R2 Filters

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From: David Taylor (david@rmit.edu.au)
Date: Tue Oct 04 1994 - 07:33:38 EDT


Hi QRP Folk!

I received my R2 kit last week and have on assembling the PCB. I'm
impressed by the compact layout and the way the components all fit easily.

Though I don't have a VFO built yet I have tested the 'back end' by
connecting the board to an existing DC Rx. on 80m. I connected the R2's
3kHz low pass filter input to a low Z op-amp output on the 80m Rx. Then I
was able to compare the performance of the filters by plugging the 'phones
from one audio out socket to the other.

The 80m Rx. uses op-amp active filters which I _used_ to think worked well!
The difference in the R2's filters is dramatic - tuning through a carrier
there is no low frequency rumble as you approach zero beat, on the high end
the note starts to diminish, then completely disappears.

By comparison, the active filters don't have the dramatic roll off and
carriers whistle up through the audio range before they are lost.

Another noticeable difference is that static crashes seem to have a lot
less energy through the R2's filters. A comparison with my FT101B receiver
was interesting - the 101B seemed to resolve weak DX on the 75m DX window
better than the DC Rx. - surprising considering the problems the 101B has.
I can only put it down to the DC Rx. receiving noise on both sidebands so
having a worse S/N ratio on weak signals.

Next I will get the image reject front end working (on the R2) and then do
a serious comparison. I suspect then the R2 will outperform the 101B.

Cheers from VK

David

David Taylor
RMIT (Bundoora Campus)
david@rmit.edu.au


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