Category: Harmonics, spurs and IMD

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Kit Projects selling outboard relay mod kit

David N8DAH from Kit-projects is marketing Gordon Gibby’s outboard relay mod as a kitset or for $5 more as a built-up board. This includes the circuit board, and all parts (including relays, diodes, inductors and caps).

Cost might seem high but David assures us he will be using quality parts.

The pre-orders were snapped up pretty quickly, but new stock will be coming shortly once the pre-orders have been shipped.

https://shop.kit-projects.com/index.php?route=product/product&path=59&product_id=119

Reference
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Summary of recommended spur mods

Jerry KE7ER,  provides summary details on the two know spurs mods:

Raj VU2ZAP recommended mod: 

Change out inductors at L5 and L7 with 1210 shielded surface mount inductors.

Here’s the post from Raj giving results for various inductor types at L5,L7:
https://groups.io/g/BITX20/message/60946

And another post from Raj, affirming that he was using a yellow S+M brand 1210 inductor.
https://groups.io/g/BITX20/message/60678

Here, Raj says that TDK-EPCOS parts work as well  (EPCOS was bought out by TDK):
https://groups.io/g/BITX20/message/61261

Unfortunately, I don’t see any complete part numbers in any forum posts
from somebody who has verified with a spectrum analyzer that spurs were reduced.

I bought a similar assortment of TDK-EPCOS-Murata parts from ebay,
have not verified results with a spectrum analyzer.

The parts they actually ship may vary with the parts they can find.
But here’s what they shipped to me

480pcs inductor coil size 1210 1008 (24x20pcs) EPCOS TDK MURATA
B82422A3470J 47nH 1210 EPCOS  20pcs
B82422A3101K 100nH 1210 EPCOS  20pcs
NL322522TR120K 120nH 1210 TDK  20pcs
B82412A3181K 180nH 1210 EPCOS 20pcs
1008CS221XG 220nH 1008 COILCRAFT 20pcs
B82422A3331K 330nH 1210 EPCOS 20pcs
B82422A3471J 470nH 1210 EPCOS 20pcs
NL322522TR56J 560nH 1210 TDK  20pcs
B82422A3681K 680nH 1210 EPCOS 20pcs
NL322522TR82J 820nH 1210 TDK  20pcs
B82412A1102K 1uH  1210 EPCOS  20pcs
FSLM2520-1R5J 1.5uH 1008 TOKO  20pcs
B82412A1222K 2.2uH 1210 EPCOS 20pcs
NL322522T3R3J 3.3uH 1210 TDK  20pcs
B82422T1472K 4.7uH 1210 EPCOS 20pcs
B82412A1562K 5.6uH 1210 EPCOS 20pcs
NLV25T6R8J 6.8uH 1008 TDK 20pcs
B82422A1822K 8.2uH 1210 EPCOS 20pcs
NLV25T120J   12uH  1008 TDK   20pcs
NL322522T270J  27uH 1210 TDK  20pcs
NLV25T330J   33uH 1008 TDK    20pcs
NLV25T680J   68uH  1008 TDK   20pcs
B82422A1104K 100uH 1210 EPCOS 20pcs
NL322522T121J 120uH 1210 TDK    20pcs

Ashhar Farhan VU2ESE mod used in v5 µBITx:

The v5 µBITx provides  a different method of fixing spurs on the v5 uBitx,
primarily the low pass filter at L31, L32, C205 near the first mixer.

That filter removes the 90mhz second harmonic coming out of the 45mhz IF amp. Other possibly significant changes on v5 with regard to spurs would be the BFR93W’s at Q20,21,22  (to replace the MMBT3904’s) and the move to 220 ohms at R26 (was 470).

Reference
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The experience of cleaning up spurs and harmonics in a V3 uBITx

v4 board at 10w on 21 MHz (overdriven)

Mike KU4QO has shared his results in trying to clean up the output of his V3 uBitx.

He thanks all the folks who spent their time on tracking down the causes and cures and shared his results with the group for others to reflect on.

His setup comprised a signal generator at 1kHz feeding the microphone input, and the radio set to SSB mode.  He applied 12.0 VDC for power. Output of the radio was fed through a 30db attenuator into a borrowed spectrum analyzer. He adjusted the signal generator’s output to vary output power in the transmitter.

Power output of the fundamental was just below the point where more microphone signal did not appreciably increase the output power:

80m  10 watts

40m    8 watts

20m    5 watts

17m    2 watts

15m    2 watts

12m    1.5 watts

10m    1 watt

As built, his V3 uBitx was marginally compliant to US FCC rules on 80, 40 and 20 meters.  Above 20m the output was messy as a result of lower frequency spurs. For example, on 15 meters there was a spur at 2.6 MHz that was down only 23dB from the fundamental.

He first completed the L5/L7 mod using surface mount 0.68uH inductors (from the Ebay seller in Poland), marked “T R68K”.  He re-ran the tests and saw no appreciable change in any of the results.

Next, he replaced the 5 relays with Axicom relays from Arrow.com. This change showed a significant change in the harmonics on the low bands. 80m, 40m and 20m went from marginally acceptable (barely -43dB from fundamental) to easily meeting FCC specs on those bands, with unwanted harmonic outputs dropping an additional 10dB or so. However (as expected), the relay change did nothing for the spurs resulting from operation on the higher bands.

Most interesting was watching the output spectrum while adjusting the microphone drive. For example, the nasty spur on 15 meters at 2.6 MHz went from 23dB down at a fundamental power of 2 watts, to below the noise at a fundamental power of 1 watt. At significantly reduced drive (power) levels, the high bands are all in compliance with FCC specs. Here are the reduced powers that provide a clean output signal:

17m   1.5 watts

15m   1 watt

12m   0.6 watts

10m   0.25 watts

Mike followed Raj’s directions for the L5/L7 mod. What should the affect of that mod have been? Should it have impacted those spurs on the high bands? He is questioning whether the inductors were actually not the correct type.

Jerry KE7ER thinks the consensus is that replacing L5,L7 should reduce coupling from the power amp back into the 45mhz IF amp where it takes another trip through the D1,D2 mixer.

Raj VU2ZAP, the originator of this mode, had a specific manufacturer in mind.  They have to be well shielded, commonly available surface mount inductors did not help at all.

Curt WB8YYY is pleased that the results show that the Axicom relay replacement provided around 10 dB improvement.   He has Gordon’s relay board installed, and he imagined his set of new relays will do at least that good.

He notes that some form of added 45 MHz filtering should help in most rigs. He first tried a xtal filter stage – but it disrupted his 40m CW output (He could restore output only by touching L5 – hence the reason for considering shielded inductors). He instead added an LC filter similar to what now appears in the stock v5 board, but he is not sure of the inductor’s Q.  He thinks it may be around 70.  According to simulation it provides more than 10 dB.  He is still waiting for verification in real life.

Curt notes that in the club’s initial measurement of many ubitx rigs, there is appreciable variation even though these are all the same board design (v4) and they were all received at the same time.

John Sutter W1JDS uploaded a spreadsheet of measurements for his µBITx for 80, 40, 30 ,20, and 10m:

1. Stock
2. Replaced L5/L7 with SMD inductors I found locally, kept C211 in circuit
3. Replaced the relays
4. Replaced the inductors with the specified Murata parts
5. Bypassed C211 per the mod

The spreadsheet can be accessed here: https://docs.google.com/spreadsheets/d/1eUFuHcprJhk_HuINxXJLhBuZ7-0DHPfaGPV-RoN8wBY/edit?usp=sharing

The mods he used are those from this website:  http://ubitx.net/spectral-purity/

Both mods helped.

To desolder the relays, he used a piece of desoldering stick,
a low temp alloy along with solder wick and a hot tip on the iron,  and this made easy work of it.  Cleanup was the worst bit.

Probably about the same as: http://www.zeph.com/lowmelt.htm

He’ll probably put the cap back in the circuit in order to be able to use 30m.

He is still needing to find a solution for 80m in order to ensure compliance.

His testing was done in CW mode with the following configuration:

  • 12V LiPo Battery pack  (link)
  • 40dB homebrew attenuator which I’ve characterized as being pretty flat
  • Agilent E4404B SA & HP 8595E SA depending on which one I could use at the time
  • 3D printed key
  • Misc cables…

    This is a v3 radio.  Right now it’s mounted in the plastic box it came in until it decides what it wants to be when it grows up 🙂   John has a JackAl board ready to go when he is happy with the harmonics.

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Warming effect of driver and pre-driver affects spurs

Lawrence Oberman mentioned an issue he has on a v4 board that looks like RF coupling or leakage when testing CW on 20m for FCC compliance.

He doesn’t see it with SSB but if the rig is placed on transmit for long enough spurs will start to show up .  He has only just got Axicom relays to replace the stock relays. Strangely enough he has not seen this behaviour on 15m, 17m or the higher frequency bands, or on the bands below  14MHz.

Others have seen similar phenomena on different bands, some with the spurs appearing on starting TX and then disappearing.

Allison KB1GMX says “Both cases come from the same source, heating in the amplifier usually the pre-driver and driver stages but it can include the finals. These stages are not thermally stable and parts heat up and their bias shifts.”

Reference
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Spectrum analyser tests of v5 uBITx board

Jim Sheldon W0EB has done some initial testing of the new µBITx board on his brand new spectrum analyser.   The results are CW only (so these results won’t show the impacts of any distortion in the mixers or IF bidi amplifiers that were observed with the v3 and v4 boards.

There are still some bands that would not meet most countries’ spectral purity requirements.

For a full set of graphs for each band click on the reference link below.

Reference
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A spur fix and BFO leak fix from Ashhar Farhan

Spurs fix (See update below)

Ashhar Farhan VU2ESE, the designer of the µBITx has been experimenting with solutions to the spurs on SSB mode above 18MHz.   He has posted to the list that one spur fix that is now consistently working for him, and that is easy to apply, is to fix the distortion in the 45 MHz amp.

He inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board.   This is on the output of the 45 MHz IF amp going to the front end of the mixer.   The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).

UPDATE But wait … he proposes a better spurs  fix a day or two later

1. Change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair’s standing current and decreases the Harmonic distortions.

2. The 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A ‘T’, rather than a Pi is used. This helps improve isolation on both the RX/TX path.

3. The LPF consists of an L-C-L of 300nH, 47pf, 300nH.

Removing the BFO leak into the local oscillator

Ashhar also found that there is cross talk inside the Si5351, whereby the 12 MHz output from the BFO leaks into the local oscillator.

Ashhar used  a 5µH inductor in series with a 47 pF capacitor placed across C202 of the LPF suppresses this. The inductor comprises 35 turns on a T30-6 toroid (to give 5µH).   However,  another constructor found this blanked out the 30m band, and calculations reveal that there may be an error with the calculation of inductance or capacitance that is waiting to be confirmed.  The filter should be tuned to 12MHz suggesting a slightly small capacitance value of around 35pF

uBITx.net will be looking for verification from constructors that these mods represent a definitive fix for the spurs and carrier leak problems.

Reference

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The cause of spurs

Ashhar VU2ESE and Raj VU2ZAP have nailed the cause of the spurs.  The 45 MHz  IF amplifier is distorting and producing a second harmonic at 90 MHz,  which mixes with the local oscillator to produce a spur at 90 MHz.
Here is an example of how it happens :
  • the spur moves down as you tune up.
  • when the radio is tuned to 28 MHz, the spur is at 17 MHz.  When the uBITx is tuned to 28.150 MHz, the spur is on 16.850 MHz.
  • At 28 MHz, the local oscillator  is at 73 MHz.
  • There is a signal X such that :

X – local oscillator = 17 mhz

Local oscillator = 73Mhz.

X – 73 = 17,  so X must be 90 MHz.

After checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.
Now to confirm a simple solution …  if the problem is in the 45MHz IF, then it may also be possible to address the IMD at the same time.
Reference
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Higher current Axicom relays

Jim W0EB has finally received his >500 mW Axicom relays, after they got misrouted in the mail.

H plugged these into his V4 test bed built up on an aluminium open chassis. Sockets are mica filled 2mm thick with silver plated flat contacts. This leaves the relays 2mm above the PC board.

With the >500mW coils (i.e. with coils that have fewer turns on them) the hope was that this would lessen  the extent of inductive coupling between the switched portion of the circuit and the relay coils.  Unfortunately this does not appear to be the case.  Jim got almost exactly the same results as with the 400 mW relays mounted the same way.

40 meters is still just barely in spec with the 3rd harmonic being -43.8 dBC. The others are also in spec but not much better with 3rd harmonic varying from -46 dBC on 10 meters, -44.5 dBC on 12 & 15 meters , -44.7 dBC on 20 meters and -57.3 dBC on 80 meters.  60 meters wasn’t tested.

Raj VU2ZAP notes that it makes quite a difference between socketed an unsocketed replacements.  It is best not to use sockets for the relays, in order to get the relays as close as possible to the PCB.

Before and after shots of relay replacement as observed by Raj follow:

Before replacement
After replacement

Raj and Jim both note that there is variation between boards in terms of improvement in performance.  The reasons for this are not known.

The conclusions are:

  • relays help, and ubitx.net recommends them, but replacing relays with AXICOM relays may not always be sufficient to reduce “blow by” on some bands or modes
  • there is no point in spending more on Axicom relays with a higher coil current
  • the best option would be to replace the filter section altogether, and there are a number of ways of doing this.
Reference
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Relay board from VK4PP given thumbs up in testing for isolation

Nick VK4PP sent one of his LPFx4  test boards to Warren WA8TOD for blow-by testing.   It looks like the bare PCB has excellent isolation between its input and output, of around 90db.

Matched with RF rated and separate in/out relays this board will perform very well.   The updated pic above shows a few tweaks and more ground stitching.

The only downside is the US$1.50 each relays (x8).    Relay K3 can potentially be reused, or you can source AXICOM relays for a bit more.

Nick is interested in feedback from constructors on the level of demand for these boards.

Reference