RF View

How Much Duplexer Isolation? TX Noise in the RX Band

By John · Updated · 6 min read

Required isolation equals the TX noise in the RX band minus the receiver noise floor, plus a margin. With -60 dBm of TX noise in 10 MHz, a -99 dBm floor and a 10 dB margin, the duplexer needs 49 dB of TX-to-RX isolation.

For practitionersNew to this? Plain-language version

For practitioners

In an FDD handset, the transmitter and the receiver work at the same time and share one antenna through a duplexer. The PA output is not clean. Along with the wanted carrier it carries broadband noise, and part of that noise falls on the receive frequencies. The duplexer's TX filter attenuates it, but some still reaches the RX port. If that leaked noise is comparable to the receiver's own noise, sensitivity drops. This page gives a direct method for finding how much TX-to-RX isolation keeps the leakage harmless.

Two different leakage problems

Two things leak from the TX port to the RX port, and they need separate specifications.

  • TX carrier leakage sits at the TX frequency. The receiver can filter it in principle, but it can block or compress the LNA and mixer. The isolation that matters here is measured across the TX band.
  • TX noise in the RX band sits at the receive frequency. To the receiver it looks exactly like wanted-band noise. No filter after the duplexer can remove it, because it is already inside the passband. The isolation that matters here is measured across the RX band.

This article covers only the second problem. The isolation figure you use is the TX-port-to-RX-port transmission of the duplexer, read at RX-band frequencies.

The receiver noise floor

The reference level is the receiver's own noise, referred to its input. At 290 K, the thermal noise power density is -174 dBm/Hz. Integrate it over the channel bandwidth and add the noise figure:

Nfloor=−174+10log⁡10(B)+NFN_{\text{floor}} = -174 + 10\log_{10}(B) + NF

Here N_floor is in dBm, B is the noise bandwidth in Hz, and NF is the receiver noise figure in dB. Any noise arriving at the receiver input from outside adds to this floor.

Setting the isolation requirement

TX noise reaches the RX port reduced by the isolation I, in dB:

Pleak=PTXnoise,RXband−IP_{\text{leak}} = P_{\text{TXnoise,RXband}} - I

Choose a margin M, the number of dB the leakage must stay below the floor. The condition is P_TXnoise,RXband − I ≤ N_floor − M. Rearranged:

I≥PTXnoise,RXband−Nfloor+MI \geq P_{\text{TXnoise,RXband}} - N_{\text{floor}} + M

P_TXnoise,RXband must be integrated over the same bandwidth B as the noise floor. PA noise is often specified as a density in dBm/Hz. If so, either convert it (add 10 log10(B)) or do the whole calculation in density form. In density form the bandwidth term cancels:

I≥PTX,density−(−174+NF)+MI \geq P_{\text{TX,density}} - (-174 + NF) + M

So the isolation requirement does not depend on channel bandwidth, as long as the TX noise density is flat across the channel. Only the noise figure and the TX noise density matter.

Worked example: 10 MHz channel, NF = 5 dB

Take B = 10 MHz and NF = 5 dB. Then 10 log10(10^7) = 70 dB, so:

N_floor = -174 + 70 + 5 = -99 dBm

At the PA output, the TX noise in the 10 MHz RX channel is -60 dBm. We want a 10 dB margin, so the leakage target is -99 − 10 = -109 dBm. The isolation is:

I = -60 − (-109) = 49 dB

TransmitterPADuplexerTXRXANTReceiverAntennaTX noise in RX band:-60 dBm in 10 MHzLeaked noise:-60 - 49 = -109 dBmLeakage: isolation 49 dBReceiver noise floor:-174 + 70 + 5 = -99 dBm(B = 10 MHz, NF = 5 dB)Leaked noise sits10 dB below the floor
TX noise path from the PA through the duplexer's 49 dB TX-to-RX isolation, which puts the leaked noise at -109 dBm, 10 dB below the receiver's -99 dBm noise floor. Concept drawing.

The figure shows the full path. The PA puts -60 dBm of RX-band noise into the duplexer's TX port. The TX-to-RX path removes 49 dB. The receiver sees -109 dBm, 10 dB below its -99 dBm floor. In dB arithmetic this is one stage: a level in dBm plus a gain in dB gives a new level in dBm.

0 dBm-60 dBmInput-109 dBmIsolation-49 dB
Gains and losses in dB add to a level in dBm: -60 → -109 dBm. Computed from the numbers.

As a cross-check in density form: -60 dBm in 10 MHz is -60 − 70 = -130 dBm/Hz. The receiver floor density is -174 + 5 = -169 dBm/Hz. The gap is 39 dB, and adding the 10 dB margin gives 49 dB. The result is the same.

Choosing the margin: what leakage costs

Noise powers add linearly. With leakage M dB below the floor, the total noise rises by:

Δ=10log⁡10(1+10−M/10)\Delta = 10\log_{10}\left(1 + 10^{-M/10}\right)
  • M = 10 dB: Δ = 10 log10(1.1) ≈ 0.41 dB of desensitization.
  • M = 0 dB: Δ = 10 log10(2) ≈ 3.01 dB. Leakage equal to the floor doubles the noise.

In the example, a duplexer with only 39 dB of isolation puts the leakage right at -99 dBm. That costs about 3 dB of sensitivity. Each extra dB of margin buys back less, so 10 dB is a common design target. It keeps the penalty under half a dB without over-specifying the filter.

Do not reuse the carrier isolation figure for this check. The duplexer's TX-to-RX transmission at TX-band frequencies and at RX-band frequencies are different numbers, set by different filter skirts. TX noise is governed by the RX-band value.

Applying the number on a real design

  • Use the worst point across the RX band. Isolation changes with frequency. The requirement must hold at the RX channel where isolation is lowest. Usually that is the band edge nearest the TX band.
  • Use the worst TX noise condition. P_TXnoise,RXband should be taken at the operating point where RX-band noise at the duplexer TX port is highest. Include anything between the PA and the duplexer that adds or removes noise.
  • Refer everything to one plane. The noise floor and the leaked noise must be compared at the same point, normally the duplexer RX port. If there is loss between that port and the LNA, refer the noise figure to the RX port. That means it includes the loss.
  • Budget for other paths. The duplexer is not the only route from TX to RX. Board coupling, shared supplies and a mismatched antenna port can all add leakage. These add in power, so the duplexer alone should not consume the whole 10 dB margin if other paths matter.

In plain words · for students and beginners

Duplexer isolation and TX noise in plain words

An everyday analogy

Picture a phone call in a room with a loud fan running next door. A wall sits between the rooms. Your own room already has a faint background hiss, and you cannot get rid of it. The question is how thick the wall must be so the fan noise coming through is clearly quieter than that hiss. If it is, your call sounds the same as before.

In a phone, the fan is the transmitter, the wall is the duplexer (the part that lets the transmitter and receiver share one antenna), and the hiss is the receiver noise floor (the lowest noise level the receiver has on its own).

Where the analogy breaks down: a real wall blocks all sounds about equally. A duplexer blocks some frequencies much better than others. Only its blocking at the receive frequencies matters here. Also, radio noise is spread across a band of frequencies, so we always count it inside a stated bandwidth, here 10 MHz.

The same example in simple steps

Some terms first. dBm is power compared with 1 milliwatt, on a log scale. dB is a ratio, such as a loss. NF (noise figure) is how much noise the receiver adds beyond the unavoidable minimum.

  1. Find the noise floor. Start at -174 dBm per hertz. For 10 MHz, add 70 dB. For NF = 5 dB, add 5. Floor = -174 + 70 + 5 = -99 dBm.
  2. Pick a target. Keep the leaked noise 10 dB under the floor: -99 − 10 = -109 dBm.
  3. Find the isolation. The transmitter makes -60 dBm of noise in the receive band. To bring -60 down to -109, the duplexer must remove -60 − (-109) = 49 dB.
-50-60-70-80-90-100-110-120Power level (dBm)Levels in the 10 MHz RX bandTX noise at PA output, -60 dBmReceiver noise floor, -99 dBmTarget leakage, -109 dBm39 dB10 dB marginRequired isolation49 dB
Level diagram for the 10 MHz example: TX noise at -60 dBm must drop 39 dB to the -99 dBm floor and 10 dB more to -109 dBm, so 49 dB of duplexer isolation is required. Concept drawing.

The picture shows the same steps. From -60 dBm down to the -99 dBm floor is 39 dB. The 10 dB safety margin takes you down to -109 dBm. Together that is 49 dB, the required isolation.

Mistakes beginners make

  • Using the wrong isolation number. A data sheet may list isolation at the transmit frequencies. For noise, you need the value at the receive frequencies.
  • Mixing bandwidths. The transmitter noise and the noise floor must be counted over the same bandwidth. Otherwise the subtraction is meaningless.
  • Aiming just under the floor. If the leaked noise equals the floor, the total noise doubles and the receiver loses about 3 dB of sensitivity. With a 10 dB margin, the loss is only about 0.41 dB.
  • Mixing up units. dBm minus dBm gives dB, a ratio. dBm minus dB gives dBm, a level.

Remember

  • Required isolation = TX noise in the RX band − receiver noise floor + margin.
  • Noise floor = -174 + 10 log10(B) + NF. For 10 MHz and 5 dB, that is -99 dBm.
  • -60 dBm of TX noise with a 10 dB margin needs 49 dB of isolation, measured across the receive band.

Frequently asked questions

How do I calculate the duplexer isolation needed for TX noise?

Take the TX noise power in the RX band at the PA output, subtract the receiver noise floor, and add your margin. With -60 dBm of TX noise, a -99 dBm floor and a 10 dB margin, you need 49 dB.

What is the receiver noise floor formula?

Noise floor in dBm = -174 + 10 log10(B) + NF, with B in Hz and NF in dB. For B = 10 MHz and NF = 5 dB this gives -99 dBm.

Why keep leaked TX noise 10 dB below the noise floor?

Noise powers add. Leakage 10 dB below the floor raises the total noise by only about 0.41 dB. Leakage equal to the floor doubles the noise and costs about 3 dB of sensitivity.

Is TX noise isolation the same as TX carrier isolation?

No. TX noise isolation is the TX-to-RX attenuation at RX-band frequencies. Carrier isolation is measured at TX-band frequencies and deals with blocking. Each needs its own number.

References and further reading

Related guides