Band 71 Duplexer, Measured: Loss and Isolation
This Band 71 duplexer, measured as a raw part, loses 1.0 to 2.8 dB in the TX band and 1.3 to 2.3 dB in the RX band, with at least 57.7 dB of TX-to-RX isolation. The worst loss sits on the band edges, not in the middle.
Engineer’s note: If you're putting this part in a link budget, don't subtract the 1.3 dB of mismatch yet. That loss only comes back once your own board gives the duplexer a proper match, so carry 2.84 dB at 663 MHz until a matched board says otherwise.
LTE and NR Band 71 is the 600 MHz band: the phone transmits from 663 to 698 MHz and receives from 617 to 652 MHz. The two bands are only 11 MHz apart, and one duplexer has to pass each of them to the antenna while keeping the transmitter out of the receiver. This page reads a real one.
The file is a three-port measurement of a Band 71 duplexer on a test board, taken on an Agilent E5071C network analyzer with a full three-port SOLT calibration. It has 20,001 points from 500 to 815 MHz, a step of about 16 kHz, which is fine enough to see every ripple in the passbands. You can download the file and open it in the online S2P viewer or in RF View.
Finding the ports from the data
The file header does not say which port is the antenna. The data does. Of the three transmission terms, S21 is the one that passes 663 to 698 MHz, S31 passes 617 to 652 MHz, and S32 is small everywhere. So port 1 is the antenna, port 2 is TX and port 3 is RX. Checking this before reading anything else avoids the most common mistake with duplexer files: quoting the isolation as the loss, or the other way round.
What the curves say
TX path (antenna to TX, S21). Across 663 to 698 MHz the loss runs from 0.99 dB at its best to 2.84 dB at its worst, and the worst point is 663.0 MHz, the lower edge of the TX band. That edge faces the RX band, so the TX filter's lower skirt starts right there.
RX path (antenna to RX, S31). Across 617 to 652 MHz the loss runs from 1.31 dB to 2.30 dB, worst at 652.0 MHz. Again it is the edge that faces the other band.
Isolation (TX to RX, S32). The highest point in the TX band is -59.1 dB at 674.5 MHz, and in the RX band -57.7 dB at 650.4 MHz. The duplexer also rejects the other band on each path: the TX path is at -56.9 dB or lower anywhere in the RX band, and the RX path at -55.5 dB or lower in the TX band.
| Quantity | Band | Measured |
|---|---|---|
| TX insertion loss | 663-698 MHz | 0.99 to 2.84 dB |
| RX insertion loss | 617-652 MHz | 1.31 to 2.30 dB |
| TX-to-RX isolation, worst | TX band | 59.1 dB |
| TX-to-RX isolation, worst | RX band | 57.7 dB |
| Antenna port return loss, worst | TX / RX band | 7.6 / 6.7 dB |
The number that is not about the filter
The last row is the one to look at twice. The antenna port return loss is only 7.6 dB in the TX band and 6.7 dB in the RX band. A good duplexer in a matched system shows 14 dB or more. This part was measured through a test board, with the calibration plane at the board connectors, so part of the loss in the table above is mismatch loss rather than loss inside the filter.
How much? The mismatch loss of a port is -10·log10(1 - |S11|²). At 663 MHz the mismatch at the antenna and TX ports together accounts for roughly 1.3 dB of the 2.84 dB, so the filter itself loses closer to 1.5 dB there. The return loss vs insertion loss page explains how the two numbers relate.
What to take from it
- Read the band edges, not the band centre. The centre of the TX band here is near 1 dB; the edge is near 3 dB.
- Check the isolation in both bands separately. The TX band matters for the receiver blocking budget, the RX band for the transmitter noise that leaks into the receiver; the isolation tutorial shows how to turn this into a requirement.
- Treat a return loss under 10 dB as a warning that the loss numbers include mismatch.
Related pages on RF View
Frequently asked questions
Which port is which in this duplexer file?
Port 1 is the antenna, port 2 is TX and port 3 is RX. You can confirm it from the data alone: S21 passes 663 to 698 MHz (the B71 uplink), S31 passes 617 to 652 MHz (the downlink), and S32 stays below about -57 dB in both bands.
Why is the loss worse at the band edges?
An acoustic filter has its steepest skirts right next to the passband, and the band edges sit on the start of those skirts. In this part the TX loss is 1.0 dB at its best but 2.8 dB at 663 MHz, the TX edge closest to the RX band, where the filter has to start rejecting.
Is 57 dB of isolation enough?
It depends on the transmitter noise in the RX band and the receiver noise floor. The isolation tutorial on this site works through that budget; with typical handset numbers about 50 dB is the minimum, so 57 dB leaves some margin.