Re: Spurs - BPF fix? #ubitx

Bill Cromwell
 

Hi Glenn,

Just a slight correction. Allison said it's the bands above 17 meters. You left that one out. I Made note of that because I use that band.

I am using mine on the bands from 80 through 17 meters with external low pass filters to reduce harmonic content. Just until a fix that I like comes along.

In addition I use my uBitx as a bench tool working on other radios. In signal generator mode the finals are disabled and no signals sent to the antenna. It's much nicer using the uBitx for this in place of a whopping big tube radio!

73,

Bill KU8H

On 08/10/2018 12:14 AM, Glenn wrote:
There seems to be a consensus from all the many posts on spurs, (not
harmonics) that a BPF following the Mixer and into the PA driver stages
is required.

From what i can glean, only the bands above 20M will require such
filters. As Allison has alluded to in a few posts, it's relatively easy
to do on 10M as the spur is some distance away. Not so easy on the others.

If this is the way to go, then we should come up with designs for such
filters. Those that are skilled in this area can obviously contribute.

If indeed it is practical then my thoughts are to mount a small pcb
vertically in the area of the existing 30MHz LPF. For control of the
filters, a simple method appears to be the use of the SN74CBT3253
multiplexer chip as used in mcHF BPF switching. To control the chip
requires 2 bits and give 4 possible filters. (enough ?) To actually
provide these two switch logic signals, my proposal is to use an I2C
expander chip, the PCA9536D with suitable firmware changes..

The picture shows a PCB with 4 BPF on it just to show how it could be
done. NOT ones for uBITX I should add. It's about 1" high and has smd
parts on it. The filters below 10M band though will require fairly hi-Q
inductors i think for good performance. Tuning the filter could be an
issue if you don't have an SA. The filters may tend to be a bit lossy,
so a buffer stage might be needed also.

glenn - vk3pe

vk3pe
--
bark less - wag more

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