A radio that cannot measure its own output power cannot control it. Power detection is the sense organ of any transmit chain: it feeds the automatic-level-control loop that keeps a cellular transmitter inside its spectral mask, it protects a power amplifier from a mismatched antenna, and it tells a test instrument what it is actually delivering. The part that does the measuring is a logarithmic amplifier or an RMS-responding detector, and it is one of the least appreciated components on the board — until a product fails type approval because the power loop drifted.
Application logic: closing the loop around the power amplifier
The classic arrangement is a directional coupler in the transmit path that taps a small, known fraction of the forward power. That sample is attenuated to bring it inside the detector's input range, and the detector's DC output goes to a controller — usually a microcontroller with an ADC — which compares it against the target set by the system and adjusts the amplifier's gain or bias. The loop is only as good as three things: the coupler's directivity, the detector's logarithmic conformance, and the temperature stability of the whole branch.
Two design choices dominate. The first is whether the detector responds to the envelope or to true RMS. An envelope detector is calibrated for one waveform, so a change of modulation — or a burst-mode transmission — moves the reading; an RMS detector is waveform-independent, which is why it is the default in modern cellular and Wi-Fi chains. The second is dynamic range. A detector with 40 dB of logarithmic range cannot regulate a transmitter that must cover 60 dB of output level; the gap has to be closed with switched attenuation in front of the detector, and the attenuation steps then become part of the calibration. The same measurement branch supports reverse-power detection for mismatch protection, which is how a transmitter avoids damaging itself into a broken antenna. This is the same engineering that shows up in cellular power-amplifier design and in the discussion of what operating past compression actually costs.
On the receive side the same part performs a different job: received-signal-strength indication. Here the requirement is accuracy at low levels and fast response, because the reading drives the AGC loop. It is worth reading how receiver noise figure budgets are built alongside this, because the detector's own noise floor sets how small a signal the loop can still regulate.
New technology in detection and power management
- Wideband RMS detectors covering 10 MHz to 8 GHz and beyond in a 2×3 mm package, removing the narrowband tuning that older logarithmic amplifiers needed.
- Waveform-independent measurement using true-RMS techniques, so a detector reads the same power for CW, LTE, 5G NR and Wi-Fi bursts.
- Integrated controller mode, where the detector compares the measured level against an external set point and drives the amplifier directly, turning a measurement into a closed loop without firmware in the path.
- Temperature compensation and factory-calibrated slope and intercept, which reduces the calibration burden on the production line.
- Single-ended RF inputs that eliminate the external balun, cutting board area and one source of loss in the sense branch.
- Digital power detectors with serial output, allowing several channels to share one bus in multi-antenna systems.
Domestic alternatives: what to analyse before switching
- Compare logarithmic conformance across the full range, not just the total dynamic range; accuracy typically degrades at the extremes.
- Check the response time for both rise and fall. Asymmetric response is normal, but a slow fall time limits how fast your protection loop can react.
- Confirm the temperature behaviour of slope and intercept, which decides whether the system needs temperature compensation in firmware.
- Verify the input impedance environment and whether an external balun or matching network is required.
- Check the output drive capability against your ADC input impedance and reference voltage; a high-impedance load can shift the reading.
- Ask for the reference design and calibration procedure — on detection branch design, the procedure is often more valuable than the part.
Indicative cost comparison: imported originals vs domestic candidates
Price figures below are indicative public-market reference ranges for moderate volumes, compiled from publicly observable listings. They fluctuate with the spot market, package option, grade (industrial, automotive, screened) and order quantity, and must not be read as a quotation. Ask us for a firm price on your exact part numbers and quantities.
| Device class | Common imported reference | Indicative imported unit price | Commonly referenced domestic candidate | Indicative domestic unit price |
|---|---|---|---|---|
| Wideband RMS logarithmic detector, DC–8 GHz, 40–45 dB range | ADI ADL5513 / AD8317 class; Linear Technology LT5534-class | USD 4–14 | Archiwave ARW28340-class (0.01–8 GHz, 40 dB RMS, DFN-8); Relmon MS2351M-class | USD 1.8–6 |
| Logarithmic detector / controller with RSSI and PA control modes | ADI AD8318 / AD8317 class | USD 5–16 | Relmon MS2351 / MS2351M (50 MHz–3 GHz, 35–45 dB range) | USD 2–7 |
| Envelope / TruPwr detector for level measurement | ADI AD8361 / AD8362 class | USD 3–9 | Domestic RMS and envelope detector families (public information is at family level) | USD 1.2–4 |
| Detection branch module (coupler plus detector plus attenuator) | Imported coupler and detector assemblies | USD 10–30 | Domestic coupler plus detector combinations, assembled to your level plan | USD 4–14 |
Two cost notes. First, detection is a low-cost function whose failure is expensive: a transmitter that fails a spectral-mask test or damages itself into a mismatch will cost more than every detector on the board. Second, the measurement branch is only as good as its calibration. When comparing prices, ask whether the cheaper part is supplied with a slope-and-intercept characterisation or whether your production line has to derive it, because that difference often decides the total cost.
Commonly referenced alternative models
| Imported original (reference) | Domestic candidate (public sources) | Why it is referenced |
|---|---|---|
| ADI AD8318 — 1 MHz–8 GHz logarithmic detector / controller | Relmon MS2351M (50 MHz–3 GHz, 35–45 dB typical dynamic range, MSOP8 / DFN8) | Publicly documented as a domestic logarithmic amplifier for RSSI and power-amplifier control loops, with measurement and controller modes. Check the upper frequency limit against your band. |
| ADI ADL5513 / AD8317 class — wideband RMS detector | Archiwave ARW28340 (0.01–8 GHz, 40 dB RMS, DFN-8 2×3 mm) | Publicly documented as a wideband RMS logarithmic detector with a single-ended input and no external balun, positioned for power measurement in cellular, Wi-Fi and instrument equipment. |
| Linear Technology / ADI LT5534-class detector | Domestic wideband RMS detector families (Relmon, Archiwave and others; verify the exact device) | Same measurement role for base-station transmit monitoring. Confirm the usable dynamic range at your modulation and the response time your protection loop needs. |
| ADI AD8361 — TruPwr RMS detector for level measurement | Domestic RMS detector families (family-level information in public sources) | Cross-referenced for waveform-independent level measurement; several domestic devices target the same behaviour without an external coupling capacitor. |
| Temperature-compensated detection assemblies | Domestic detector plus PIN-attenuator combinations | Worth considering when the level plan needs switched attenuation; a domestic PIN attenuator such as the ALM-series parts we stock can sit in front of the detector. |
We supply detector, attenuator, coupler and termination parts as a kit for a complete sense branch, including the AD8361ARTZ-RL7 RMS detector, ALM-32220-TR1G PIN-diode attenuator and ALM-38140-BLKG PIN-diode attenuator, drawn from the Broadcom / Avago collection and the RF and interface chip collection. Tell us the power range you must resolve and we will build the branch around it.
Frequently asked questions
Frequently asked questions
What does a log detector actually measure?
It converts an RF input into a DC output that is proportional to the logarithm of the input amplitude, so a wide dynamic range compresses into a voltage your microcontroller can read with a single ADC channel. In RMS-responding types the output tracks true root-mean-square power, which makes the reading independent of the waveform's peak-to-average ratio — the property that matters when the modulation changes or when you measure a burst.
Why not just use a diode detector?
A simple diode detector is cheap but its output follows the envelope rather than the power, it drifts with temperature, and its dynamic range is limited to a few tens of decibels around the square-law region. Logarithmic and RMS detectors are temperature-compensated and offer 40–60 dB of usable range with a defined logarithmic slope, which is what closed-loop power control needs.
Where are these parts used besides transmit power control?
Receive signal-strength indication and automatic gain control, voltage-standing-wave-ratio and load mismatch protection, power monitoring in RFID and radar, level measurement in test instruments, and health monitoring of an antenna system. Anywhere a system must know how much RF power is present and act on it.
Can you supply detectors together with the couplers and attenuators around them?
Yes. We quote logarithmic and RMS detectors, coupler and attenuator parts, terminations and the surrounding gain blocks, from imported brands and from domestic suppliers. Give us the frequency band, the power range you must resolve and the required accuracy, and we will propose a complete detection branch.
About us & trademarks: JTDZ Tech (Shenzhen Jingteng Electronics Technology Co., Ltd.) is an independent distributor of electronic components. We are not an authorised distributor or representative of the imported manufacturers named in this article. Manufacturer, brand and part names are used for identification purposes only; all trademarks remain the property of their respective owners. Cross-reference information in this article is based on publicly available sources, is subject to change, and is not a substitute for your own engineering validation. Always verify specifications against the manufacturer's datasheet before design-in or purchase.
