Diodes D_F_1 and D_R_1 should be placed on the same board as the directional coupler to minimize the effects of stray inductance from long wires.
Diode D_F_1 rectifies the RF forward voltage sample V_F_0 into the DC voltage sample V_F_1. Diode D_R_1 rectifies the RF reflected voltage sample V_R_0 into the DC voltage sample V_R_1. Germanium 1N34A diodes are used to minimize losses. However, both RF voltages will be under 1V when transmitting at low power levels, and this causes a substantial diode drop loss.
Both D_F_1 and D_R_1 must be matched in order to enable the SWR meter to provide good accuracy at high SWR values. The diode drop loss varies from one individual diode to another, and the smaller the voltage to be rectified, the greater the difference between the rectified voltages. Instructions on how to match these diodes are provided later in this article.
The next step is to compensate for the diode drop losses in the rectifiers with logarithmic non-inverting op amps. Two noninverting logarithmic LM324 operational amplifiers are configured to provide gain that varies according to the input voltage levels. The resistances of feedback diode D_F_2 and resistor R_F_2 determine the gain of the forward voltage compensating amplifier, and the resistances of feedback diode D_R_2 and resistor R_R_2 determine the gain of the reflected voltage compensating amplifier. When V_F_1 is low, D_F_2 has a high resistance, and the forward voltage compensating amplifier has a high gain. When V_R_1 is low, D_R_2 has a high resistance, and the reflected voltage compensating amplifier has a high gain. When V_F_1 is high, D_F_2 has a low resistance, and the forward voltage compensating amplifier has a gain near unity. When V_R_1 is high, D_R_2 has a low resistance, and the reflected voltage compensating amplifier has a gain near unity.
D_F_2 and D_R_2 must also be matched in order to enable the SWR meter to provide good accuracy at high SWR values. Diode resistance at a given voltage varies from one individual diode to another, and the smaller the voltages involved, the greater the difference between one diode and another.
R_F_2 and R_R_2 must both be the same value, but the optimum value of these resistances CANNOT be known in advance and will vary from one individual QROP meter to another. Instructions on how to determine the optimum resistance value are provided later in this article.
Capacitors C_F_2 and C_R_2 are used to minimize oscillations. When there is no significant reflected or forward voltage sample to amplify, the gain will be high due to the large diode resistances. Adding these capacitors in parallel with the diodes will limit the amplifier gain for AC signals. Omitting these capacitors will allow unwanted AC noise (such as 60 Hz noise) to amplify. The AC noise may cause intermittent unwanted oscillations and cause parts of the rest of the QROP Meter to behave strangely. As is the case with the power supply decoupling capacitors, watch your capacitor polarities.
.01uF decoupling capacitors are needed to minimize RF ripple in V_F_3 and V_R_3.
I use an LM324 op amp in the active rectifier due to its low cost, its widespread availability, its ability to withstand a 10V input in the absence of a power supply, and an input range that includes ground.