[Update] Transmit LO Leakage (LOL)—An Issue of Zero-IF That Isn’t Making People Laugh Out Loud | zeros lol – Vietnamnhanvan

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Introduction

There are several major advantages to zero-IF architecture. However, there are also some challenges that need to be overcome. Transmit local oscillator leakage (referred to as transmit LOL) is one such challenge. Uncorrected, transmit LOL will produce an unwanted emission within the desired transmission, potentially breaking system specifications. This article discusses the issue of transmit LOL and examines the techniques used to eliminate it, as implemented in ADI’s RadioVerse™ transceiver family (which includes the AD9371; see ADI RadioVerse Website for more details). If transmit LOL can be reduced to a low enough level that it no longer causes system or performance issues, perhaps people can learn to laugh out loud about LOL!

What Is LOL?

An RF mixer has two input ports and one output port, as shown in Figure 1. The ideal mixer would produce an output that is the product of the two inputs. In frequency terms, the output should be FIN + FLO and FIN – FLO, nothing else. If either input is undriven there will be no output.

Figure 1. Ideal mixer.

In Figure 1, FIN is set to FBB with a baseband frequency of 1 MHz and FLO is set to FLO with a local oscillator frequency of 500 MHz. If the mixer were ideal it would produce an output that comprises two tones: one at 499 MHz and one at 501 MHz. However, as shown in Figure 2, a real-world mixer will also produce some energy at FBB and FLO. The energy at FBB can be ignored because it is far away from the desired output and will be filtered out by the RF components located after the mixer output. Regardless of the energy at FBB, the energy at FLO can be a problem. It is very close to or within the desired output signal and difficult or impossible to remove by filtering, since the filtering would also filter the desired signal. It is this unwanted energy at FLO that is referred to as LOL. The local oscillator (LO), which drives the mixer, has leaked to the mixer’s output port. There are also other paths for the LO to leak to the system output, such as through power supplies or across the silicon itself. Regardless of how the LO leaks out, it can be referred to as LOL.

Figure 2. Real-world mixer.

In a real-IF architecture where only one sideband is to be transmitted, it is possible to resolve LOL by using RF filtering. In contrast, in a zero-IF architecture where both sidebands are to be transmitted, the LOL sits in the middle of the desired output and presents a more difficult challenge (see Figure 3). Conventional filtering is no longer an option, because any filtering that would remove the LOL would also remove portions of the wanted transmission. Therefore, other techniques must be used to eliminate it. Otherwise, it will likely end up becoming an unwanted emission within the overall desired transmission.

Figure 3. Unwanted energy at FLO shown in red. This unwanted energy at FLO is called LOL.

Eliminating LO Leakage (also Called LOL Correction)

The elimination of LOL is achieved by generating a signal that is equal in amplitude but opposite in phase to the LOL, thus cancelling it, as shown in Figure 4. Assuming we know the exact amplitude and phase of the LOL, the cancellation signal can be generated by applying dc offsets to the transmitter’s inputs.

Figure 4. LO leakage and cancellation signals.

Generation of the Cancellation Signal

The complex mixer architecture lends itself well to the generation of the cancellation signal. Because quadrature signals at the LO frequency exist in the mixer (they are at the heart of how the complex mixer works),1 they allow for the generation of a signal at the LO frequency with any phase and amplitude.

The quadrature signals that drive the complex mixer can be described as Sin(LO) and Cos(LO)—these are orthogonal signals at the LO frequency that drive the two mixers. To generate the cancellation signal, these orthogonal signals are added together with different weights. In mathematical terms, we can produce an output that is I × Sin(LO) + Q × Cos(LO). By applying different signed values in place of I and Q, the resulting sum will be at the LO frequency and can have any desired amplitude and phase. Examples are shown in Figure 5.

Figure 5. Examples of any phase and any amplitude cancellation signal being generated.

The desired transmission signal will need to be applied to the transmitter’s inputs. By applying a dc bias to the transmission data, the output of the mixer will contain both the desired transmission signal and also the desired LOL cancellation signal. The intentionally generated cancellation signal will combine with the unwanted LOL and they will cancel, leaving only the desired transmission signal.

Observing the Transmit LOL

The transmit LOL is observed using an observation receiver, as shown in Figure 6. In this example, the observation receiver uses the same LO as the transmitter, so any transmit energy at the LO frequency will appear as dc at the output of the observation receiver.

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Figure 6. Basic concepts of observation and correction of TxLO leakage.

The approach shown in Figure 6 has an inherent weakness: by using the same LO to transmit and observe, transmit LOL will appear as dc in the observation receiver’s output. The observation receiver itself will have some amount of dc due to component mismatch in the circuit, so the total dc output by the observation receiver will be the sum of the transmit LOL and the native dc offset that exists in the observation receiver. There are ways to overcome this issue, but a better approach is to use a different LO frequency for observation, thereby separating the native dc in the observation path from the transmit LOL observation result. This is shown in Figure 7 below.

Figure 7. Using different LOs to transmit and observe.

Because the transmission is being observed using a frequency other than transmit LO, energy at the transmit LO frequency will not appear at dc in the observation receiver. Instead, it will appear as a baseband tone whose frequency is equal to the difference between the transmit LO and the observation LO. DC native in the observation path will still appear at dc, so there will be total separation of observation dc and transmit LOL measurement results. Figure 8 illustrates this concept using single-mixer architecture for simplicity. The input to the transmitter is zero in this example, so the only output from the transmitter is transmit LOL. Frequency shifting is done after the observation receiver to move the transmit LOL observed energy to dc.

Figure 8. Separating observation receiver dc from Tx LOL.

Finding the Necessary Correction Values

The required correction values are found by taking the observation receiver’s output, dividing it by the transfer function from transmit input to observation receiver output, and comparing this result to the intended transmission. The transfer function in question is shown in Figure 9.

Figure 9. The transfer function from transmitter input to observation receiver output.

The transfer function from the transmitter baseband input to the observation receiver baseband output is comprised of two components: amplitude scaling and phase rotation. Each is explained independently in more detail in the following sections.

Figure 10 shows that the amplitude of the transmit signal reported by the observation receiver may not represent the actual amplitude of the transmit signal being transmitted if the loopback path from transmit output to observation receiver input has gain or attenuation in the path, or if the gain of the transmitter circuit were different from the gain of the observation receiver circuit.

Figure 10. Amplitude scaling due to attenuation in the loopback path.

Now let’s consider phase rotation. It is important to realize that signals do not travel instantaneously from point A to point B. For example, signals travel through copper at approximately half the speed of light, meaning that a 3 GHz signal travelling along a copper strip has a wavelength of approximately 5 cm. This means that if the copper strip is probed with multiple oscilloscope probes spaced a few centimeters apart, the oscilloscope will show multiple signals that are out of phase with each other. Figure 11 illustrates this principle, showing three scope probes that are spaced out along a copper strip. The signal seen at each point is at a frequency of 3 GHz, but there is a phase difference between the three signals.

Note that moving a single scope probe down the copper strip would not show this effect, as the scope would always trigger at 0° phase. It is only by using multiple probes that the relationship between distance and phase can be observed.

Figure 11. The relationship of distance and phase, a 5 cm trace, a 3 GHz signal, and probe points at 0 cm, 2 cm, and 4 cm.

Just as there is a phase change along the copper strip, there will be a phase change from transmitter input to observation receiver output, as shown in Figure 12. It is essential that the LOL correction algorithm knows how much phase rotation has occurred for it to compute the correct correction values.

Figure 12: Phase rotation due to physical distance in the loopback path.

Determining the Transfer Function from Transmit Input to Observation Receiver Output

The transfer function shown in Figure 13 may be learned by applying an input to the transmitter and comparing it to the output from the observation receiver. However, some points need to be kept in mind. If a static (dc) signal is applied to the transmitter input, it will produce an output at the transmit LO frequency and the transmit LOL will combine with it. This will prevent the transfer function from being learned correctly. It should also be noted that the transmit output may be connected to an antenna, so intentionally applying signals to the transmitter input may not be allowed.

Figure 13. Determining the transfer function from transmitter input to observation receiver output.

To overcome these challenges, the ADI transceivers use an algorithm that applies a low level dc offset to the transmitted signal. The offset level is adjusted periodically and these perturbations will show up in the observation receiver’s outputs. The algorithm then analyzes the deltas in input values compared to the deltas in observed values, as outlined in Table 1. In this example there is no user signal being transmitted, but the method still holds in the presence of user signal.

Table 1. The Deltas in Input Value Compared to the Deltas in Observed Value

Tx Input Signal
Tx Output Port
Observation Receiver Output

Case 1
DC offset 1
TxLO 1 + Tx LOL
(TxLO 1 + Tx LOL) × transfer function

Case 2
DC offset 2
TxLO 2 + Tx LOL

(TxLO 2 + Tx LOL) × transfer function

By performing a subtraction of the two cases, the constant transmit LOL is eliminated from the equation and the transfer function can be learned. The number of cases can be expanded to more than two, giving many independent results that can be averaged to increase the accuracy.

Summary

The LOL correction algorithm will learn the transfer function from transmit input to observation receiver output. It will then take the observation receiver’s output and divide it by the transfer function to refer it to the transmitter’s input. By comparing the dc levels in the intended transmission to the dc levels in the observed transmission, the transmit LOL will be determined. Finally, the algorithm will compute the necessary correction values to eliminate the transmit LOL and apply them as a dc bias to the desired transmission data.

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This article provides an overview of one aspect of the algorithms used in ADI’s RadioVerse transceivers. For a broader understanding on the concepts of zero-IF and algorithms, see this article on complex RF mixers.1

References

1. David Frizelle and Frank Kearney. “Complex RF Mixers, Zero-IF Architecture, and Advanced Algorithms: The Black Magic in Next-Generation SDR Transceivers.” Analog Dialogue, Vol. 51, February, 2017.

[Update] Fix: League of Legends FPS Drop | zeros lol – Vietnamnhanvan

League of Legends (LoL) is an online battle arena video game developed by Riot Games for numerous platforms including Windows, Linux, macOS, etc. The game has immense popularity in the market and is a direct competitor of multi-million game Dota.

Recently, there have been several reports by users where they have massive random FPS drops while playing. This is even the case for users with high-end PCs having GTX 1080 and Core i7 8700k processors. This rules out the possibility that the FPS drop is because of low-end PCs and graphics hardware.

What Causes the League of Legends FPS to Drop?

After researching from the users and testing the phenomenon on our systems, we concluded that the FPS drop was not because of a single reason but a combination of several different factors. The reasons why League of Legends FPS drops are but not limited to:

  • Overlays: Overlays of Discord, GeForce Experience, etc. were known to cause issues with the game.
  • Overclocking: Even though overclocking is meant to enhance performance, it was seen to do the opposite in the case of LoL.
  • Full-screen optimization: League has a history of having issues with Window’s module of Full-screen optimization.
  • Game configuration: The game configuration files may either be corrupt, unusable or set improperly. This may cause issues with the game.
  • High-end graphics options: These options enhance the player’s experience by improving the graphics output but sometimes they are the reason why the FPS drop occurs in the first place.
  • Frame rate cap: This is an option provided in the game menu and allows users to set their FPS cap. This module is known to cause issues and be the reason for FPS drops.

Before we proceed with the solutions, make sure that you have an active internet connection and are logged in your computer as a computer administrator.

Solution 1: Set FPS Cap

FPS cap is a feature in League of Legends that allows users to set the cap for the game’s FPS. You can set a custom value according to your needs. By default, the cap is set as uncapped. Even though this might seem okay but this option was discovered to cause issues with the FPS. We will set the FPS cap of the game and see if this fixes the issue.

  1. Open LoL and enter a Custom game. Once you are in a custom game, access the options by pressing the Esc button.
  2. Select the Video tab present at the left side of the screen and scroll down to find Frame Rate Cap box.
  3. Change the setting to 60 FPS.
  4. After saving the changes, click on the Game tab present on the left side of the Options window. Locate Movement Protection under ‘Gameplay’ and make sure the option is unchecked.
  5. Press Okay to save changes and exit. Restart your game and see if these options do the trick.

Solution 2: Disable Overlays

Overlays are software components that allow users to access certain parts of third-party software while playing the game. This is usually a small bar at the bottom of the screen which can be accessed by pressing a specific hotkey. Overlays might seem appealing but are the cause of FPS drop in League of Legends.

Below is the method on how to disable Discord overlay (if you are using one). Make sure that you disable all other overlays present on your computer.

  1. Launch Discord and open its User Settings. Now select the option Overlay from the left navigation tab and uncheck the option Enable in-game overlay.
  2. Restart your computer completely so changes are implemented and check if the FPS drop is fixed.

Solution 3: Disable Overclocking, GeForce Experience, and Third-Party Apps

If the above solutions don’t work, we will move ahead and broaden our troubleshooting area to third-party applications and hardware. Let’s talk about Overclocking first. Overclocking is an act of increases the clock rate of your processor until it reaches its threshold temperature. Once it does, the overclocking is stopped. Even though this is there to increase performance, there are several cases where this causes the FPS drop.

So disable overclocking and even uninstall MSI Afterburner. You should also check for third-party applications such as NVIDIA GeForce Experience. Make sure that you uninstall it temporarily. Next, you should check the third-party applications running on your computer while you have the game opened. Make sure that you disable all of them and check each time whether the application is solved.

Note: Also try disabling Overlay from GeForce Experience and Windows game bar feature.

Solution 4: Setting Maximum Performance Power Options

Laptops are known to have power options embedded inside them which allow the user to select power options according to his need. By default, this option is set as balanced. We need to change it to maximum performance.

  1. Press Windows + R and type “control panel” in the box and press Enter.
  2. Once in the control panel, select Large icons and click on Power Options.
  3. Once in the Power Options, select Maximum performance using the window at the right side. Save changes and exit.

  4. Now try launching League of Legends again and see if the drop issue is solved.

Solution 5: Change Game Config File

If all the above methods fail to improve the FPS drop of the game, we can try changing the game configuration file. The game configurations are stored in an external file that is extracted every time League of Legends is launched. It might be the case that these configurations are not set properly or are corrupt. We will try to replace them and see if this does the trick.

  1. Close all League of Legends processes.
  2. Download the ZIP file from (here). Save it to an accessible location.
  3. Navigate to League of Legends installation folder and open the folder Config.
  4. As you can see, there is already a ‘game.cfg’ file present in the folder. Copy it to another location so you can always backup when needed. Now open the downloaded folder and open Config. Here you will see several different settings. Select the one according to your hardware specifications and copy the config file from here to the location of the config file in the LoL installation folder. If asked to replace, click Yes.

  5. Restart your computer properly and check if the FPS drop is fixed. You can always change the config file to another graphics setting.
  6. Also, you can add the following code at the end of your config file. (You can use Notepad to edit Config file)
    [UnitRenderStyle]
    
    Inking=0
    
    AdvancedReflection=0 < default 1 change to 0
    
    PerPixelPointLighting=0 < default 1 change to 0

Solution 6: Change Full-screen Optimization and DPI Settings

Full-screen optimization function in Windows helps applications to have their executable optimized to the full-screen of your display. This is a very helpful feature and is enabled by default. We will disable this and also change the DPI setting. You can revert the changes later on if they don’t work.

  1. Navigate to the Installation folder of League of Legends. Type ‘.exe’ in the search dialogue box and press Enter.
  2. Now lots of different executables will be returned in the results. Right-click on the first one and select Properties.
  3. Select the Compatibility tab from the top and check the option Disable full-screen optimization. Now select Change high DPI settings.

  4. Now check the option Override high DPI scaling behavior with the second option chosen as Application. Save changes and exit.

  5. Repeat these steps for all executables of League of Legends which returned in the results. Save changes and exit.

Restart your computer and check if the FPS drop is resolved.

Solution 7: Update Graphics Drivers

If all the above methods don’t work, your graphics drivers need to be updated. Drivers are the main mechanisms connecting hardware and software and make use of your resources. If these are outdated or broken, you will experience issues such as FPS drops.

Look up Civilization 5 won’t Launch and refer to Solution 3 where the entire method of reinstalling the latest graphics drivers is listed. Make sure that you uninstall the current driver first using DDU before attempting to install the latest driver released by your manufacturer.

Solution 8: Enable Low Specs Mode

League of Legends has a low spec mode to help users with low specs to play the game. The low-specs mode will automatically lower all the computer’s graphics settings and set the entire performance to low. Enabling this mode and then playing LOL will clarify if the problem is happening due to PC specs/Internet speed. If the game gets better, it probably means that you need to update your graphics hardware.

  1. Launch League of Legends.
  2. In the upper right corner of the window, click on Settings (the gear icon).
  3. Now click on the checkbox of Enable Low Spec Mode and click Done.
  4. Close League of Legends and launch it again after restart your computer. Check if the FPS issue is resolved.

 


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