How does the RX attenuator work?

W9IP
Posts: 43
Joined: Mon Apr 10, 2017 11:57 am

How does the RX attenuator work?

Postby W9IP » Tue Jan 18, 2022 8:29 pm

Dear friends,
I am having trouble understanding how the drop-down RX attenuator works. I would expect that adding attenuation would lower RX levels; this is how an external (physical) attenuator works. You add a 10 dB attenuator to the RX input and all received signals are reduced by 10 dB. In Thetis, it seems to do the opposite. Adding 10 dB from the drop-down box raises everything by abut 10 dB. Could someone explain?
Thanks,
W9IP
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: How does the RX attenuator work?

Postby w-u-2-o » Tue Jan 18, 2022 8:48 pm

In Thetis (and PowerSDR), the approach used is just like that done in laboratory test equipment. That approach references all measurements to the rear panel connectors. Example: if you have a signal that is -80dBm at the rear panel connector and the attenuator is set to 0dB, it will displayed as -80dBm on the panadapter. Then if you add 10dB of attenuation, the software knows this and it will adjust the panadapter (and S-meter) scaling so that it still displays as -80dBm, even though it is now -90dBm at the ADC input.

One thing you might not have realized is that there is a fixed gain, undefeatable preamp in front of the ADC that is part of the openHPSDR receiver design. On the Orion MKII board (7000 and 8000) it is 14dB, on all other hardware it is 20dB. The software removes that gain when displaying signal strength as well.

You can argue whether or not this is the correct way to do it, but it removes any ambiguity with respect to what the true conditions are at the input connector (which is why laboratory test equipment works like this, too).
W9IP
Posts: 43
Joined: Mon Apr 10, 2017 11:57 am

Re: How does the RX attenuator work?

Postby W9IP » Tue Jan 18, 2022 9:23 pm

Scott -
Thank you for (another) clear explanation.
73,
W9IP

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