Category: Harmonics, spurs and IMD

Posted on

Testing removal of spurs with additional 45MHz filter

The photo above shows an additional 45MHz filter (15khz passband) inserted in place of R27 (you can’t see the centre wire on the filter, which is attached to the ground end of R13).

Early indications are that this removes all of the offending spurs.   This will make it  a uBITx.net ESSENTIAL MOD.  The mod has, however, yet to be tested by uBITX.net.

Warren WA8TOD has completed spectrum analysis plots for each band, and these have been reproduced below.  The plots show removal of all unwanted spurs.

Conditions for the test:

  •  eBay filter in place of R27. No other changes.
  • Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

Yet to be verified:

  • 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance.  It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.
  • Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance.  Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Reference
Posted on

10dB spur reduction mod

Raj, VU2ZAP has come up with a fix that reduces the spurs by up to 10 db  and requires ONLY ONE part to be added.   Farhan VU2ESE has come up with an alternative modification.

These mods result in a significant change in the level of spurs above 10MHz with some improvement below this frequency as well.

With Raj’s mod CW may not work anymore and will need some more mods.  With Farhan’s modification CW will still work.

Raj VU2ZAP Instructions

  1. T2 – desolder the transformer wires that go to pin 3 and 5. Pin 1 has a square pad.
  2. Bring out the two wires above board and join them together and solder.
  3. Take a 45Mhz filter- 45M15 or  similar 2 pole  (one crystal only) and solder one end of the filter to the wires of T2 pulled out. The centre filter wire to ground at one end of R26. Check which end of the resistor is grounded.
  4. Solder the third wire of the filter to C10/R27 junction.

This mod prevents the leaked TX signal that gets amplified by the 1st bidirectional amplifier from getting into the first mixer and creating havoc.

Farhan VU2ESE Instructions

  1. Remove R27
  2. Solder the 45Mhz filter two extreme ends to the pads of the resistor.
  3. Solder the center lead of the filter to the nearest ground. R13 is very near with a ground via.

Using the first method (Raj’s solution) the extra filter will work in RX mode as well as TX, but CW is disabled.  In  the second approach, the filter is only used in the TX path.

Folks with DSA815 or better please share your feed back. The filter may work better properly terminated.

Reference
Posted on

Cleaning up the transformers in the output stage may fix harmonics

Jim AB7VF suggests that much of the harmonic cruft in the µBITx is from DC current flowing through the L7 and L8 ferrites that effectively lowers the inductance as current increases and allows RF to go everywhere.

Jim replaced the electrolytics as can be seen from the picture ..

He suggests that the transformer is effectively a centre tap.  He wanted tighter coupling between the two windings ….  The electromagnetic field set up by the DC passing through the coil will bias the ferrite “magnetic domains” causing a shift in the B-H field resulting in loss of inductance and the generation of spurs .

When feeding the center tap – current flows up toward the “dot” or start of the top winding “left hand rule” will give you the polarity of the magnetic field around the top coil … current also flows through the bottom coil away from the dot or start of the bottom winding creating a magnetic field opposing the one created by the top coil. The net result is no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.

The following photo shows the 80 meter output of Jim’s unit after doing the L7,L8 mod and the output transformer mod.

Jim suggests putting a proper inductor on the IRF510’s that is NOT affected by DC current flowing through it and you will get legal output on 80m CW.

uBITx.net would be interested in whether this approach works for others in cleaning up the harmonics, because it will be a lot cheaper and easier than sorting the LPFs.

Reference
Posted on

80m harmonic fix by changing the capacitance of the LPF

Howard WB2VXW previously mentioned that he would try to reduce the harmonics by changing the output to 25 ohms and adding a step up transformer to go back to 50 ohms for the output. This would allow tripling the values of the capacitors at the input and output of the filters, reducing the effect of the stray relay and layout capacitance.

In the end after more simulation, Howard decided just to change the filter characteristics and retain the 50 ohms termination impedance.

Howard was able to come up with a filter that doubles the capacitance.  At least on 80 meters, with this new filter design, the harmonics don’t exceed -45 dB in his tests. Not as much margin as he would like, but legal.

Howard changed the 3 inductors to 1.6 uH by adding 2 turns on each.  He added an extra 1000 pF cap in parallel with the input and output caps, bringing the total to 2000 pF.  He also added a 620 pF cap in parallel with the one of the two paralleled 1000 pF caps in the middle two sections for a total of 2620 pF. (750 pF would have been better, but he didn’t have one).

Howard is asking others to give this ago.   After validation of the 80m LPF redesign he plans to try a similar solution for the 40 meter band.

Reference
Posted on

Kees LPF and Relay external board

This is Kees K5BCQ 6x LPF/BPF board showing one LPF plug-in. The LPF parts were removed from the uBITX and moved to the plug-in board. This is easy with a hot air rework station like the ones all over eBay for about US$50. The relay removal is also “relatively ” easy with no pulled pads (using the large square tip which heats 4 pins at a time).

The plug-in footprint is the same as QRP Labs uses on their LPFs/BPFs.  Kees mounted the pins on the other side of the board because it reduces the overall height. You can solder the coax to the SMA connector footprint or use SMA connectors. Many options.

The photo below shows the four transplanted filters from the main µBITx board and the main board LPF corner that is now stripped bare.

Reference
Posted on

Single-ended relay board for existing LPFs 

Gordon has been testing out his mult-relay single ended board.  The concept here is that the existing LPFs and relays are reconfigured on the µBITx main board and the ends of the LPFs are connected to a new daughter board attached behind the LPFs and relays to provide for greater RF separation between LPFs.

And this photo shows the cuts made to the top of the board:

And showing the connections to the add-in board

And a different view showing the wiring to the main board:

And the relay power on connections:

The results seem promising.

A separate board with all LPFs mounted on the external board  (e.g. this one) would not be much larger overall and would require fewer connections to the main board.

A document with all the explanations of how to create this modification, including photos of traces to cut can be found here.

Reference
Posted on

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 …

Posted on

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.

Posted on

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