Orion Hi Cut / Low Cut Explained

W4PA in 2004:

I'll try to explain Hi Cut / Lo Cut succinctly:

We (Ten-Tec) had a long discussion about whether HI CUT / LO CUT is a bandwidth value or a passband tuning value when deciding how to update the display on the Orion - it's actually some of both.

For an example of why LO CUT (and HI CUT) are both a PBT and a BW value, try this:

Tune in a CW signal. Put your CW offset (using SPOT) at 600 to 700 Hz. Tune the CW signal to match the SPOT tone. Put the BW value at 1000 Hz, PBT at 0, select LO CUT.

As you rotate the LO CUT control clockwise, the PBT value increases as one side of the filter is moved towards the center of the passband. Because this also has the effect of narrowing the total filter bandwidth, the BW value is also updated to reflect the total BW available. As the CUT is narrowed, however, it is also possible to move past the center frequency of the filter until the CW signal you are listening to eventually disappears. The BW will still show a value of maybe 450 Hz or so, as that is now the BW of the available "distorted" filter after moving one side in towards or past the original center frequency.

If only the BW was adjusted, not the HI CUT or LO CUT or the PBT, when the filter was narrowed to 450 Hz you'd still hear the CW signal in the center of the filter. As opposed to PBT, where the filter BW is constant and the filter is then moved back and forth across the center frequency. HI or LO CUT moves one side of the filter or the other towards or past center frequency.

The PBT and BW values are supposed to update on screen graphics and text - unfortunately compounding the confusion is a bug that is present when HI CUT is adjusted. LO CUT shows the adjustment of PBT and BW values simultaneously, while HI CUT only shows the BW update, not the PBT value. This needs to be corrected.

John Henry in 2012:

PBT allows the operator to move the signal down the slope of the filter to get away from interference.

High cut only narrows the top half of the DSP filter and there is no need to change PBT because the signal is not being moved inside the filter pass band. So on Hi Cut only the narrowing of the high end of the filter is shown.

On low cut it is necessary to move the signal with PBT to keep it centered inside the filter pass band. PBT and the band width on the display change because this is actually what is occurring with the filter. As low cut is moved not only is the low end of the filter being narrowed but the signal is constantly being moved by the PBT to keep it centered in the filter pass band.

If you do watch when you change HiCut, you will see that the top limit of the filter moves and the bottom limit does not move. If you do watch when you change LoCut,you will see the low end of the filter move, and whereas the top end moves a tad, if you continue to move the low cut, you will see that the top is relatively the same. Shuffling a little up/down but basically close to the same point.

It may be confusing a bit to see lo cut also adjusting PBT, but that is actually part of what is happening to keep the passband the same.

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