Corrective Tuning of an RF Duplexer with a Vector Network Analyzer

What is an RF Duplexer?

An RF duplexer is a passive device that enables a transmitter and a receiver to share a common antenna while operating simultaneously on different frequencies. It provides isolation between the transmit (TX) and receive (RX) paths, preventing high-power transmit signals from saturating or damaging sensitive receive circuits. The duplexer typically achieves this isolation through carefully designed frequency-selective notch filters, such as cavity resonators or ceramic filters.

Common Applications for RF Duplexers

RF duplexers are critical in many communication systems. Key applications include:

• Land Mobile Radio (LMR) Systems (public safety, transportation, utilities)
• Cellular Base Stations (GSM, LTE, 5G)
• Repeater Systems (UHF/VHF radio repeaters)
• Aviation Communication Systems
• Military Tactical Communication Systems

Why Tuning is Required

Duplexers are highly frequency-specific devices that require precise tuning to ensure optimal performance. A common assumption is that once a duplexer has been tuned and put into service it can be forgotten, left to run without ever needing to attend . In reality, the internal components age and factors such as temperature, humidity, and mechanical stresses will contribute to shifts in resonator characteristics. Tuning plungers, finger stocks, and other components within the cavities may oxidize. Additionally, components such as air trimmer capacitors can deteriorate over time due to the overall impacts of heat generated by high levels of RF power from transmitters. Lightning strikes can also cause loop assemblies and tuning circuits within duplexers to fail or fall out of specification. Because of all such possibilities, routine maintenance or troubleshooting to optimize isolation and performance become a necessity.

Measurement Setup and Analysis

Each of the duplexer halves are designed to pass their frequency of interest while specifically blocking the frequency of the opposite half. For instance, Figure 1 shows a duplexer whose

The duplexer used in the steps that follow is shown in Figure 1. The side marked LOW is configured to pass 462 MHz while blocking 467 MHz. Conversely, the side marked HIGH is configured to pass 467 MHz while blocking 462 MHz. Each half is composed of three cavities containing adjustable components that allow the user to adjust the frequencies of interest.

There are three main specifications that are of interest to the user (see Figure 2):

1. Suppression is the desired attenuation for the frequency that is to be blocked.
2. Insertion loss is the desired attenuation for the frequency that is to be passed.
3. Voltage standing wave ratio (VSWR) or return loss is the expected or acceptable match at the connection ports of interest.

Since it is increasingly common to quantify mismatch in terms of return loss in place of VSWR, you will want to convert the 1.3 value, preferably using an online or spreadsheet calculator tool like the Bird RF Calculator. Using the recommended tool, you will find that the return loss equivalent is 17.69 dB.

Since the user must make connections to all three port of the duplexer, choosing a VNA with more than two ports is ideal. Note that the measurements may be made with a 2-port VNA, but the unused duplexer port must be terminated with a 50 Ω load and additional connect and disconnect cycles will be required.

To measure this duplexer, set an appropriate frequency range of 460 to 470 MHz so that both points of interest can be monitored. You will follow this with a calibration that includes the cables to be used during the test. For those not familiar with measurement calibration, see the application note entitled, “Why Calibration Matters”.

You can examine both suppression and isolation loss of the LOW side by connecting Port 1 of the VNA to the LOW input, Port 2 to the ANT input, and Port 3 to the HIGH input (as in Figure 3).

You will then configure the VNA to make an S21 parameter measurement so that VNA Port 1 performs a source sweep that is measured by Port 2, with markers set to our points of interest at 462 and 467 MHz. Because this is the LOW side, the expectation is that very little loss will be witnessed at 462 MHz (< 1 dB) and extreme loss will be witnessed at 467 MHz (< 80 dB). See Figure 4 for illustration.

The noise around 467 MHz can be reduced to give a cleaner look to the curve and help in more accurately adjusting the suppression point. To do this, you can enable both averaging and smoothing, adjusting their settings to meet your needs.

Once the VNA settings are to your liking the notch filter attributes may then be viewed on the display. Note that while Figure 5 achieves the specified < -80 dB of suppression at 467 MHz, the isolation loss at 462 MHz falls short of expectations: -1.1410 dB instead of < -0.8 dB.

Tuning for correction is performed using the set screws at the end of each of the three cavities that comprise the LOW side of the duplexer.

This can be a delicate process that requires you to move methodically between each of the set screws to get just the right balance to meet the application needs. Notice in Figure 7 below that while we start getting the isolation (or insertion) loss at 462 MHz closer to optimal, we start to sacrifice some of the centering and magnitude for the suppression at the 467 MHz point. Similar might be encountered if you were adjusting the suppression point from 467 to 467.5 MHz.

Proper tuning can prove to be tedious and time consuming, requiring the utmost in patience from the operator.

Once you have reached the levels of suppression and isolation at the points of interest, it is important to check the match at the LOW input port. This is done by enabling an S11 measurement with a VWR format and verifying that the 462 MHz point is <1.3:1 (as in Figure 8 below).

Note that the same can be achieved by looking at return loss at this same point, ensuring that it is >17.69 dB (the equivalent of a 1.3:1 VSWR).

Once the LOW side meets acceptable performance limits, the same procedure is conducted on the HIGH side with respect to the ANT port using S23 measurements for suppression and isolation and S33 measurements for VSWR or return loss. See Figures 9a & 9b for illustration.

Once satisfied with the adjustments on the individual halves, a final verification should be performed using a full sweep inclusive of both LOW and HIGH paths. Both share the ANT port connection and changes to one side of the duplexer do have an impact on the other. This check, using either an S13 or S31 sweep, helps to confirm suppression points of the notch filters are within specifications at the desired frequencies and to verify LOW-to-HIGH isolation is sufficient.

Additionally, you should check the ANT port return loss (measuring S22) is within acceptable range.

Conclusion

Proper tuning of an RF duplexer is essential to ensure high-quality performance in any system where shared antennas and simultaneous transmit/receive operation are required. Whether commissioning a new system, moving to new frequencies, or maintaining existing equipment, careful and precise tuning of the duplexer maximizes isolation, minimizes insertion loss, and protects both the transmitter and receiver. Following a methodical tuning process and understanding common tuning challenges will help ensure reliable system operation and long-term performance.

Said tuning can be achieved using a vector network analyzer, configuring it for transmission loss measurements (S21 and S23 as in the above examples) to help quantify the suppression and insertion loss at the frequencies of interest. VSWR (or return loss) must also be monitored to help ensure adequate match at each of the available ports.

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