RF Log Detector Symbol
Definition: The RF Log Detector symbol represents an integrated circuit block that converts RF or IF signal power into a DC output voltage proportional to the logarithm of the input power, providing a wide-dynamic-range received-signal-strength indicator (RSSI) or power-measurement function in circuit diagrams.
Also known as: RF log detector, RSSI detector, log detector IC, AD8307 detector, power detector, logarithmic detector.
What the RF Log Detector symbol means
The RF Log Detector symbol denotes an analogue IC or functional block that accepts an RF or IF signal at its RF In pin, internally detects and logarithmically compresses the signal amplitude, and delivers a calibrated DC voltage at its VOUT pin that is linear in decibels. The GND pin provides the circuit reference. Representative devices include the Analog Devices AD8307 (500 MHz, 92 dB range) and the AD8310.
In schematic diagrams the symbol marks the point where an RF chain transitions to a measurable DC level for power monitoring, automatic gain control (AGC), or spectrum analysis. The log-linear output simplifies firmware: a linear ADC reading maps directly to power in dBm without floating-point computation.
How to identify the RF Log Detector symbol
The symbol is drawn as a rectangular functional block labelled 'RF Log Detector', 'Log Det', or by the specific IC part number (e.g. 'AD8307'). Three pins are shown: RF In on the left (the high-frequency signal input), VOUT on the right (the DC log-linear output), and GND at the bottom (signal/power ground). Supply voltage (VCC or V+) may be shown on top but is sometimes omitted in block diagrams.
Function in a circuit
The RF log detector internally contains a cascade of limiting amplifiers and rectifier cells. Each amplifier stage contributes to the detection of signals over a segment of the dynamic range, and a summing stage combines all rectified outputs to produce a current proportional to log(P_in). A transimpedance stage converts this to a voltage at VOUT. The result is a DC voltage linearly related to input power in dBm, with a calibrated slope (e.g. 25 mV/dB for the AD8307) and intercept (the input power at 0 V output, typically around −84 dBm).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated symbol for an RF log detector; the device is represented as a functional block per IEC 60617-13 (amplifier and signal-processing elements), with a text label indicating its logarithmic detection function. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 likewise provides no dedicated glyph; the RF log detector appears as a general functional block with a text qualifier such as 'LOG DET' or the IC part number. |
| Key difference | Both IEC and ANSI standards treat the RF log detector as a labelled functional block; there is no difference in glyph form between the two standards for this component. |
Terminals / pins
| Pin | Name |
|---|---|
| rf_in | RF In |
| vout | VOUT |
| gnd | GND |
Typical values
Frequency range: DC to 500 MHz (AD8307) or up to several GHz for newer devices. Dynamic range: 92 dB (AD8307). Output slope: 25 mV/dB. Output voltage range: 0.4–3.5 V (AD8307, 5 V supply). Supply voltage: 2.7–5.5 V. Input impedance: 1.1 kΩ differential (AD8307).
Where the RF Log Detector symbol is used
- RF receiver RSSI circuits in mobile, Wi-Fi, and IoT transceivers for automatic gain control
- Spectrum analysers and power meters requiring calibrated power readout in dBm
- Radar and sonar systems measuring echo return power across a large dynamic range
- Cable TV (CATV) signal-level monitoring and alignment equipment
- Antenna-beam-forming systems where individual element power must be measured
- Microwave link and satellite ground-station receiver power monitoring
Example
In an AD8307-based RF power meter, the RF In pin of the log detector symbol connects through a 52.3 Ω termination resistor to the RF source; VOUT goes to an ADC input on a microcontroller; GND connects to the PCB ground plane. The microcontroller reads the ADC voltage, applies the calibrated 25 mV/dB slope and the device intercept (approximately −84 dBm at 0.4 V), and displays the input power level in dBm on an LCD, covering the range −75 dBm to +17 dBm.
Key facts
- The RF Log Detector symbol represents an IC block that converts RF input power to a DC voltage linearly proportional to power in dBm, providing a wide-dynamic-range RSSI output.
- The device operates on the successive-detection principle: a cascade of limiting amplifiers and rectifier cells each contribute to the log-linear output over a portion of the dynamic range.
- The AD8307, the most widely referenced RF log detector IC, covers 500 MHz with a 92 dB dynamic range and produces a 25 mV/dB output slope from a 5 V supply.
- Three key pins define the RF log detector symbol: RF In (signal input), VOUT (DC log-linear output), and GND (reference ground).
- The output slope and intercept are temperature-compensated in modern IC log detectors, maintaining calibration accuracy across −40 °C to +85 °C.
- No dedicated IEC 60617 or ANSI Y32.2 / IEEE 315 glyph exists for an RF log detector; it is represented as a labelled functional block in both standards.
- RSSI (Received Signal Strength Indicator) is the most common application: the VOUT reading in volts divided by the slope in V/dB gives power in dBm relative to the device intercept.
Frequently asked questions
What does the RF log detector symbol mean in a circuit diagram?
The RF log detector symbol represents an IC or functional block that measures RF or IF signal power and outputs a DC voltage linearly proportional to the input power in dBm. It is used to provide an RSSI (received signal strength) reading, enabling automatic gain control or power monitoring across a wide dynamic range (typically 60–100 dB).
What does the RF log detector symbol look like?
The RF log detector symbol appears as a rectangular block labelled 'RF Log Detector', 'Log Det', or by part number (e.g. 'AD8307'). It has three primary pins: RF In on the left for the high-frequency signal, VOUT on the right for the DC output, and GND at the bottom for circuit ground. The symbol is visually similar to a functional amplifier block.
What are the pins on an RF log detector IC?
The primary pins are RF In (the RF or IF signal input, typically 50 Ω or matched to the source), VOUT (the DC output voltage proportional to input power in dBm), and GND (circuit ground). A supply pin (VCC or V+) is also present but is often omitted from block-diagram representations.
What is the output slope of an RF log detector and what does it mean?
The output slope defines how many millivolts the DC output changes per dB of RF input power change. For the AD8307 this is 25 mV/dB. A 10 dB increase in input power produces a 250 mV rise in VOUT. This linear-in-dB characteristic allows an ADC reading to be converted to power in dBm by a simple arithmetic calculation: P_dBm = (V_out − V_intercept) / slope.
What is the difference between an RF log detector and a log amplifier?
An RF log detector is specifically designed to measure power at RF/IF frequencies and outputs a DC voltage indicating signal level in dBm; it includes envelope detection inside the device. A log amplifier (log amp) typically processes a DC or low-frequency signal, producing an output proportional to log(V_in), and does not inherently include RF rectification.
What standard defines the RF log detector symbol?
Neither IEC 60617 nor ANSI Y32.2 / IEEE 315 provides a dedicated glyph for an RF log detector. Both standards represent it as a general functional block (IEC 60617-13) with a text qualifier such as 'LOG DET' or the IC part number labelling the block's function.
What is a common RF log detector IC used in hobbyist and professional circuits?
The Analog Devices AD8307 is the most commonly referenced RF log detector IC in both professional and hobbyist schematics. It operates from DC to 500 MHz, provides a 92 dB dynamic range, and outputs 25 mV/dB from a single 2.7–5.5 V supply, making it easy to interface with Arduino or STM32 ADC inputs.
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