Re: Understanding Spurious Emissions
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Caution here seems to be that if you have modified your uBITX from original,
if you are using an amplified microphone, or if you have pushed the gain
excessively you may have increased the spurious output.? Unmodified uBITX
operating within stated specification should be okay.? Tests were performed
during initial design and prototype testing to make sure that this is the case.
However, if you have applied any of the many modifications that are described
on this venue, or any other modifications, then it would be your responsibility
to insure that these changes to the original design do not cause it to operate '
outside of original specifications.
Arv
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On Fri, Jun 22, 2018 at 10:59 AM Jerry Gaffke via Groups.Io <jgaffke= [email protected]> wrote: I wouldn't operate the transmitter on 15m,12m,10m without
either a fix for the spur or some way to monitor it.
Monitoring could be done with a second receiver plus a step attenuator.
Transmit into a dummy load while monitoring the spur at 44.995mhz-Fop through a step attenuator,?
where Fop is your operating frequency.
So if you are operating at? Fop = 29.0 mhz, the spur is at 44.995-29=15.995mhz, give or take a couple khz.?
Determine the signal strength of the spur, perhaps using the receiver S-meter.
Then tune in your operating frequency, and use the step attenuator to reduce the strength of your main signal
into the dummy load until you get the same S-meter reading as you did for the spur.
The additional attenuation required tells you how many dB below your main signal that spur is.
A good source for a step attenuator is the "Attenuator Kit #13" from Kees:??
If you can't be bothered to do the procedure above, it may not be a good idea to be transmitting?
on the 15m, 12m and 10m bands using the uBitx.
This is especially true if you have jacked up your mike gain somehow to get more power out,
or are running your uBitx into a high powered external amplifier.
Some do report that a properly adjusted uBitx can have that spur down near the legal limit
of 43 dB below the main signal.? But that is pretty much the best possible case.
Your best possible case could be much worse, the gain of the 45mhz amp in particular will be
all over the map depending on the characteristics of the mmbt3904's that happen to be in your rig.
And it will definitely get worse if you raise the mike gain, or shout into the mike.
We seem to know the best case, I would want to know my worst case?
before attempting to operate on these upper bands.?
There are several solutions to this.
We could call it a rig suitable for operation from 160m to 17m, forget the upper bands,
replacing the top transmit LPF with one suitable for 160m.
We could replace the 30mhz LPF at L1,2,3,4 with different filters for each band.
This is what pretty much all other HF transceivers do.
Or we could raise the first IF frequency from 45mhz to something well above 60mhz,
perhaps the 86.85mhz of post? ?/g/BITX20/message/52460
Since 86.85mhz - 30mhz puts the lowest possible frequency for the spur at 56.85mhz,
the spur won't get through the stock 30mhz LPF at L1,2,3,4
I'd prefer to see a fix that replaces the 45mhz IF, everything there including those two diode ring mixers.
This could be a small daughterboard that mounts into the holes remaining when the mixer transformers are removed.
All 6 mmbt3904's would be replaced with BFR106's, the 86.85 mhz PX1002 is used as the filter,
the mixers redesigned for 86mhz.? Gains would be adjusted in both IF amps to minimize the spurs,
and also minimize the carrier we have leaking through.
Parts count and topology remain the same as the stock uBitx, no added complexity,
though we could add some sort of RF AGC attenuator for some extra pennies.
The better transistors will allow an even drive to the PA, giving us a better shot at getting
the same power out to the antenna on 10m as on 80m.
Total cost of parts can be under $10, so it might be a kit that sells for $20 at sufficient volume.
If hfsignals decides to move in this direction, the only significant hit on cost or procurement
would be the $6 PX1002 (volume pricing) vs the custom built 45mhz crystal filter (whatever that costs).
This has not yet been tried.
Jerry, KE7ER
On Fri, Jun 22, 2018 at 05:18 am, ajparent1/KB1GMX wrote:
No, your analogy would be right if you were on the wrong side of the road.?
If you get near even fully driving the modulator the last mixer is easily 10+ db
past saturated as result of there being enough IF gain to assure that.
For those that never go above 20Mhz its adequately filtered and not an issue.
Allison
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Howard Fidel? June 21? ?#52728
Got it. Thanks. I never saw on my uBitx the numbers you reported, but I don't think it is a serious problem for a barefoot uBitx, which is the main point I was trying to make. It seemed like the group was spending a lot of energy on something that is like driving 60 MPH in a 55 zone.?
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