What Is a Duplexer?

The duplexer is the main accessory of the inter-frequency duplex radio and repeater. Its role is to isolate the transmitting and receiving signals and ensure that both receiving and transmitting can work normally at the same time. It is composed of two sets of band-pass filters with different frequencies to prevent the local transmission signal from being transmitted to the receiver.

General duplexer ^[1]

The duplexer should be customized according to the radio transmitting and receiving frequency. The working bandwidth of a 10 megahertz duplexer with a difference between 400 megabits of transmit and receive frequency is between plus and minus 250 kHz, which guarantees an isolation of about 90db, and a single-frequency working isolation of up to 120db. When the use frequency exceeds the rated bandwidth of the duplexer, the transmit and receive isolation will decrease sharply, and the standing wave of transmission will increase, and the receiving circuit will not work normally due to the influence of the transmitting part. The U section of the amateur wireless repeater generally has a transmission and reception difference of 5 megahertz. The duplexer used is a narrow-band design, which can ensure that the isolation does not decrease but the operating bandwidth becomes narrow, which is plus or minus 100KHz. Practice has proved that using a duplexer is better than using two antennas to send and receive. ^[1]

The duplexer is a special two-way three-terminal filter. The duplexer needs to receive weak signals

The operating frequency range of the duplexer should not be narrower than the operating frequency range of the radiotelephone itself. Generally speaking, the bandwidth we refer to is the input bandwidth of the receiver and the output bandwidth of the transmitter after the radiotelephone is equipped with a duplexer. For a duplexer, it is the stopband bandwidth of two equivalent bandstop filters, not the passband bandwidth. From its frequency response curve, it is the frequency range of the two stopbands at a certain attenuation. As everyone knows, today's VHF and UHF radiotelephones themselves, the high-frequency input bandwidth of the receiver can generally be within Above 5MHz, the high-frequency output bandwidth of the transmitter is above 10MHz. In other words, the high-frequency input and output bandwidth of the radiotelephone itself is relatively wide, so the bandwidth of the duplexer should also be a certain width, in order to overcome the trouble caused by the bandwidth problem when the user declares the frequency. However, according to our country and international countries

Isolation of duplexer ^[2]

The insertion loss of the duplexer refers to the loss of the useful signal corresponding to the passband frequency point in the channel. Can be expressed by the formula: 10LgPi / PO or 20lgUi / Uo. Among them, Pi and Ui represent the power or level of the signal entering the duplexer; Po and Uo represent the power or level of the signal coming out of the duplexer. It goes without saying that, for a duplexer, the smaller the insertion loss, the better, especially for the transmission channel. The smaller the insertion loss, the better the output power and efficiency of the whole machine. Calories. The domestic duplexer index is below 1.2db, and the actual insertion loss of the duplexer in some imported products is about 0.5db. For example, for a repeater that requires RF power of 25W, if the selected duplexer has an insertion loss of 0.5db, then the entire RF power amplifier is required to output 28.1W of power. But if the selected dual The insertion loss of the device is 2db, and the entire RF power amplifier is required to output 39.6W of power. The difference is more than ten watts. Therefore, the insertion loss, especially the insertion loss of the input channel, is also a very important indicator. ^[2]

The frequency stability of a duplexer should include two meanings: one is the stability of its structure, which makes its distribution parameters have a certain stability; the other is its temperature stability.

As mentioned earlier, the duplexer is a cavity filter with distributed parameters, and its frequency response curve is determined by the cavity's distribution parameters. We require that the duplexer used in mobile communication should have considerable frequency stability in the temperature range of -25 ° C to 55 ° C, so as not to affect the communication effect of the radiotelephone, that is, the frequency response curve of the duplexer should be Maintain its required spectrum position and curve shape in the corresponding temperature range. The usual duplexer should have a stability better than + 55ppm in the operating temperature range to meet the wireless

In addition to the indicators discussed above, the duplexer also has characteristic impedance, maximum input power, and standing wave ratio. The maximum input power is the maximum input power that a duplexer can withstand, and it is a safety indicator for the use of a duplexer. The input impedance of the antenna of the radiotelephone and the output impedance of the transmitter are both 50. Therefore, the impedance of the duplexer should also be 50. To ensure the safety and communication effect of the whole machine, the standing wave ratio of the duplexer is usually 1.4. the following.

It is worth pointing out that the impedance of the duplexer also changes with its distribution parameters. Therefore, impedance matching is one of the purposes of debugging. The adverse effect caused by impedance mismatch is even greater than the adverse effect caused by some other factors.

In short, there are many indicators of the duplexer that are worth considering, but these indicators are interconnected from the perspective of the duplexer itself, and should be comprehensively considered when selecting. ^[2]

The duplexer should confirm whether the duplexer's working frequency is consistent with that of the radiotelephone before joint tuning of the whole machine. If the difference is large, it should be readjusted and re-tuned to avoid burning the input high-amplification circuit.

The initial adjustment of the duplexer is generally performed by the manufacturer of the duplexer. After the adjustment is completed, the operating frequency is indicated before leaving the factory. But before using the duplexer, if it is really necessary, it can be readjusted according to the actual working frequency. Connect the instrument with the debugging method and connect a 50 terminal dummy load. Repeat the adjustment and swap the dummy load and the spectrum analyzer. Tighten each adjustment screw.

During the whole machine debugging phase, after the transceiver circuit board is adjusted, the duplexer must be carefully fine-tuned to maximize the transmit power and minimize the degradation of transmit and receive sensitivity. During joint tuning, we must pay attention to small, slow adjustments to prevent burning out of the satellite TV. Because the duplexer has a higher working frequency and is a component whose distribution parameters determine its characteristics, it is very sensitive when adjusting; and to prevent the optimal point of the original adjustment from running away during the locking process after adjustment. Some duplexers use self-locking screws, which do not need to be tightened with nuts, which greatly facilitates debugging and saves man-hour costs. ^[2]