SCU bandpass filter override

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  • Question
  • Updated 4 years ago
  • Answered
Is there a way to override the SCU bandpass filters, such as to force it off, or to force it to stay on a particular band regardless of how wide the panadapter is 

I often have trouble when operating down at the bottom of 160m where the bandpass filter clicks off unless my panadapter is  adjusted so my transmit slice is right at the edge of the panadapter., which is rather annoying.
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Greg Zenger [N2GZ]

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  • frustrated

Posted 4 years ago

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Photo of Tim - W4TME

Tim - W4TME, Customer Experience Manager

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Official Response
Bandpass filtering or the preselectors are not dependent on the slice frequency.  The SmartSDR software dynamically enables the band specific preselector based on the bandpass filter (preselector) bandwidth and the panadapter viewable bandwidth; turning a bandpass filter on when it will properly reduce out of band signal energy (remember, the SCU is sampling all RF energy from 70 kHz up through the 6m band depending on radio model).

To disable the preselector, either open another panadapter in a different band or zoom out until the panadapter viewable area exceeds the bandwidth of the preselector.  These conditions will prevent the preselectors from becoming enabled.

Can you force on a band specific preselectors?  The answer is no; there is not a way to force on a preselector and circumvent the logic that dynamically enables it.  

SmartSDR has logic to dynamically enable or disable the preselectors depending on the viewable bandwidth of all panadapters associated with a particular SCU, and the actual bandwidth of the preselector (bandpass) filter.  Forcing a particular preselector on when it should not be would degrade the receive performance of the radio.  We have put a lot of time and effort in making the radio perform at an optimal level with minimal user interaction and configuration (the smart in SmartSDR).

"I often have trouble when operating down at the bottom of 160m where the bandpass filter clicks off unless my panadapter is  adjusted so my transmit slice is right at the edge of the panadapter., which is rather annoying."

This is because the preselector on 160m is very tight around the band edges due to the presence of strong BCB signals just below it.  The function of a preselector is to reduce the total spectral energy getting to the SCU (the wide band AD converter).

On 160m, if the viewable area displayed, is no less than ~1.790 MHz and the upper frequency does not exceed ~2.1 MHz, the 160m preselector is enabled.  I suspect that what is happening is as you tune towards the lower band edge, you end up pan edge tuning, meaning the panadapter is tuning to a lower frequency with the slice resulting in the lower frequency of the panadapter going below 1.790 MHz.  As you have noted, judicious zooming of the panadapter (adjusting the panadapter bandwidth) will allow you to tune a slice at the band edge without disabling the preselectors.  

Rather than using the stepped zoom controls, you can click and drag the frequency display of the panadapter to adjust the viewable bandwidth without centering the active slice in the middle of the panadapter viewable area.