Circuit functions and advantages

The circuit shown in Figure 1 accurately measures return loss in wireless transmitters from 1 GHz to 28 GHz without requiring system calibration.

The design is implemented on a single circuit board and uses a non-reflective RF switch, a microwave RF detector and a 12-bit precision analog-to-digital converter (ADC). In order to evaluate the circuit over the widest possible frequency range, a two-port directional coupler with an SMA connector was used instead of a narrowband surface-mount directional coupler.

The circuit can measure return loss up to 20 dB over an input power range of 25 dB (return losses in excess of 20 dB can be measured over a smaller input power range).

A unique feature of the circuit is that the return loss can be calculated using a simple ratio of the digitized voltages from the RF detector, eliminating the need for system calibration.

Circuit description

An RF signal in the 1 GHz to 28 GHz range is fed through an RF coupler (Marki Microwave C10-0226) to a matched 50 Ω load or antenna, as shown in Figure 1. The forward and reverse coupling ports connect to the HMC547, which is a single-pole double-throw (SPDT) non-reflective switch. The switch input switches between the forward and reverse coupled ports while terminating the opposite port at 50 Ω so that both coupled ports always see a 50 Ω load.

The output port of the RF switch drives the ADL6010, a microwave RF detector that operates from 500 MHz to 43.5 GHz. The output voltage of the detector is proportional to the amplitude of the input signal. The ADL6010 is a linear V/V detector with a nominal slope of 2.1 V/V.

The AD7091R 12-bit ADC samples the power detector output voltage at 1 MSPS. (Lower sampling rates can also be used, resulting in lower power consumption in the ADC).

The AD7091R converts the analog voltage into a digital code. The EVALSDP-CB1Z (SDP-B) interface board then uses Serial Peripheral Interface (SPI) communication to control the ADC and send the results to a computer for system evaluation and return loss calculations. Then, the VSWR, return loss, and reflection coefficient are calculated using the ratio of the forward coupling voltage sampled by the ADC to the reverse coupling voltage.

Return loss calculation

The following derivation shows the relationship between the ratio of forward and reverse voltages and system return loss. This relationship plays a key role in the calibration-free nature of the system.

The system transfer function of the detector in its linear operating region can be expressed by the well-known straight-line equation:

Where:
m is the slope.
c is the intercept.

Using real circuit parameters,

As mentioned before, m is nominally 2.1 but may vary depending on frequency and device. The value of c is usually close to zero.

Rewrite Equation 1 in terms of V _{IN ,}

Convert the equation to power,

Then convert to dBm,

If ADC is included, the equation becomes

Where:
m' is the slope of the detector and ADC combined signal chain.
c' is the intercept of the combined signal chain of the detector and ADC.

Return loss is the difference between forward and reverse power, in dBm:

Since c' is close to zero and CODE _F and CODE _R are generally much larger than c', the formula can be simplified as

The derivation in this section shows that return loss can be calculated without calibration because the formula does not include the slope (m') or intercept (c') of the signal chain.

RF switch

The HMC547 is a non-reflective SPDT RF switch with a frequency range from DC to 28 GHz. As shown in the block diagram of Figure 2, the switch internally terminates either input at 50 Ω while the other input feeds the RFC output. The fast switching time of the switch is typically 6 ns. The A and B logic inputs of the switch are controlled by negative voltage logic with a high level of −5 V and a low level of 0 V. The HMC547 data sheet includes a recommended control circuit. The circuit consists of a 5.1 V Zener diode level shifter driving a 74LV04AD inverter. The inverter's supply voltage range is −5 V to 0 V, not 0 V to +5 V. The complete power circuit is shown in the detailed schematic included in the CN-0387 Design Support Package at www.analog.com/CN0387-DesignSupport .

Power detector

The ADL6010 power detector has linear V/V characteristics, which is key to this application. To drive the device, a +5 V DC voltage is applied to the VPOS pin and rounded to the COMM pin, as shown in Figure 3.

As shown in Figure 4, the output voltage changes with frequency. This change in the transfer function with frequency does not have any adverse effect on circuit performance because the return loss calculation relies on a ratiometric calculation at a specific frequency.

Analog to Digital Converter

The AD7091R is a 12-bit successive approximation register (SAR) ADC with throughput rates up to 1 MSPS. Although an ultra-accurate external voltage reference can be used, this is not required for this application. In this circuit, an internal reference voltage of 2.5 V is used, resulting in an LSB size of

Since the output voltage of the ADL6010 can reach a maximum value of approximately 3 V, this voltage must be attenuated using a 200/340 resistor divider between the detector and the ADC, as shown in Figure 1. This resistive divider has a nominal attenuation ratio of 1.6.

Directional coupler

Directional couplers couple a portion of the forward or reverse signal to the power detector for measurement. Generally speaking, a coupler has 4 ports, as shown in Figure 6.

In the configuration of Figure 6, the input signal is coupled to port 4, and port 3 is terminated at 50 Ω to achieve non-reflective coupling of the signal. If port 4 is terminated at 50 Ω instead of port 3, the reflected signal will be coupled to port 3.

In this circuit, the 50 Ω termination shown earlier for directly connecting the ports is not used. Instead, both ports feed into the RF switch input. Therefore, the coupler can be considered bidirectional because the 50 Ω termination resistance is applied internally by the HCM547 to either port 3 or port 4, depending on the state of the switch.

The RF coupler selected for this circuit is the Marki Microwave C10-0226 stripline coupler. This coupler has a 10 dB coupling capability, meaning the coupled signal is 10 dB less than the input signal. This circuit uses a directional coupler with an SMA connector to demonstrate its operation over the widest possible frequency range. Surface-mount couplers can also be used; however, these devices generally have a narrower frequency range.

data analysis

The EVAL-SDP-CB1Z System Demonstration Platform (SDP) board is used with the evaluation software to capture data sampled by the ADC.

The software calculates the return loss using Equation 8 derived previously. Both reflection coefficient and VSWR are derived from this equation.

Figure 7 shows the results display pane of the software GUI.

Detector Sampling Strategy

In order to accurately measure the return loss of a system, when measuring forward and reverse voltages, the time delay between the forward and reverse measurements must be short. Figure 8 shows the sampling sequence performed during continuous sampling.

When the RF switch receives a signal to switch the switch, the switch position changes, and the forward or reverse coupling port signal is fed into the power detector. In the return loss calculation step, the average of 500 forward samples and 500 reverse samples is calculated, and the return loss is calculated based on the ratio of the average forward and reverse voltages.

The sampling rate of the ADC is 1 MSPS. Therefore, measuring 500 samples takes 500 μs. It takes approximately 400 μs to switch bits using the general-purpose input/output (GPIO) of the SDP-B interface between forward and reverse cycles. The timing diagram is shown in Figure 9.

Return loss, reflection coefficient, and VSWR are calculated using the average of the forward and reverse voltage measurements. To clearly read the results before updating, the 50 samples are averaged before displaying the results on the GUI results pane.

Complete documentation for the EVAL-VSWR-SDZ board, including schematics, layout, Gerber files, and bill of materials, can be downloaded from the CN-0387 Design Support Package at www.analog.com/CN0387-DesignSupport .