Some radio amateurs are very keen on accurate RST reports, others (mostly the VHF/UHF fraternity) never look at the S-meter on the receiver, and are satisfied as long as they can hear the other station. This circuit is the first group.
Traditionally, one S-point corresponds to a 6-dB increase in signal strength, while ‘S9’ is defined as 50 uV into 50 K2. Unfortunately, very few receivers these days have a calibrated S-meter, hence the confusion among radio amateurs about the interpretation of the signal strength reports they exchange and write on QSL cards.
The logarithmic-to-linear converter contained in the NE604 from Valvo (Philips Components) is used here to build an accurate S- (signal strength) meter for short-wave receivers. The amplifier in the NE604 is tuned to the intermediate frequency (IF) of the receiver with the aid of Ll and C2. Here, the circuit is dimensioned for an IF of 455 kHz, which is applied to input capacitor C1.
The output of the field strength detector in the NE604 supplies a current of 0 to 50 pA at pin 5. This current is converted into a voltage of 0-5 V by a 100 k.C2 resistance, R2+R3. Note that two E96-series 1% resistors are used here, plus a diode, D1, instead of a single 100 kΩ resistor. This is done to compensate temperature effects which would cause degrading of the linear output voltage. If the specified E96 1.1 resistors cannot be obtained, R2 may be replaced by a parallel combination of two 120 Icf2 1% resistors, and R3 by a parallel combination of a 39 Icf2 1% resistor, and a 1 lic2 1% resistor. It should be noted that the usable range of the log-to-lin convener in the NE604 is roughly from 5 pA to about 40 pA of output current, corresponding about 70 dB, or 0.5 V to 4 V at pin 6 of IC2.The lower level is caused by the background noise of the IF amplifier in the *NE604, and the upper level by limiting and saturation effects. Fortunately, the effective range of the converter is large enough for the present applications, bearing in mind that S-meter readings lower than S3 are rare and of little meaning in the short-wave bands.
Components R4, C9 and C10 suppress ripple and noise. Opamp IC2 is set to provide unity gain, i.e., its output voltage is 0 to 5 V. The moving-coil meter is connected between two presets. P1 is adjusted until the meter reaches full deflection at a voltage of 4.5 V measured at pin 6.
Parts list
Resistors:
R1 = 5.6 kΩ
R2 = 60.4 kΩ(E96 – see text)
R3 = 40.2 kΩ(E96, see text)
R4, R5 = 1 MΩ
R6 = 2.2 kΩ
P1 = 10 kΩ preset H
P2 = 500 Ω preset H
Capacitors:
C1 = 10 nF
C2= 12 nF
C3, C4-C7, C9, C13, C14 = 100 nF
C9 = 47 uF. 16 V, radial
C10 = 4.7 nF
C11, C12 = 1 nF
Semiconductors:
D1 = 1N4148
Integrated circuits:
ICi = NE604A
IC2 = CA3130E
Miscellaneous:
S1 = on-off push-button
L1 = 10 uH choke, axial
M1 = MC meter, 1 mA
