Category: Hardware

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Update on fixes for uBITx spurs and harmonics

The number of posts on the BITX20 IO Groups list hasn’t gone down very much over the last month,  but the ideas that swirl are all about potential fixes to the µBITx spurs and harmonics.  And a new problem has been identified … very poor intermodulation results.

The µBITx is simply not putting out the nice pure signals we would like.   We are now trapped in a pattern of waiting for the  “magic bullet” solution to our woes, but sadly this seems unlikely to result.   There are just too many issues that need to be resolved.

Somewhat understandably after months of exciting developments, new contributions from constructors with substantive mods and hacks making the whole package sing are now no longer emerging.  We had become somewhat used to regular exciting new updates on processor or screen additions, or simple fixes to niggly issues (such as low audio, or distorted audio, or the annoying pop on toggling between RX and TX), and now we are becoming a bit more despondent about those more serious problems of compromised output.

Some progress is being made in solving spurs and harmonics.  We know that the built-in LPF filters and relays in the output stage can simple be disconnected and bypassed by adding a new external filter set (e.g. using Hans G0UPL’s filter board and filter kits for LF through VHF) on the antenna output.  After doing this the rig should work just fine on CW operations on all bands.   Of course, it may need either a hack to the firmware to get automatic band switching to work, or we can just use a rotary switch.

Alternatively, there has been work on modifying the filters in situ.  By changing the relay switching logic (e.g. mounting the relays under the main board and rewiring the switch points) and at the same time adding a second set of relays on an add-in daughter board we can restore effective functioning of the existing LPFs.   They work fine, but the switching system creates unwanted blow-by.  However, this involves quite a bit of hacking on the main board that many constructors may baulk at.

And none of this solves the issues with the spurs on SSB on 15m and above, that requires yet another fix of a second filter board at the output of the 45MHz mixer, and then there is the probably unrepairable issue with intermodulation products are probably way above what would normally be expected.

So where does this all leave us?   We have had a lot of fun on the journey to date.  We all acknowledge that most of the fun has been in the journey, but what’s the point in a journey that has no end?

Should we wait a bit longer hoping for a fairy godmother and a magic bullet solution to appear?   We  could spent more money to acquire the new QRPLabs transceiver  kit when it comes out (hey, all bands all modes and around US$150!)?    However, we already know that Hans G0UPL won’t release the firmware source code – therefore, there will be no add-ons and feature enhancements unless they come from Hans.

We could now just bite the bullet and hack our µBITx to turn it into a CW rig?  But it is supposed to be a phone rig as well.

Hold in there a bit longer, the “magic bullet” may eventuate.  Who knows? And I am sure there are plenty of other projects under the workbench for you to get on with in the meantime.   At this QTH there’s a couple of mag loops under construction (Teensy controlled) and a bench power supply project …

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Low power on 80m?

Allison KB1GMX notes that if you have a v4 board and experiencing low power output on lower bands (80m and 40m) you may want to check the emitter of Q90 to see if a .01uf or a 470pf or 220PF is there.

There was a change to lower the value from .01 to 470 pf to level the output
power so 80M wasn’t 15-20W when 20m was maybe 8 watts output.

Some have experienced  that low bands work very poorly with a 470pF or 220pF capacitor.  You may want to increase the capacitor back to 0.01µF or perhaps 1000pf.

Reference
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Biteensio with LCD touch screen coming

Jim W0EB is working on a 2.8″ touch panel for the triumverate’s BITeensio.  That will make four different alternative touch screen options for constructors:  VU2SPF Mega2560 attached 2,4″ LCD (first up way back in January),  KD8CEC Nextion screen (in a range of sizes from 2.4″ to 7″) , BiTeensio 2.8″, and the long awaited JackAl (in 5″ and 7″ sizes).

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Summary update on efforts to find solutions to harmonics and spurs

There have been many postings on the BITX20 IO Groups list following the raising of concerns by a couple of list members about harmonics and spurs emanating from the µBITx.

It is still rather early to be sure that these issues apply to every µBITx produced.   They certainly don’t apply to single-band BITx designs like the BITx40.   Raj VU2ZAP suggests that his µBITx is pretty clean (except 10m) and has provided spectrum screenshots to demonstrate this.  However, several have found issues with their boards.  The issues seem to lie with design deficiencies (layout issues, relay switching design and the use of a double balanced mixer without specific BPFs) suggesting the issues can be overcome with future revisions to the µBITx main board, but will require hardware mods on the existing boards in service.

The spurs, as identified by Allison KB1GMX appear on SSB on higher bands (above 18MHz) as a result of unwanted mixer by-products and exacerbatedn by overdriven audio.  There may be ways of reducing the spurs sufficiently by modifying the circuitry that injects the VFO output via the si5351a to combine with the 45MHz IF signal.  Higher injection levels from the si5151a are being experimented with by Ian KD8CEC, but it is unclear as yet whether this approach could reduce spurs.  Others are pursuing alternative hardware solutions, including incorporation of filters (BPFs) to replace the 0-30Mhz LPF following the mixer stage which is in circuit on both receive and transmit.

Several others  are working on circuit board options and design suggestions that would address the odd harmonics (3rd, 5th, 7th, etc.) found particularly on CW.  This involves reworking the final LPF matrix (four LPFs).  This could be a daughter card that plugs into the main board or, alternatively could be a completely separate stand-alone circuit board.  There is discussion about whether these should be plugged-in manually, switched using a standard wafer switch, or be relay driven, as in the µBITx design.  Several options may well result.

If you are concerned right now about a fix for the harmonic generation on CW, you can use outboard LPF filters.  For example, the filters that Hans G0UPL has a 5 LPF board  kit available for sale, and also sells individual filter kits for each band.  Minor changes to firmware would be required to select the correct filter for each band.  You would also need to rescue some additional ports (e.g. by using an i2c display).   Alternatively you could insert an LPF in the antenna line and change it out when you change band.

You can use Hans G0UPL’s filter board for switching in and out Band Pass Filters as well.  These are best used to replace the existing 0-30MHz LPF.   While you may be kept busy for two or three evenings winding toroids, the solutions to these problems already exist.   You will end up with a cleaner transmitter and receiver after doing this.

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Ashhar Farhan VU2ESE responds to comments on spectral purity

Ashhar Farhan VU2ESE responds to recent posting on the BITX20 IO Groups list surrounding issues with spectral purity for the µBITx.   His comments are reproduced in full below

Peeps,

Let me put some of the discussions on harmonic and spur purity of ubitx in context. This is going to be a long read, so bear with me as we plod through this. At the outset, I must thank Allison, Arv, Raj and Warren for their deep dives into these challenges.

Before I deep dive into these numbers, let me sum it up for those who just want the headline :

The ubitx put out harmonics less than 2 milliwatts on all the bands. That’s well below what a ‘commercially designed’, well behaved, FCC compliant, 100 watt commerical set would spew out. There are some easy fixes.

Details

Now, onto the details:

Let’s consider a no-name, standard issue, 100 watt HF transceiver with the harmonics below the required -43 dbc level. 100 watts is +50dbm. +50dbm – 43 dbc = +7dbm. That is about 5 milliwatts in harmonics. This is what we live with on air with all those factory built rigs. 5 milliwatts of harmonics. If you add a linear it can even get worse. An SB-220 after an Elecraft K2, will put out almost a watt of harmonics.

Now, consider our 10 watt ubitx. You can see the scans done by Warren. They are on https://groups.io/g/BITX20/album?id=65861. He has helpfully drawn a green line across the scans. The green line is the -43dbc line. Apart from the actual signal, everything else must stay below the green line as per the FCC guideline.

Let’s sum up the report :

10 meters : SSB & CW are good  (https://groups.io/g/BITX20/photo/65861/0?p=Name,,,20,1,0,0)

12 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/1?p=Name,,,20,1,0,0)

15 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/2?p=Name,,,20,1,0,0)

17 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/3?p=Name,,,20,1,0,0)

20 meters: SSB harmonics are higher by 4db, CW is good (https://groups.io/g/BITX20/photo/65861/4?p=Name,,,20,1,0,0).

30 meters: CW harmonics are 4db above the required level. SSB is not allowed anyway (https://groups.io/g/BITX20/photo/65861/5?p=Name,,,20,1,0,0)

40 meters: SSB is 3db above the green line, CW is even higher by 10 db (https://groups.io/g/BITX20/photo/65861/6?p=Name,,,20,1,0,0)

80 meters: SSB is within the spec, CW is out of spec by 5 db (https://groups.io/g/BITX20/photo/65861/8?p=Name,,,20,1,0,0)

To sum it up:

SSB: it is within the rulebook everywhere except on 20M & 40M by 3 db.

CW: it is within the rulebook except on  on 30M, 40M & 80 M.

So, what’s the problem and how is it cured?

First: the trouble seems to be more with CW than SSB harmonics. We can reduce the CW level by decreasing the CW drive level. This is determined by the amount of current that flows into the front-end diode mixer by changing the R105 to a higher value resistor. I guess one can also do this in software by reducing the CLOCK #2 current to 2 ma from 8 ma when in CW. This will clear CW for all the bands.

Second: the alignment as it comes out from HF signals will show far better harmonic suppression than being reported here. For instance, the -38 dbc on 20 meters and -30 db on 40 meters will almost disappear if you balance it out with the bias on the IRF510s. The factory alignment works like this : First crank up both the IRF510s for 100 ma standing current on each, then tweak one of them to null out the harmonics. It is like balancing out the carrier on diode modulator.

Third: below 14 MHz, the IRF510s are distorting with too much drive. If you back off the drive to adjust to a leve of 7 watts, the harmonics will climb down to be within the spec. (Remember that the harmonics are not present at the output of the 45Mhz-to-HF mixer, they are generated in the power chain). This was also Allison’s suggestion.

Future revisions

We use 3 section low pass filters made out of micrometals’ toroids and  C0G caps from AVX. Most commercial sets use only 2 section low pass filters.  And yes, we can change the relays. I doubt if the problem is the relays. If it were so, it would be worse on the higher bands. The fact that the harmonics are climbing at the lower frequencies points to overdrive, not filter design. The filter layout etc is less critical on the lower bands as opposed to higher bands.

I guess that we we really need is a good transmit side ALC to cut the drive on lower bands. The gain varies by as more than 10 db across the spectrum from 3 Mhz to 30 MHz. Given that we want to keep the design to use what is available in a regular junkbox, this is a challenge to overcome with IRF510s and general purpose transistors. Allison has tried using 2N2222As with some success.

So, in a nutshell, here is the summary:

The ubitx remains less polluting than a 100 watt commercial rig that works within the FCC requirements. The harmonics at 14 Mhz and below can be controlled by backing off the drive level.

I would like to hear contrary views and figures. As I said before, unlike a commercially made radio, ubitx will thrive on critical feedback and design suggestions. This only gets better with time, like any good open source design.

Thanks all for the inputs.

– f

Reference
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A crisis that gets worse … uBITx woes!

A full blown mid-life crisis can sometimes only get worse!

Further investigations of the issues surrounding µBITx odd harmonics on CW on the lower bands are showing that the problem is from board layout issues in the relay switching and LPF section of the board.

Allison KB1GMX has cut up her board and done testing on the LPF/relay section.  This shows the following:

  • On 3.5 MHz in TX mode, the 3.5 MHz filter is selected the blow by limits us to barely 25DB of filtering of harmonics at 30 MHz
  • On 7.0 MHz in  TX mode,  the 7 MHz filter is  selected, blow by limits us to maybe 25db  of filtering.’
  • On 21.0 MHz  in  TX mode with the 21 MHz filter selected the blow by limits us to about 30db at 30MHz of filtering.

While the “blow by” may vary by individual unit and it is posisble to argue over the magnitude,  over all the filters are simply unacceptable.

Allison says “At this point I’d rip out the filters and the relays and even the TR relay (KT1-3 and K3) and route everything to an external low pass board. At that point I think we have a chance with the external board performance being unknown but for certain it cannot be worse.”

Stay calm

For many of us, all of this is simply more bad news.  However, we should stay calm at this point and start using our commercial rigs or other kits for a while.  With such a large community of constructors,  there will be solutions found to some of these problems.   You may well need to do some mods though for your µBITx to be fully legal in your country.

Summary

There are two issues known to date:

  • Spurious emissions (spurs) generated in the double balanced mixer at 45 MHz that exceed US emission standards on SSB above 18 MHz.  This issue is exacerbated by higher audio drive levels (e.g. if you have added a mic preamp or voice compressor).  This issue is an unnoticed design issue.

It is likely that this issue can only be resolved by adding additional filtering to replace the existing 30MHz LPF following the mixer, or by redesign of the µBITx with a higher IF frequency (maybe 70MHz or higher).

  • Harmonic output on 3rd, 5th, 7th, 9th harmonic, etc. that exceeds US emission standards on CW on some bands.   This issue is caused by “blow by” in the LPFs and the associated relay switching.  This is a design issue with the layout of the board and the relay switching design (using only one relay to switch both ends and the sequential routing of relays on lower bands).

It is likely that this issues can only be fixed by disconnecting the existing LPFs and reconstructing the filtering and relay switching system on a new daughter board.

Reference #1
Reference #2
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A microphone pre-amp with no additional power requirement

Karsten DB7JB provides this simple circuit to boost mic gain (perhaps if you are quietly spoken) that does not require any modification of the μBitx board. It is powered off the existing microphone line (which already provides bias for the electret microphone).   It could be built right into your microphone housing, given it has only seven parts and the electrolytic capacitors are very low voltage components.

With the supplied electret microphone in the kit you will get additional gain for fully modulating your SSB signal. However, you will need to be careful not to overdrive the mixer, which as Allison has indicated in her post, is easily over-driven, which can result in spurious emissions from unintended mixer products.  

 

 

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A mid-life crisis

uBITx.net followers will remember the concerns that Allison KB1GMX raised about the spurs generated by the µBITx above 18 MHz on SSB (but not on CW), at least on her ‘Sample of One’.

“No one is going to want to hear this” says Warren WA8TOD, “But the design of the PA output filter is probably fatally flawed and it is likely due to board layout issues.”

Warren was puzzled how the high order harmonics from 80 meter operation, for example, could possibly survive a 7th order filter which should have well over 50 dB of attenuation at the 5th harmonic. So he set out to test the results with a 9th order filter, figuring more is better. The harmonics remained. But in troubleshooting that installation the source of the issue was revealed.

In the attached screenshot the purple trace is the 80 meter CW output with the harmonics clearly far in excess of the green -43 dBc legal threshold (NB The line on the screen is actually at -53dB).

The blue trace is the output with L20 removed which disables the 80 meter filter and blocks RF output from the finals from reaching the output connector. (Don’t worry….. the tests were carried out with the drive reduced so the output was only one half watt. And I have a bag of spare IRF510s on hand……). The harmonics remain! How can that be?

The circuit diagram shows the path 80 meter RF follows through all three switching relays in order to reach the filter. It is a very novel and inventive concept to use three relays to switch both ends of 4 filters. Most manufacturers would require 8 separate relays to do this job. And for a very good reason.

In order for a filter to provide the required 50+ dB of attenuation, the filter itself must have 60 dB or more of input-to-output isolation. If it does not then the unwanted products simply bypass the filter and go directly to the output. That appears to be exactly what is happening in the filter complex of this unit.

There is no easy fix for this. No amount of on board filtering and tweaking is going to improve harmonic suppression until the path around the filters is blocked. I suspect that path is via the relays themselves but board layout often is critical as well in such cases. Warren’s guess is that the only solution would be either extensive external filtering or a carefully laid out daughter card to replace the existing filters and relays.

Reference
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New build uBITx with Nextion screen

Jim Reagan W0CHL has made a compact build using a Nextion display.

He used a Context Engineering (Fry’s Electronics) case. It’s a tight fit and not for the beginner. I had to “mill out” (dremel) areas for the stand offs (they are nicely recessed on the front panel).   He also chopped off (with a bandsaw) off back 1 3/4″ of the case. It’s using the 3.2″ Nextion display and has a quiet  24 volt fan to keep the finals cool.  The final size of the case is just 3 x 6 x 6 1/2″.

Reference