Handset RF Front End Explained: Antenna to Transceiver
Receive runs antenna, switch, filters, LNA, transceiver. Transmit runs transceiver, PA, filters, switch, antenna. Loss ahead of the first LNA adds dB for dB to noise figure: 1.5 dB of loss plus a 1 dB receiver gives 2.5 dB.
For practitioners
The map: two paths sharing one antenna
A phone front end is the set of RF parts between the antenna and the transceiver. The transceiver is the chip that converts between RF and baseband. The front end has two jobs that pull in opposite directions. On receive it must handle a very weak signal without spoiling it. On transmit it must push a much stronger signal out to the antenna without wasting it or leaking it into other bands.
The two paths, in signal order:
- Receive: antenna → switch or multiplexer → band filters → LNA → transceiver.
- Transmit: transceiver → power amplifier (PA) → filters and switch → antenna.
The antenna is shared. The switch or multiplexer decides which path is connected to it, and in which band.
Block by block
Antenna. It converts between a guided wave on the board and a radiated wave. Everything else in the front end exists to serve it. A mismatch here shows up as reflected power, which is the S11 of the antenna port. See the S-parameter page for the definitions.
Switch or multiplexer. It routes the antenna to the right path. Its insertion loss sits directly in both the receive and the transmit path, so it has to be low. Its isolation, the signal that leaks between ports, has to be high.
Band filters. Filters pass the wanted band and reject unwanted signals outside it. On receive they keep strong out-of-band signals away from the LNA. On transmit they keep the PA's out-of-band products from reaching the antenna. Each filter has a passband insertion loss and a stopband rejection, and both are two-port quantities, S21 in the passband and in the stopband.
LNA. The low noise amplifier amplifies a weak received signal while adding little noise. It is placed as early as the filters allow, because its gain reduces the effect of every noisy stage after it.
Power amplifier. The PA raises the transmit signal power. It works at much higher power than anything on the receive side, so its efficiency and its output spectrum both matter. Its output goes through filters and the switch, which cost some of that power.
Transceiver. On receive it takes the amplified signal and converts it down for demodulation. On transmit it generates the modulated signal that drives the PA.
Why the order matters
The order is not arbitrary. Each rule follows from how gain and loss combine.
Gains and losses in dB add. A signal level in dBm rises or falls by the sum of the stages it passes through. The dB and dBm page covers this in detail.
On transmit, any loss after the PA subtracts from the power that reaches the antenna. A PA that delivers 25 dBm into a filter and switch with a combined 1.5 dB of loss leaves 23.5 dBm at the antenna. The lost 1.5 dB is dissipated as heat in those parts. That is why transmit-side filters and switches are chosen for low loss.
On receive, the same loss does something worse. It does not only lower the signal. It adds to the noise figure.
Noise figure and the first stage
For a passive, matched component ahead of the first LNA, at the 290 K reference temperature, the insertion loss adds directly in dB to the receiver noise figure:
The Friis formula for a cascade explains why the position matters. Each stage's noise contribution is divided by the total gain ahead of it:
A lossy part ahead of the LNA has a gain below 1, so it divides by a number below 1 and makes every later contribution larger. A lossy part after the LNA is divided by the LNA gain, so its effect is reduced.
Worked example
Take a receiver whose noise figure is 1 dB with nothing in front of it. Place a switch and filter with 1.5 dB of combined insertion loss ahead of it.
NF_total = 1.5 dB + 1 dB = 2.5 dB.
The 1.5 dB of loss costs 1.5 dB of sensitivity, dB for dB. The same loss placed after a 15 dB gain LNA would be reduced by that gain and would add far less. This is the reason designers fight for every tenth of a dB in the switch and filter ahead of the LNA, and why the LNA sits as close to the antenna side as the filtering allows.
Filters must still come before the LNA in most designs, because they protect it from strong out-of-band signals. Low loss and good rejection are in direct tension. The order sets the tradeoff.
Putting it together
Reading a front end from left to right, ask three questions at each block. What does it add or remove in dB? Where does it sit relative to the first gain stage? What does it keep out? The answers explain nearly every layout choice in the chain.
In plain words · for students and beginners
The front end in plain words
An everyday picture
Think of a front desk at an office building. One door (the antenna) serves visitors going in and staff going out. A doorkeeper (the switch) sends each person the right way. A security check (the filters) lets in only the people on the list. A listening aid (the LNA) helps the receptionist hear a quiet visitor. On the way out, a loudspeaker (the power amplifier) makes the announcement carry.
The picture breaks down in one place. A real door does not add noise to what the receptionist hears. In a radio, every part you pass through also adds its own hiss, and that hiss is what the engineers are fighting.
The same example in simple steps
- A receiver on its own has a noise figure of 1 dB. That number says how much extra hiss it adds. Lower is better.
- Put a switch and filter in front of it. Together they lose 1.5 dB of the signal.
- The loss comes before the amplifier, so nothing has boosted the signal yet. The loss is added straight onto the noise figure.
- Total: 1.5 dB + 1 dB = 2.5 dB.
- On the transmit side the same loss after the power amplifier simply takes power away. If 25 dBm goes in, 23.5 dBm comes out.
Mistakes beginners make
- Thinking order does not matter. A loss before the LNA and a loss after it are not equal. The first hurts far more.
- Mixing up dB and dBm. dB is a ratio, a gain or loss. dBm is a power level. You add dB to dBm to get a new dBm.
- Forgetting the filters' job. Filters are not only there to clean up your own signal. They also protect the receiver from strong signals in other bands.
Remember
- Receive path: antenna, switch, filters, LNA, transceiver. Transmit path: transceiver, PA, filters, switch, antenna.
- Loss before the first LNA adds dB for dB to the noise figure.
- Loss after the PA takes power away from the antenna.
Related pages on RF View
Frequently asked questions
What is the order of blocks in a phone RF front end?
On receive: antenna, switch or multiplexer, band filters, LNA, transceiver. On transmit: transceiver, power amplifier, filters and switch, then the antenna.
Why is the LNA placed before most of the receiver?
The received signal is weak. The LNA amplifies it while adding little noise, so later stages, which add more noise, matter much less.
How does insertion loss before the LNA affect noise figure?
For a passive matched component at 290 K, its loss in dB adds directly to the receiver noise figure. NF_total = L(dB) + NF_receiver(dB).
What do the filters in a handset front end do?
They pass the wanted band and reject unwanted signals outside it, including strong signals that could otherwise overload the receiver.