designs for a change in frequency plan
- Bandlimited to a single octave (versus multiple decades with the PBR)
- Often expensive because they are hand crafted
- Custom designs often have long lead times
- Very high rejection filter with steep skirts could be hard to realize
- High Rejection Cavity Duplexer
The final design in the list is a high rejection cavity duplexer. A high rejection cavity duplexer is an all in one duplexer system that can achieve very high isolation and is very commonly used. This is typically the most expensive option available however it is also extremely simple to implement as it is an all in one drop in solution. If the frequencies you are duplexing are common frequencies, a common off the shelf model may be available for your design. More often than not however, a custom design will be necessary which can have long lead times and an even higher price associated with it.
- Simple implementation of a single component
- High Isolation
- Widely used
- High Q therefore high rejection of unwanted signals
- Most expensive option on the list (can cost thousands of dollars)
- Changing frequency plan would require buying a different duplexer
- Physically large
- Custom design requirements make for long lead times
Ultimately, the best duplexer for a given system depends on the requirements of the system. Some systems may favor a higher loss in exchange for a smaller size, or reduced system complexity in exchange for a higher price. Below is a table comparing the most common figures of merit of the four duplexer systems discussed in this article.