The circuit is quite straightforward as the bulk of the functions are performed by IC1 & IC2.
The device power can be from either a battery or separate supply. It should be in the range of 7V to 12V DC.
After the Power On/Off switch, and reverse polarity diode D4 the +5V rail for IC2 is derived by R1 & D1. The CAL trimmer potentiometer is also supplied from this rail.
RF signals in the range of 1-500MHz applied to the RF_INPUT. Diodes D2 & D3 provide some input protection, however the maximum input level should not exceed 1W.
IC2 takes the RF input signal, amplifies it, and produces a DC voltage from Pin 4.
IC2 the AD8307 is a precision logarithmic amplifier, so the DC output from the IC is logarithmically related to the input signal level.
IC1 is a display driver IC.
The DC voltage from the AD8307 is applied to SIGIN Pin 5.
IC1 consists of a series of voltage comparators which drive the individual display LED's.
One input to each comparator is the SIGIN voltage, and the other from an potential divider internal to the IC.
RHI Pin 5 supplies the reference voltage to the internal potential divider, and the measurement range is fixed by the external potential divider of R4 & R5 into ADJ Pin 8.
The CAL potentiometer is adjusted until, with no signal input, until only LED 1 (Green - Power On) is lit.