Preselectors, the ALEX button, and RX2

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w-u-2-o
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Preselectors, the ALEX button, and RX2

Postby w-u-2-o » Tue Mar 20, 2018 2:25 am

This post will try to address the mysterious ALEX button, and how to manage the receive preselector, i.e. band pass filter (BPF), array.

Once upon a time, before there was any Apache Labs radio hardware, there was a board called "Alexiares". This board was designed as a filter array for the early openHPSDR family of SDR hardware. For some reason this board was controlled by firmware in the SDR FPGA, not by the software, i.e. PowerSDR.

1. The ALEX button was born out of the this legacy of hardware that predates the ANAN-10 that requires the use of the "firmware" mode of operation.

2. When ALEX is ON, there is a hard-coded table in firmware that looks at the frequency of VFO-A and sets the preselection BPF in accordance with the frequency. If both RX1 and RX2 are being fed by ADC1 then this may often be in conflict with the frequency of VFO-B.

3. When ALEX is OFF, the software, PowerSDR, looks at the frequency of VFO-A and sets the preselection BPF in accordance with that frequency per the settings in Setup > General > Ant/Filters. Again, if both RX1 and RX2 are being fed by ADC1 then this may often be in conflict with the frequency of VFO-B.

4. Neither mode, ALEX on or off, is actually "manual". This is a confusing misnomer. They are both completely automatic unless one goes into Setup > General > Ant/Filters and manually takes control there.

5. To turn off preselection filtering completely, set ALEX to OFF and

- On the 100 and 200 series radios, check both the "ByPass/55 MHz HPF" and "6M/Bypass on RX" boxes
- On the 7000 and 8000 series radios, check the "ByPass/55MHz BPF" box (not 100% sure about this on the 7000)

6. It can be convenient to preset the settings in Setup > General > Ant/Filters as per (5) above and use the ALEX button to switch between firmware and software (aka "manual") modes of operation. In this way you get an easy way to bypass all preselection filtering from the main console user interface.

7. In all hardware the 55MHz LPF is not defeatable (nor should it be!)

Editorial note: IMHO the firmware modes of operation should be deprecated for Apache Labs radios, as philosophically all control should be from the software in a software defined radio.

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