AD8318: Using it at 8.5GHz

Question I would to enlarge the frequency operating range up to 8.5GHz 1. Is it recommended by you even the device is up to 8GHz(refer to datasheet) 2. The influence on the electrical performance. Answer Operation at 8.5 GHz is possible. The gain stages of the AD8318 start to fail above 10.5GHz. Note that parasitics of the evaluation board, as well as the package, contribute to this performance decay.

Question I want to control the PA by using this part in control mode. I tested the EVB and got some results both measurement mode and control mode. The measurement mode results are OK. But I can not know whether the control mode results are correct. Could you please help to check it? Could you please send me some test results in control mode or the principles? Answer In order to test the AD8318 in controller mode you need to have an additional device, such as a VGA or a PA, to control. Simply removing the 0 ohm resistor from VOUT to VSET is not enough. Controller mode places the AD8318 in a closed loop system with the PA or VGA. Without the PA or VGA the AD8318 is operating open loop. I suggest reading the Controller Mode section of the AD8318 datasheet starting on pg.18. You can also see an example of using the AD8318 in Controller Mode in the ADL5330 datasheet, pg.16. The data you provided suggests that the AD8318 will operate correctly in controller mode. If the AD8318 operates correctly in Measurement Mode it should operate correctly in Controller Mode.

Calculating the Resistor Values to Alter the Slope of a RF Log Amp or RMS Detector

Q: I want to increase the slope of the AD8319 log amp. Is there a formula to cacluate the resistor values needed? -------------------------------------------------------------------------------------------------------------- A. The required resistor values can be calculated from the equation: R2 = ((SLOPEnew/SLOPEnom) - 1).( Rext||Rin) where SLOPEnew is the desired slope SLOPEnom is the nominal slope of the device when Vout is directly to Vset Rext is the resistor connecting Vset to ground R2 is the resistor that connects Vout to Vset The input impedance of VSET on the AD8319 is 40k ohms. Slope setting resistors should be kept below approximately 10k ohms to prevent this input impedance from significantly affecting the resulting slope. The table below shows resistors required and the resulting voltage swing for different slopes. It is also important to set these resistor values so that Vout is not required to source significant current. In the examples below the load current is < 1 mA. This equation applies to any log or rms responding RF detector that has a scaleable output. Input Range (dBm) Desired Slope Actual Slope R2 K ohms Rext K ohms Voltage Swing -60 - +5 -22 mV/dB -21.968 mV/dB 0 - 1.52 V - 0.37 V -60 - +5 -44 mV/dB -46.445 mV/dB 4.75 4.75 3.22 V - 0.777 V -60 - +5 -66 mV/dB -68.199 mV/dB 4.75 2.37 4.72 V - 1.14 V

ADL5902 RMS Detector: Operation at Frequencies greater than 5.8 GHz

Q. The datasheet for the ADL5902 RMS detector shows only limited data above 5.8 GHz but the data sheet front page says it works up to 9 GHz. Can it really operate above 5.8 GHz? --------------------------------------------------------------------------------------------------------------- A. Let's start by talking a look at the plot of performance at 5.8 GHz. As you can, see the temperature drift at this frequency along with the device to device variation are already fairly high (as evidenced by the vertical thickness of the red band at about 3 dB). At higher frequencies, this temperature drift will get worse. So while the ADL5902 RMS detector will work at higher frequencies (see below), it would have to be operating in an environment where the temperature is fairly stable (maybe +/-10 or -/-20 degC but not -40 degC to +85 degC). For high frequency applications (i.e. 6 GHz or greater) where the environmental temperature is not controlled, I would recommend a device such as AD8318, AD8317 or AD8319. These devices are well characterized at these frequencies over temperature. Unlike the ADL5902, they are not rms responding. So if the modulation scheme changes, a correction factor needs to be applied for each modulation scheme. However, if the system runs with a single modulation scheme, these devices will work fine without correction as long as they are calibrated with the modulated carrier.

Long Term Drift of RF Detectors

Q. Do you have any data on the long-term drift of RF Detectors? ------------------------------------------------------------------------------------------------------------ A. The work that we have done suggests that the drift of RF Detectors over time is very low and may be within the margin of error of the measurement system used. Testing was performed on a number of AD8362 devices (high dynamic range RF rms detector). The device were operated continuously (5V power supply applied with a fixed RF input power) for approximately 260 hours (11 days). Some of the devices were operated at 25 degC while others were operated at 110 degC to try and emulate an accelerated lifespan (both temperatures were tightly controlled so as not to introduce temperature drift as a source of error). The results show that in all cases (i.e. devices at 25 degC and devices at 110 degC), over the first 50 hours, the devices' output voltages drifted by the equivalent of a between 0.05 dB and 0.1 dB. After the first 50 hours, the drift flattened out (see illustration below). Because these numbers are so small, the observed drift may be partially caused by the measurement equipment. That being said, every precaution was taken to ensure that the there was little or no drift in the test equipment. Long soak times were used where the equipment was operated continuously before the start of the 25 degC and 110 degC tests. Also, the source RF power to the aD8362 was continuously monitored with any observed variations over time being factored out.

Pulse Response of Linear-in-dB RMS Detectors with no RMS Averaging Capacitor Present

Question: RMS detectors with a Linear-in-dB transfer function such as AD8362, AD8363, AD8364, ADL5902 and ADL5906 produces a peaking in detection voltage when driven by RF Bursts when there is no rms averaging capacitor present. Adding an external rms averaging capacitor seems to eliminate the peaks. Could you explain what could possibly produce this kind of peaking? Answer: Think of the rms detector as a VGA/AGC loop (RF Input drives VGA, VGA drives low range rms detector, output of low range detector is compared to setpoint, difference drives an integrator which drives the gain control input of the VGA). When there is no input signal, the VGA gain is very high. When you apply a bursted signals, the VGA gain initially remains really high which produces a large signal at the VGA output and at the output of the squaring cell. This causes the integrator output to rise sharply (overshoot) if there is no rms filter cap present (the rms filter capacitor sets the response time of the integrator). The loop will eventually settle and the integrator output voltage (which is also the rms output) settles to a steady (but incorrect) final value. The attached step response plot is from the AD8362 datasheet. Operating a linear-in-dB rms detector with no external rms averaging capacitor is not really meaningful. With no filter cap present, there is very little rms averaging going on and the device will give an incorrect output voltage. In order for the device to give the correct rms answer there must by R(oot), M(ean) and S(quare root) going on. If you don't do enough M(ean), that is, averaging, you rms computation is not valid and the dc voltage at the output will be incorrect. So a minumum value of filter capacitance must be present. On the newer rms detector datasheets such as ADL5902 and ADL5906, we have experimentally determined the minimum required rms averaging capacitor size for various modulation schemes. The minimum filter cap size is generally in the 1nF to 10nF range. Once you add a reasonably sized rms filter cap, you also notice that the Vout spike will not be present (you have effectively reduced the loop bandwidth of the AGC loop, preventing the integrator it from over-reacting). If you want a detector with a fast response time and no spike, then a log amp might be better (e.g. AD8318, AD8319, AD8317 or ADL5513). It does not give you a correct rms answer (but neither does the rms detector when operated with an rms averaging capacitor that is not present of set too small).

Switching an RF Detector between Measurement Mode and Controller Mode

Question: Is it possible to configure an RF Detector so that it can be switched between Measurement Mode and Controller Mode. Answer: The attached schematic shows one possible implementation where the RF detector can operate in Measurement Mode or Controller Mode. The detector used in this example is the ADL5902 (65 dB broadband, rms-responding RF detector) but this idea could be applied to any RF detector which has a Measurement Mode and a Controller Mode. In this example, the RF detector is being used to control the output power of a Variable Gain Amplifier (VGA) or Voltage Variable Attenuator (VVA). A directional coupler sniffs the output power of the VGA and this signal is fed to the ADL5902 rms detector. With the switch configuration shown, the rms detector is operating in Measurement Mode, that is, Vout is shorted to VSET and the VOUT/VSET voltage is proportional to the log of the rms input level. This voltage is fed to an Analog-to-Digital Converter such as AD7466 (see Circuit Note CN0178 for more information on this interface, www.analog.com/cn0178 ). In this example, the gain of the VGA/VVA is controlled by a DAC such as AD5621. This configuration assumes that the reading from the ADC influences the setpoint voltage on the DAC. So this digitally controlled Automatic Power Control Loop would be completed by a or micro-controller which interfaces to the ADC and DAC. By flipping the switch setting on both analog switches (a dual single-pole-double-throw switch such as the ADG436 is suggested for this application), the RF detector switches into Controller mode. As before the directional coupler sniffs the output power of the VGA/VVA and drives the input of the rms detector. However, the ADL5902's VOUT and VSET signals are no longer shorted together. Instead, VSET is driven by the DAC and VOU'T drives the input of the VGA/VVA. In this mode of operation, the voltage from the DAC is compared to the equivalent power level at the input of the detector. If this relationship is out of balance (the relationship is simply the Vout vs. Pin plot that you get when operating in Measurement Mode), Vout with integrate up or down which adjusts the gain and brings the system into balance.

RF Detector Flatness vs. Frequency

Question: How can I figure out the output flatness vs. frequency of an RF Detector such as the AD8363 Answer: Using the Slope and Intercept specs it is possible to get a sense for and RF Detector's flatness vs frequency. Let’s take the example of the AD8363. Let’s consider the easy-to-calculate case where the input power is equal to 0 dBm. The general equation for the output voltage of a linear-in-dB RF Detector is: Vout = Slope x (Pin – Intercept) So for Pin = 0 dBm, this reduces to Vout = - Slope x Intercept Using the published slope and intercept specs of AD8363, we get 100 MHz Vout = 2.99 V 900 MHz Vout = 3.0044 V 1.9 GHz Vout = 2.86 V 2.14 GHz Vout = 2.8188 V 2.6 GHz Vout = 2.5921 Since the slope of the AD8363 is around 52 mV/dB, this corresponds to a variation vs frequency (over this freq range) of approximately 8 dB. On a newer device ADL5511 whose output voltage is linear in V/V, we have measured the variation and found it to be much smaller. In this case, we see that the device is quite flat up to around 4 GHz. The trade off with this device (compared to AD8363 or AD8318) is that it has less range (47 dB). For more information on this, take a look at Figures 57 and 58 in the Rev. A Datasheet. The plot below shows the relative errors of various RF Detectors vs. frequency after they have been calibrated at a single frequency. Unsurprisingly, the ADL5511 that was referenced above has the flattest overall performance (this plot has the brown tick marks in the plot). This device also has another advantage in applications where RF power needs to be measured over a wide frequency range. Many of ADI's RF Detectors have external temperature compensation nodes. These inputs allow for the temperature drift of the device (which can vary with frequeny) to be compensated. Since the drift can vary with frequency, the required compensation voltage will also vary. In the case of the ADL5511 RF Detector, no external temperature compensation is required.

AD8318 Temperature sensor output characteristic

Q. Can you provide information about the output voltage characteristic of the temperature sensor embedded in the AD8318? --------------------------------------------------------------------------------------------------------------- A. The figure attached is a plot of the output voltage of the sensor as a function of temperature. Temperature Sensor section on the product datasheet (page 14) provides detailed information about the operation of the sensor.

RF & Microwave

Documents AD8318: FAQ

AD8318: FAQ

Q: ADL5902 RMS Detector: Operation at Frequencies greater than 5.8 GHz

Q. The datasheet for the ADL5902 RMS detector shows only limited data above 5.8 GHz but the data sheet front page says it works up to 9 GHz. Can it really operate above 5.8 GHz? --------------------------------------------------------------------------------------------------------------- A. Let's start by talking a look at the plot of performance at 5.8 GHz. As you can, see the temperature drift at this frequency along with the device to device variation are already fairly high (as evidenced by the vertical thickness of the red band at about 3 dB). At higher frequencies, this temperature drift will get worse. So while the ADL5902 RMS detector will work at higher frequencies (see below), it would have to be operating in an environment where the temperature is fairly stable (maybe +/-10 or -/-20 degC but not -40 degC to +85 degC). For high frequency applications (i.e. 6 GHz or greater) where the environmental temperature is not controlled, I would recommend a device such as AD8318, AD8317 or AD8319. These devices are well characterized at these frequencies over temperature. Unlike the ADL5902, they are not rms responding. So if the modulation scheme changes, a correction factor needs to be applied for each modulation scheme. However, if the system runs with a single modulation scheme, these devices will work fine without correction as long as they are calibrated with the modulated carrier.

Q: Long Term Drift of RF Detectors

Q. Do you have any data on the long-term drift of RF Detectors? ------------------------------------------------------------------------------------------------------------ A. The work that we have done suggests that the drift of RF Detectors over time is very low and may be within the margin of error of the measurement system used. Testing was performed on a number of AD8362 devices (high dynamic range RF rms detector). The device were operated continuously (5V power supply applied with a fixed RF input power) for approximately 260 hours (11 days). Some of the devices were operated at 25 degC while others were operated at 110 degC to try and emulate an accelerated lifespan (both temperatures were tightly controlled so as not to introduce temperature drift as a source of error). The results show that in all cases (i.e. devices at 25 degC and devices at 110 degC), over the first 50 hours, the devices' output voltages drifted by the equivalent of a between 0.05 dB and 0.1 dB. After the first 50 hours, the drift flattened out (see illustration below). Because these numbers are so small, the observed drift may be partially caused by the measurement equipment. That being said, every precaution was taken to ensure that the there was little or no drift in the test equipment. Long soak times were used where the equipment was operated continuously before the start of the 25 degC and 110 degC tests. Also, the source RF power to the aD8362 was continuously monitored with any observed variations over time being factored out.

Q: AD8318 Temperature sensor output characteristic

Q. Can you provide information about the output voltage characteristic of the temperature sensor embedded in the AD8318? --------------------------------------------------------------------------------------------------------------- A. The figure attached is a plot of the output voltage of the sensor as a function of temperature. Temperature Sensor section on the product datasheet (page 14) provides detailed information about the operation of the sensor.