CN0366

The circuit shown in Figure 1 uses an ADL6010 RF and microwave power detector to convert an ac waveform to a scaled output voltage that corresponds to the amplitude of the input waveform. The output voltage is linear-in-voltage, having a slope with units of V/V rms. The ADL6010 can extract RF signal envelopes with bandwidths of up to 40 MHz. However, in most power meter applications, the output voltage is a settled dc value that represents the amplitude of the input waveform.

The AD7091R 12-bit, 1 MSPS ADC samples the detector output, and the data is processed through a data capture board and sent to a PC for further processing and analysis. The ADC has an internal 2.5 V reference voltage that can be used to set the full-scale voltage. The internal reference can be overridden if a larger full-scale voltage is needed.

The system must be calibrated because the output voltage is dependent on the frequency of the input waveform. A correction factor is also needed when measuring modulated signals. PC based software with a simple graphical user interface is provided to perform the computations (CN-0366 Evaluation Software).

Power Detector

The ADL6010 is 45 dB envelope detector that operates from 500 MHz to 43.5 GHz. It has a linear in volts slope of approximately 5.9 V/V rms and an absolute detector input range from −30 dBm to +15 dBm or −43 dBV to +2 dBV in a 50 Ω system. The detector cell uses a proprietary eight Schottky diode array followed by a novel linearizer circuit that creates a linear voltmeter with an overall scaling factor (or transfer gain) of nominally ×5.9 relative to the rms voltage amplitude of the input. With an output averaging capacitor, the ADL6010 can detect a signal with a varying envelope, but a correction factor must be used to compensate for the change in output voltage for the same given input power. The output voltage is related to the rms input voltage by

where:

V_OUT is the voltage on the V_OUT pin.
Slope is approximately 5.9 V/V rms at 10 GHz.
V_RFIN is the rms input voltage.
Intercept is the y-axis value that the data crosses if extended.

Figure 2 shows a functional block diagram of the ADL6010.

Figure 3 shows the variation in the transfer function with frequency. There is approximately 300 mV of voltage deviation in the output between 1 GHz and 40 GHz. The temperature variation is less than ±0.5 dB over the entire frequency range. Figure 4 and Figure 5 show the temperature variation at 10 GHz and 40 GHz, respectively.

Analog-to-Digital Converter

The AD7091R is a 12-bit, 1 MSPS ADC with an input voltage range between 0 V and V_REF, where the reference voltage is either provided by the internal 2.5 V reference or by an external reference that overrides the internal reference. The external reference can be as high as 5 V. For a 2.5 V full-scale voltage (V_REF = 2.5 V), the LSB size is

The output voltage of the ADL6010 is approximately 25 mV to 4 V; therefore, a 200 Ω/340 Ω resistor divider with an attenuation of approximately 1.6 reduces the amplitude of the signal so that it is always within the range of the AD7091R when using the internal 2.5 V reference.

Data Analysis

The EVAL-SDP-CB1Z system demonstration platform (SDP) board is used in conjunction with software based on the AD7091R evaluation board control software to capture the data being sampled by the ADC. The software has a power meter readout and calibration option. The power meter display shows the power applied to the input of the ADL6010. To take an accurate power measurement with the ADL6010 and the AD7091R, apply two known input powers at different levels to the input of the ADL6010, then read the corresponding output ADC code. These four values make up two points on a plot and must be stored for later use in the calibration procedure. The two points are

• Point 1: (V_LOW, CODE_LOW)
• Point 2: (V_HIGH, CODE_HIGH)

From these two points, a slope and an intercept can be found and used to calibrate the system at the particular frequency of operation.

Figure 6 shows the software power level display.

System Transfer Function

The slope and intercept of the system from the input of the detector to the output of the ADC are

where:

Slope_SYS is the system slope.
CODE_HIGH, CODE_LOW are the high and low code outputs, respectively, from the ADC.
V_HIGH, V_LOW are the high and low RF voltages, respectively.
INT_SYS is the system intercept.

The overall system transfer function is

where V_IN is the rms voltage of the input RF signal.

Solve for V_IN using

Therefore, the power in dBm, PIN, can be expressed as

For a 50 Ω input impedance, this equation simplifies to

User Calibration Algorithm

The CN-0366 Evaluation Software performs a one-time calibration at the particular frequency of operation. Calibration is achieved via the Calibration tab, shown in the window of Figure 6. The calibration routine is as follows:

1. Set the RF power to high level (V_HIGH).
2. Measure the code from ADC (CODE_HIGH).
3. Set the RF power to low level (V_LOW).
4. Measure the code from ADC (CODE_LOW).
5. Calculate the system slope (units of codes/V).
6. Calculate the system intercept (units of codes).
7. Store the slope and intercept as calibration coefficients.
8. Measure the ADC code with an arbitrary input RF power.
9. Calculate the input power using the code, slope, and intercept.

Measurement Results for Complete Signal Chain Including ADC

Using the CN-0366 Evaluation Software, measurements were taken across several frequencies. Each frequency was calibrated before the measurement took place. The results are shown in Figure 7, Figure 8, and Figure 9. In Figure 7, note the dependency of the error on the calibration power levels. Choosing the proper calibration levels may require some trial and error.

Figure 7 through Figure 9 show the input power measured vs. the actual power applied to the input of the ADL6010 and the error between the two. Data was taken over temperature at 0°C, 25°C, and 70°C, and the results are shown in Figure 10, Figure 11, Figure 12, and Figure 13 for frequencies of 1 GHz, 10 GHz, 20 GHz, and 30 GHz. Note that some measurements were taken using an early version of the software that did not have an averaging feature, which is why there is a large ripple at the lower input power levels.

Summary of Equations

The following is a summary of the equations in the CN-0366, as well as additional equations to provide insight into the function of this power meter circuit.

The transfer function of the ADL6010 is given by

where:

V_OUT is the dc output voltage of the detector.
Slope_DET is the gain/slope of the detector, in V/V rms.
V_IN is the rms voltage of the input RF signal.
INT_DET is the intercept of the detector, in volts.

Solve for V_IN using

The ADC transfer function (when being driven by the ADL6010) is given by

CODE = V_OUT/LSB
LSB = V_REF/4096

where:

CODE is the ADC output code, a unitless number that can range from 0 and 4096.
V_REF is the full-scale reference voltage of the ADC, in volts. LSB is the smallest quantized voltage that the ADC can resolve, in V/step or V/bit.

Solve for V_OUT using

V_OUT = CODE × LSB

Substituting this expression in the previous V_IN equation yields

Referring everything back to input power yields

where R is the impedance on the RFIN pin of the ADL6010. Substituting V_IN into the power equation yields

Simplifying the equation yields

This equation is in terms of the ADL6010 detector output slope and intercept, which is useful for conceptual purposes to show the interactions of the system. However, for practical purposes, the transfer function of the system is required in terms of the overall system slope and intercept, where the slope has units of codes/V and the intercept has units of codes. The final transfer function can be derived as follows.

The derivation of the system transfer function in terms of system slope (slope_SYS) and system intercept (INT_SYS) begins with Equation 2, which has the input power in terms of detector slope (slope_DET) and detector intercept (INT_DET). This derivation is achieved by multiplying both the numerator and denominator of the V_IN expression (Equation 2) by 1/LSB, as follows:

Substituting this expression for V_IN into Equation 2 yields Equation 1.

INT_DET has units of volts, Slope_DET has units of V/V rms, and LSB has units of V/codes. Multiplying by 1/LSB converts INT_DET into its equivalent ADC code and converts the detector slope into the system slope with unit of codes/V rms, yielding the following relations:

INT_SYS = INT_DET/LSB
Slope_sys = Slope_DET/LSB

A complete set of design files, including schematics, layouts, Gerbers, and bill of materials for the CN-0366 circuit are available in the CN-0366 Design Support Package.