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Alexey

Tuner bandwidths in v2.4

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In WBT v2.4 there is this new feature that the tuner span can be set to some arbitrary value (instead of a predefined list). For this there are WbtCommon::Data::RadioFrontend::getMinBandwidth() and WbtCommon::Data::RadioFrontend::getMaxBandwidth() methods. Currently they return values 0.7 and 42. However, in the Session screen of the WBT device the threshold at which the tuner config is switched to "Combined" is 40Mhz.

The app has a screen similar to the Session screen and has to mimic its behavior. What is the proper way to determine when the tuners have to be combined/uncombined?

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What getMinBandwidth and getMaxBandwidth are actually returning is the min and max sample rates, rather than the bandwidths. This is a known issue that will be fixed in a future release.

To determine whether the Tuners are in combined mode, the best thing to do at the moment use WbtSystem::getTunerSettings() to get the current Radio Settings (WbtCommon::Data::RadioSettings). This contains the _combined member which holds the RF parameters of the combined mode. If the Center Frequency for combined mode is greater than 0, then you can safely say you are in combined mode. If it is not greater than 0, then you are not in combined mode.

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What getMinBandwidth and getMaxBandwidth are actually returning is the min and max sample rates, rather than the bandwidths. This is a known issue that will be fixed in a future release.

To determine whether the Tuners are in combined mode, the best thing to do at the moment use WbtSystem::getTunerSettings() to get the current Radio Settings (WbtCommon::Data::RadioSettings). This contains the _combined member which holds the RF parameters of the combined mode. If the Center Frequency for combined mode is greater than 0, then you can safely say you are in combined mode. If it is not greater than 0, then you are not in combined mode.

Why is the Session screen threshold 40MHz? Where is this 2MHz difference coming from?

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Why is the Session screen threshold 40MHz? Where is this 2MHz difference coming from?

The Session screen is displaying the Bandwidth rather than the sample rated. The bandwidth will generally be 9/10ths of the Sample rate except at the upper range where the Maximum Bandwidth forces that difference to be smaller. For example, a bandwidth of 9 MHz would have a calculated sample rate of 10 MHz. A sample rate of 40 MHz would calculate a sample rate of 44.444 MHz, but the max single tuner sample rate is 42, and so the sample rate is set to 42 MHz, the 2 MHz difference you see here.

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The Session screen is displaying the Bandwidth rather than the sample rated. The bandwidth will generally be 9/10ths of the Sample rate except at the upper range where the Maximum Bandwidth forces that difference to be smaller. For example, a bandwidth of 9 MHz would have a calculated sample rate of 10 MHz. A sample rate of 40 MHz would calculate a sample rate of 44.444 MHz, but the max single tuner sample rate is 42, and so the sample rate is set to 42 MHz, the 2 MHz difference you see here.

OK, thank you! Will wait for the patch then.

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The Session screen is displaying the Bandwidth rather than the sample rated. The bandwidth will generally be 9/10ths of the Sample rate except at the upper range where the Maximum Bandwidth forces that difference to be smaller. For example, a bandwidth of 9 MHz would have a calculated sample rate of 10 MHz. A sample rate of 40 MHz would calculate a sample rate of 44.444 MHz, but the max single tuner sample rate is 42, and so the sample rate is set to 42 MHz, the 2 MHz difference you see here.

There is an analog, low-pass, anti-alias filter (AAF) in the tuner before the ADC. The Nyquist sample rate is a theoretical value that assumes the AAF is a perfect filter (its response falls to the noise floor at the transition point).  A filter’s bandwidth is typically specified as the point where the amplitude is 3 dB below the pass-band as it transitions from pass-band to stop-band.  It is necessary to oversample relative to the ideal rate and apply a digital filter to diminish the data representing the roll-off near the AAF transition point. (The digital filter doesn’t change the effective sample rate.) The difference between the ideal sample rate and the sample rate depends on the filter’s shape factor and the flatness requirement.

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