The voltage readings are taken at each power level in the spreadsheet and the voltage indicated  by the DVM entered into the third column (C). From the second and third columns (B&C), a xy chart can be constructed that is similar to Figure 1.3. 

Figure 1.3

1.5) Finding the Voltage Correction Constants

    It can be seen in Figure 1.3 that the linearity is poor below an input level of about 200 mV rms. The curve has now become a near square law and this can be used to advantage if a power measurement is being made. However, we are measuring voltage and in order to make the voltage detection more linear using calculation, we first find the constants for the formula that fit the curve. This can be performed in Excel using the LOGEST Function. Simply select the function and enter the DVM reading and the input voltage as the x and y data respectively. Excel will calculate results in the form y=b*m^x which translates into Actual Voltage = b*m^(DVM Reading). Whenever a reading is taken from the voltage detector output, just feed the DVM voltage reading into this formula with the appropriate constants and the correct signal voltage level will be calculated.

1.6) Principle of the Current Detection Circuit

    The input signal to the circuit is passed through a current transformer as can be seen in Figure 1.4. The resultant current in the secondary of the transformer develops a voltage across a damping resistor to ground. This resistor flattens the frequency response and matches the transformer output impedance to the amplifier input impedance. The amplifier produces an output voltage level similar to the voltage level present at the signal input to the current transformer. This ensures that the detection diode on the output of the amplifier has adequate voltage to perform mainly in the linear part of the response curve and works in a similar voltage region to the voltage detection circuit previously described.

Figure 1.4

1.7) Plotting the Current Curve

    The Voltage and Current Detector is connected directly to the output of the Signal Generator, or with an adaptor and is terminated with a 50 Ohm load as shown in Figure 1.2. The Signal Generator output is set to +13 dBm (or 1 Volt rms) at a frequency of 50 MHz. The DVM is connected to the Voltage and Current Detector, current output and should indicate approximately 200 mV. This represents ten times the rms current of 20 mA in the primary of the current transformer ( If the output potentiometer of the detector circuit is adjusted to give a 20 mV reading it will be difficult to get a good reading at low input power levels when the detected dc voltage approaches the noise level of the DVM test leads).

    The input current is calculated from simple Ohms law using the input voltage calculated in the second column (B) of Spreadsheet_01.

I = V / R                               

Change into mA and impedance                             I = (V x 1000) / Z    mA                Formula 1.3

For an input of +13 dBm or 0.9988 Volts this is:-

             I = (0.9988 x 1000) / 50    = 19.98 mA

     We now use Formula 1.3 to calculate the input current for the range of signal generator output voltages in the second column (B) in the spreadsheet. The calculated input current is placed in the fourth column (D). Current readings are taken at each power level in the spreadsheet and entered into the fifth column (E). From the fourth and fifth columns (D&E), a xy chart is generated that should appear similar to Figure 1.5. 

Figure 1.5

1.8 Finding the Current Correction Constants

    We now find the formula that fits the current curve. This can again be performed in Excel using the LOGEST function. Simply select the function and enter the DVM reading divided by ten, and the input current as the x and y data respectively. Excel will calculate the constants for the formula in the form y=b*m^x, which now translates into actual current = b*m^(DVM reading/10). Whenever a reading is taken from the current detector output, simply feed the result into this formula with the appropriate constants and the correct current is calculated.

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