The All-band HF Amateur Radio Transceiver: Just waiting to be modified
Dave KD0UYH has made a microphone from an old desk lamp purchased from a “resell it” shop. He says, “Not much to look at, but it serves my purpose. Used some vacuum tubing to attach the mic element and hot glued a handful of nuts into the base to give it some weight.”
Alison KB1GMX has advice to constructors on the spurs on the higher bands:
For bands below 20Mhz spurs are NOT an issue as the low pass filters catch it.
Spurs are only a problem for SSB and frequencies greater than 20mhz.
NOTE: due to the way the uBTX does CW it is never an issue on any band.
The short form is when you mix two frequencies you get a third, in a perfect world.
The diode mixers used are handy but they can present conundrums. If any of the three ports (IF, LO, and RF) are mismatched (think SWR) The signal can be reflected back in. Since DBMs are omnivorous in that any port can be input or output and if mismatched both! This does not include effects of distortion in the source signals.But in the real world things like this exist.
Double balanced mixers also suffer from overload, too many and too strong and you get a plethora of signals. What that means for lots of simple and complex reasons you can get “spurs” or spurious outputs that are undesired.
Basic math, addition and subtraction:
If you mix 45Mhz with 73mhz you get 28mhz. We want that and the radio needs that. However if any of the 28 gets reflected back into the DBM where it originated it mixes with 45mhz and we get 17mhz.With those four signals you get mixtures of those like:
Those are “first order” as they do not involve harmonics. They will be the strongest, but not always equal strength.
Both inputs can have harmonics like 90mhz and 146mhz and the 34 and 56 coming out can have harmonics too. If you add and subtract all the possibles you get an increasing sea of signals some weak some stronger. We will not cover the possibles as the first order ones are the most troublesome.
The solution traditionally applied is band pass filters or if it isn’t between 28 and 29.9999 the filter strongly attenuates it. But you need a band pass filter for most every band… uBitx takes the path of below a certain frequency you only need low pass filters and fewer of them. And it generally works well especially for 80 though 17M…
But at 20mhz and up the low pass filter passes everything below 30mhz and if you overdrive the rig slightly you get a spur on the tech window on 10m where the spur is 16.5 to 16.7mhz and there is no filter for that. What makes this worse is some radios are very poor at 10M putting out maybe 2W so pushing the audio to get more invites the problem to be greatly worse.
There is no setting we can safely give that absolutely assures there will
be no problem that is consistent with maximum achievable power.
As a licensed amateur radio operators we are responsible for signal quality and also not generating signals outside our assigned bands.
There are two solutions one is bandpass the other is high pass filter.
Either way the rig must be modified to allow those and there are side effects.
One side effect is you need extra switching not provided. The other is any filter has a loss though it and that would further reduce power out.
Short of that, keep the power right down on the higher bands and go for it…
Kees K5BCQs has 403 paid orders for AGC and/or Anti-pop Mini-Kits and 399 of these have been shipped. He has additional boards and parts and will continue to supply these Mini-Kits as long as there is demand. Details are in the “Files” section of the BITX20 webpage under his call K5BCQ.
Kees was somewhat surprised to be able to process such a quantity over a short period of time. That indicates there are opportunities for others to join in with their ideas. As Kees notes, the uBITX is a great vehicle for modifying and improving.
One of the challenges with digital modes is their higher duty cycle than phone or CW. However, Ashhar Farhan VU2ESE, designer of the µBITx, assures us that on FT8 the standard heatsinks supplied with the kit are adequate. FT8 has a 50% duty cycle.
This may not apply with other digital modes such as WSPR.
Jon Titus, KZ1G has provided a first pass at preparing “µBITX TUNEUP” instructions, and is welcoming your comments. He has aimed for ease of understanding, particularly for newcomers to our interesting hobby.
The first draft can be downloaded here; uBITX tune up instructions.
Email Jon your comments
There is a Facebook group for the µBITx in addition to the BITX20 list that represents the primary source of news articles on this website.
One of the members of the Facebook Group (Ufi Auttorri) has produced a diagnostic guide for constructors who are facing problems with their µBITx. While it is still a work in progress, and won’t cover all issues arising with the rig, it is an excellent starting for constructors who have run into issues. You can find it here:
https://drive.google.com/file/d/1PBK-cdn4-9EunFHl2bXBSUnx1RFP9Z_f/edit
Ian KD8CEC is working on a protocol to allow Nextion LCD touch displays to communicate with the µBITx.
He is implementing this a little differently to most support for Nextion LCDs.
The firmware will handle communication between the uBITX arduino and Nextion LCD using template files.
There are quite a few variables in the Nextion LCD. If the status of any parameters in the uBITX changes, the variables sent to the Nextion LCD will also be changed at the same time (and vice-versa). This should allow any Nextion display and any configuration of the display’s User Interface to interact with the µBITx. This means constructors can customise their µBITx display using the Windows GUI used with the Nextion to configure the “look and feel”.
For example, when uBITX’s frequency changes, it is transferred to a specific variable on Nextion LCD.
Example code
Below is an example of simple frequency and mode display changes
//Frequency
if(vc.val!=-1)
{
nFreq.val=vc.val
vc.val=-1
}
//Mode
if(cc.val!=-1)
{
if(cc.val==2)
{
tMainMode.txt=”LSB”
}
else
{
}
cc.val=-1
}
Alison KB1GMX has been exploring ways to flatten the TX output across bands over the last couple of weeks, but says,
“Here is the problem… Improving the amp leads to acknowledging other issues. In particular … the spurs!”
When transmitting on 15m and 10m the LPF has a cutoff frequency over 30 MHz, so effectively the amp will pass everything through below 30 MHz.
If you overdrive the mixers especially the 45mhz output mixer you get more than one product from the mixer. For a desired output at 28 MHz, you actually get two other outputs, one at 11.995 MHz (that is 50 to 60db down on the desired output) and another at about 17 MHz, which if the mixer is not over-driven is around 40dB down. However, if the audio is pushed a bit harder, it may be only 25db down from the desired frequency. The amplifier has no filtering below 30MHz and will gladly amplify all of the signals through the pre-driver, driver and finals stages and pass them out to your antenna.
This is not an issue until you hit around 18 MHz as the output low pass filters that are switched in help attenuate the spurs.
The Maths!
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 33-35 MHz cutoff and the spur is now down around 24 and 17 MHz respectively. They will be about 40-50db down until the point where 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 worse.
The is due to the nature of Double Balanced Mixers: 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 will exist.
This was determined, without the amp operating, by breaking the circuit at C200 to allow a spectrum analyzer to be inserted. We can’t blame the amplifier chain for this as it would just do its designed duty and amplify everything.
Right now Allison has no obvious solution to present.
Commercial radios implement a switched filter at the point were L1 though L4 (and C200 to C204) are present and a filter would be switched in for 15M, 12M and 10M. Allison is investigating other ideas.
This issue does not show up in CW mode as the radio is keyed in a different manner and there is no 45 MHz contribution.
Jerry KE7ER comments that the spur is not a problem from 3.5 MHz up to 18MHz. So for operators that never use the 15, 12 and 10 meter bands on SSB, this should 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 18 MHz, perhaps some sort of external band-pass or high-pass filter is in order between the µBitx 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. Jerry assumes it should be 43 dB below the primary signal to be compliant in the US, so it’s marginal at best. The worst of this problem could be avoided 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 would be sufficient?
The µBitx has a 30 MHz low pass filter between the mixers and the power amp. As Allison says, most multiband rigs would have switched bandpass filters in that position. Signals are quite low level, so this 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 45 MHz minus the operating frequency. If the first IF is raised from 45 MHz to 70 MHz or more, that should remove the spur for all frequencies from 3.5 to 30 MHz. That 70MHz IF strip would definitely require something better than the 2n3904’s that are in the µBITx!
John VK2ETA has uploaded his latest version of a variation on Ian KD8CEC’s software with a default of “stock standard” which means it should run on units “factory” wired.
It includes the “OPTION_ALC” which use a table and interpolation to set the attenuation for 3 levels (Low, High, Max) for SSB power output.
Not a complex code at all (just search for the “option_alc” keywork). This should give up to 50dB of attenuation. This should be plenty to control the power even for WSPR low power users.
You will probably need to attenuate the signal for “Max” power on 40 and 80m otherwise you will get well past the targeted 16-17W on the lower frequencies.
Note that on 10m, you will not get 10w out. You will need to follow Allison’s work on the power chain to see if a solution can be found to the fall-off in power out on 10m.
John’s software can be found here.
Gordon KX4Z has gerber files for a Raduino replacement and a full file note about his design. You can read about it here:
http://arrl-nfl.org/wp-content/uploads/2018/05/KX4ZDDSApplicationNote.pdf