Background
The Rx gain table is a table of gain values that are used to control the gain of the receiver chain of ADRV9001. The table is generated by the Transceiver Evaluation Software (TES) based on the selected settings in the program. Using the gain indices from the table, the ADRV9001 gain can be set manually via API, pin or set automatically using Automatic Gain Control (AGC) system. The external LNA itself can then be controlled by the device’s AGPIO output, based on the values generated in the gain table. The gain table contains values from 255 to 187, with 255 having 0 dB of gain with the lowest gain index containing the maximum gain of –30 dB. Every gain step is 0.5 dB of attenuation. Adding an external Low Noise Amplifier, for example a 14 dB LNA, the maximum total gain can be increased past the –30 dB limit of ADRV9001. Using that 14 dB LNA, we can have a total gain of –44 dB. See image of a simplified example version of a gain table below. On the left table, that is the maximum gain that ADRV9001 can produce, without using any external components. The right table shows a gain table when introducing external 14 dB gain LNA. The maximum gain increases to –44 dB.
Figure 1: simplified example version of a gain table
However, introducing an external component introduces different things to consider and keep in mind such as gain delays when switching from no external gain to having external gain, which in some cases, can take up to microseconds.
In the case of this guide, the receiver gain table used the LNA at 0 dB gain (bypass in this case) for gain indices from 255 to 195 and sets the external gain control to 0. When the gain entries are less than 195, the Low-Noise Amplifier (LNA) is adjusted to a gain of -14 dB, corresponding to gain table index 195, which itself corresponds to a gain of -30 dB. The subsequent gain index, 194, corresponds to a total gain of -30.5 dB, with an additional 14 dB gain from an external LNA. Consequently, the ADRV9001 gain becomes -16.5 dB.
Example Implementation
For this guide, an example of the ADL8111 bypass Low Noise Amplifier (LNA) was used to provide 12.5 (± 1.5dB) gain and 0dB gain. To provide the switching between internal bypass mode and amplifier mode, an external circuit was created that lets an AGPIO pin control the state of the LNA, as required for the gain table.
1) Connect your signal generator to the LNA input and connect the LNA output to “RX1A_IN” on your ADRV9001 Evaluation Board.
2) In TES go to the “Configure” tab and select “Board Configuration,” scroll to the bottom and select the checkbox “External LNA Gain Control”.
Figure 2: External LNA settings from TES
AGPIOs for the external gain control, are assigned in groups of 4 as they share a single source control between them. Two bits (nibble) are used for each Rx, i.e. two bits for Rx1 and two bits for Rx2. So, AGPIO_0 to AGPIO_3 share a single source control, AGPIO_4 to AGPIO_7 share a single source control, and so on. In this case, only one AGPIO is used as there are only 2 gain steps, so a 1 or a 0 is written to only one AGPIO.
3) AGPIO_4 was arbitrarily used in this example, so “Nibble 07-04” is selected, as it contains the number of our selected AGPIO. Select “Rx1 lower crumb/Rx2 upper crumb” so the signal is at the lower GPIO pin, AGPIO_4 in this case, rather than AGPIO_7. The AGPIO is used to control the external LNA gain. In this case, toggling AGPIO_4 ‘high’ turns the LNA off and setting the AGPIO ‘low’ turns the external LNA on.
4) The settling delay and the LNA digital gain delay are described in the ‘External LNA Timing’ section. Initially, these can be set to 0.
5) ADRV9001 can also support up to 3 different LNA’s. Each LNA can provide any gain up to 29 dB, which is the maximum ADRV9001 can support. This allows a maximum gain of -117 dB to be achieved using 3x 29 dB LNA’s and ADRV9001. More than a single LNA can be selected in the “LNA Type” dropdown box. The purpose of using multiple LNA’s is to allow higher overall gain without compromising the ability to keep 0.5 dB step sizes.
As can be seen from the illustration below, there are 3 LNA’s that either have a gain of 0 or -29 dB, and ADRV9001, which can support any value in 0.5 dB steps in the range of 0 dB to -30 dB. For example, to get -69 dB of total gain, 2 of the 3 LNA’s need to be set to -29 dB, one LNA set to 0dB, and add -11 dB of gain from ADRV9001. To get -7.5 dB of gain, all LNAs need to be turned off, and ADRV9001 must provide –7.5 dB of gain. Other examples can be seen in the picture below.
Figure 3: Example LNA lineup
6) In this case, “LNA Type” was set to single and the “Number of LNA Gain Steps” was set to 2 as our system has two gain steps: 0 dB and 12.5 dB gain. Hence, 14000 mdB was entered in the “LNA Gain Steps” as the LNA had a small offset of about 1.5dB. Enter the gain of your external LNA in the corresponding step box, TES only accepts gain in 0.5 dB steps. ADRV9001 can support up to 4 gain steps total, with each step containing an extra integer multiple of 0.5 dB, as can be seen below, taken from the “Receiver Datapath” section in the “Receiver Gain Control” chapter in the User Guide.
Figure 4: Explanation of LNA control words from user guide
7) Select “Program” at the top menu and wait until it says “Programmed” on bottom right of the TES. Ensure that signal generator output is off during programming.
8) You can check if your settings were applied by going to the “Gain Control” tab and scrolling down to the gain table. The gain table is automatically generated based on the selected LNA settings from steps 3-5 in this guide. Your gain index values of 194 and lower should contain a “1” in the External Gain Control field.
Figure 5: Generated gain table
9) Now go to “Receive” tab and press play and turn on your signal generator. The signal should be seen on the spectrum. There is also an option called “Interface Gain” which can be applied to avoid saturating the signal due to the limitation of the bit-width of the data port.
Figure 6: Interface gain screenshot
Also, see Enhanced SSI In TES - Documents - TES GUI & Software Support ADRV9001 – ADRV9007 - EngineerZone (analog.com) to view gain index in real time.
10) Lower your “Rx Gain (Index)” until 195, and when you lower it one more time, the LNA will turn off if everything was configured correctly, and there should be a smooth, continuous drop of 0.5 dB. If the signal jumps or lowers dramatically, tweak the “LNA Gain steps” in 500mdB steps, back in “Board Configuration”, to ensure a continuous transition. An example of a smooth step of approximately 0.5 dB can be seen below, with the first picture containing the spectrum at gain index 195, and the second picture the index is lowered to 194, to power on the LNA. As can be seen, it's approximately 0.5 dB difference, meaning it is a smooth transition.
Figure 7: Rx Signal at Gain Index 195
Figure 8: Rx Signal at Gain Index 194
External LNA Timing
External LNA timing can be controlled directly by ADRV9002 for enabling LNA output power and LNA gain change timings. Figure 9 shows the external LNA timing for an example Rx frame. Delays are used to compensate for path delay latency to ensure timings of the external LNA are aligned with ADRV9002. These delays are described fully in the following sections. Note that there is a distinction between ‘AGC Settling Delay’/’Digital Gain Delay’ and ‘LNA AGC Settling Delay’/’LNA Digital gain delay’ as these cases are managed differently.
Figure 9: LNA Signals Timing Diagram
1) Timing of LNA output power
ADRV9002 AGPIOs can be used to enable LNA output power timings. For LNAs connected to Rx1, AGPIO1 is used and for Rx2, AGPIO9 is used. The AGPIOs corresponding to each channel are not user configurable. Figure 10 shows enabling this option in TES, in the ‘External LNA’ section.
Figure 10: External LNA output power settings
The timing for these AGPIOs is configured by the enablement delays. The LNA enablement timing refers to the “Rx On (LNA Power Out)” line of the diagram in Figure 9 and described fully in the “TIMING PARAMETERS CONTROL” section of the user guide. The AGPIO will trigger some time after the Rx Enable rising and falling edges as controlled by the ‘Rise to On’ and ‘Fall to Off’ delays, respectively.
The intention of this signal is to be used to power-on an LNA at the correct time to align with the Rx framing on air (as per Figure 9).
2) Timing of LNA Gain Change
Figure 1 shows an example Rx frame, with 2 sub frames. The gain update period aligns with the subframe boundaries. The gain change timing will behave differently when there is a change to the LNA gain and when there is no change to the LNA gain.
In this example, the first gain change, there is no change to the gain update period. The gain change will be delayed by the ‘digital gain delay’ parameter which is configured internally. The ‘AGC settling delay’ is controlled by user in the AGC settings (slowLoopSettlingDelay), the delay to start the next AGC measurement.
On the second gain change, there is an LNA gain change. The delays for this gain change are configured via the LNA settings as highlighted in Figure 11.
Figure 11: LNA delay settings
To allow time for the LNA to change gain and signal to propagate, an extra ‘LNA Digital Gain Delay’ is included which is to be measured by the user. This delay is defined as the path delay between the LNA/External Gain and the ADRV9002 front-end attenuator as shown in Figure 12. Measure this by setting the ‘LNA Digital Gain Delay’ to 0, triggering an LNA gain change and observing the delay in gain change of the Rx captured data.
Figure 12: Path delays block diagram
For similar reasons, an ‘LNA Settling Delay’ is included for the AGC. This settling delay will override the settling delay set in AGC settings when there is an external LNA gain change. This delay is recommended to align with ‘Digital Gain Delay’ + ‘LNA Digital Gain Delay’ as shown in Figure 9 i.e. after the gain change has fully propagated through the Rx data path.
