Allison, many thanks for looking into that.
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The nasty spur is not a problem from 3.5mhz up to 18mhz.
So for operators that never use the 15, 12 and 10 meter bands, this is not of concern.
For many of us, a $129 rig that covers 80-17 meters is still a very good deal.
For those wishing to use the rig above 18mhz, perhaps some sort of external band-pass
or high-pass filter is in order between the uBitx and the antenna.
Some (but not all) antenna tuners might be sufficient.
If the audio from the mike amp is kept reasonable, Allison reports that the spur is 40+ dB down.
I assume it should be 43 dB below our primary signal, so it's marginal at best.
But we might avoid the worst of this problem by somehow monitoring the transmit signal level
at the mixers. Allison has found that gain with the 2n3904's can vary on the upper bands
depending on your particular device characteristics, so it currently is not sufficient to just
check the level at one point in the chain. However, if gains can be made more predictable,
perhaps monitoring at the top of RV1 using a diode RF probe is sufficient?
The uBitx has a 30mhz low pass filter between the mixers and the power amp at
As Allison says, most multiband rigs would have switched bandpass filters in that position.
Signals are quite low level, could probably be done with analog switches rather than relays.
So yet another possibility is to add a daughterboard with switched bandpass filters to
replace the filter at L1,2,3,4. We only need 3 or 4 filters there and parts can be small and cheap,
so not so bad.
The spur is at the first IF frequency of 45mhz minus the operating frequency.
If the first IF is raised from 45mhz to 70mhz or more, that should remove the spur for all
frequencies from 3.5 to 30mhz. Will definitely require something better than the 2n3904's
within that 70mhz IF strip.
Anybody see any other possible solutions?
On Fri, Jun 1, 2018 at 08:18 pm, ajparent1/KB1GMX wrote:
Here is the problem...
There are a few actually. Improving the amp leads to acknowledging other issues.
SSB CASE only:
For the case of transmit for 15 and 10 M the biltier used has a cutoff over 30Mhz
so effectively the amp will pass though everything it hears. If you overdrive
the mixers especially the 45mhz output mixer you get two things. The desired
output say 28Mhz you get a second actually two others one at 11.995mhz about
50 to 60 db down and the other at about 17mhz and if the drive is low its 40+ db
down, push the audio and it can be 25db down from carrier out. The amplifier
having no filtering below 30mhz will gladly amplify all of them and does! This
is not an issue for 3-about 18mhz as the output low pass filter that are switched
in helps attenuate the spurs.
The math is:
Output is IF(about 45mhz) + LO (48 to 75mhz)= 3 to 30 mhz
The spur case is IF(45mhz) - Output frequency (42 to 15mhz) Its going down while the dial frequency is going up.
For up to 18mhz the output lo pass filters and the inside 33mhz low pass filter are at work doing the job.
However for 21 and 28 mhz both filters are roughly 33-35mhz cutoff and the spur is now down around
24 and 17 mhz respectively. They will be about 40-50db down until the 45mhz mixer overloads and
then the spurs get very much stronger at a frequency well below the cutoff of the low pass filters.
Since power output is low on 10M people will likely push the audio and the spur will get significantly
The is due to the nature of DBMs, they are three port devices and all three can receive input or deliver outputs.
Also inside the mixer all possible sums and differences for the base frequencies and their harmonics exist.
This was determined without the amp operating by breaking the circuit at C200 for a spectrum analyzer.
We can't blame the amplifier chain for this as it would just do its designed duty and amplify everything.
Right now I have no solution. The usual one commercial radios implement is a switched filter
at the point were L1 though L4 exist (and C200 to c204) and the filter would be for 15M, 12M and 10M.
I'm looking at and for other ideas.
I've stopped work on the amp (I've had success but the spur took me on a divergent path) till a
solution can be had.
This does not show up in CW mode as the way the radio is keyed there is no 45mhz contribution.