RAJNI ELECTRONICS SERVICES

Low Level (L²) RF Detector

(VU2PPP)

RF enthusiasts have always been on lookout for simple RF Detectors for detecting low level RF signals. Traditional RF detectors using diodes have been in use since ages but these have limited use when it comes to detecting signals in the range of tens of milli-volts.

I had designed a simple low level RF detector using FET around 1977, naming it ‘L-Square’ detector and using in a variety of applications. Recently I used this for null detection in my Antenna Impedance Analyzer and thought of sharing it with radio hobbyists and professionals.

Fig.1 shows the operating principle of low-level FET detector capable of detecting signals down to less than 10mV levels with ease.

The FET is biased at Pinch-off, where it passes very little current, allowing its source to drop to a near zero level. The positive portion of the incoming signal pulls the device into conduction and rectifying effect is obtained.

The graph shows that the input signal of even less than 10 mV produces detectable current in the meter and the detection is quite linear up to about 15dbm.

These properties make the L² detector very useful for a variety of applications like RF Probes, VSWR meters, Field Strength meters, Null Bridges and scores of other applications requiring detection of RF signals.

For single supply (battery) operation, this detector has been modified to provide negative bias to FET for pinch-off by biasing the source positive wrt to gate. The variable positive voltage bias also serves as the sensitivity control for the current meter.