Low-cost RF sniffer finds 2.4-GHz sources
Whether you measure or use RF circuits that operate in the popular 2.4-GHz ISM (industrial/scientific/medical) band, cordless telephones, Wi-Fi access points, Bluetooth devices, and microwave ovens can radiate RF signals, causing unwanted interference. A spectrum analyzer remains the instrument of choice for detecting and identifying interference sources, but analyzers are expensive, bulky, and sometimes not readily available.
The circuit in Figure 1 shows an easily assembled, low-cost, and portable RF "sniffer" that provides a quick and reliable reading of the ambient-RF-signal level in the 2.4- to 2.5-GHz frequency band. At the circuit's heart, a Linear Technology general-purpose LT5534 RF-power detector, IC1, measures RF-signal strengths from –55 to –5 dBm and provides an RSSI (received-signal-strength-indicator) dc-output voltage (Reference 1).
An antenna for this frequency band drives FL1, a Toko filter (Part No. TDFU2A-2450T-10A), which restricts the circuit's passband to 2.4 to 2.5 GHz and limits out-of-band interference. The filter drives IC1, whose internal circuitry comprises a cascade of RF detectors and limiters. The detectors' and limiters' summed outputs generate an accurate logarithmic-linear voltage proportional to the RF input in decibels. A single discrete transistor, Q1, converts IC1's RSSI output to a current that drives a low-current-LED signal-strength indicator. You can connect a digital voltmeter to IC1's RSSI output to provide a digital readout of signal strength or rely on the lighted LED to visually indicate an RF signal. Two 1.5V alkaline batteries or three nickel-cadmium cells provide 3V power for the circuit.
The LT5534's frequency range of 50 MHz to 3 GHz covers the VHF, UHF, 800-MHz-cellular-telephone, 902- to 928-MHz-ISM, 2-GHz-PCS (personal-communications-system)/UMTS (Universal Mobile Telecommunications System), and 2.4-GHz-ISM bands. For the 2.4- to 2.5-GHz range, use a Laird Technologies BlackChip antenna or a Toko dielectric antenna (Part No. DC2450CT1T). To build a sniffer for the 915-MHz band, replace the antenna with Part No. ANT-916-JJB-ST from Antenna Factor and replace the input filter with a Toko 4DFA-915E-10 ceramic filter that provides 26 MHz of bandwidth centered on 915 MHz.