The All-band HF Amateur Radio Transceiver: Just waiting to be modified
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Joe KD2NFC is building his new µBITx into one of the cases from www.amateurradiokits.in. In this case it is for use with the 5″ Nextion screen.
Joe says, “These cases are awesome and really add to the fun and creativity of building the uBitx. I am still waiting for the 5” LCD but here are some images of my progress.”
Jim W0EB, TSW Project Coordinator, bought two of these kits for his 2 uBITX Version 5 radios. First off, the directions for installing this board are brief, but they are easily followed and the boards are easy to install.
Not wanting to drill a hole for the included switch and run a bunch of wires to it, he just wired the common pad to the “fast” pad for always on, Fast AGC. A “Via” hole was identified in the trace between R70 and “Vol HI” and Jim ran a wire from there to the “VOL” pad on the AGC board. This worked great.
Even though the “S-meter” output of this AGC system was designed to work with the CEC software, we found it worked with TSW’s BITeensio board as well.
The BITeensio uses the A19 analog input on the Teensy 3.6 for the S-meter. This little AGC system drives the S-meter routines just fine on the BITeensio.
A 50 microvolt (-73 dBm) signal was fed into the antenna connector and Jim adjusted the software’s S-meter routine’s division ratio so the touch screen’s display read S9 as it should with a 50µV input. The rest of the S units were so close to correct that no further tweaking was deemed necessary.
He also found that adjusting the on board RF gain control for max recieved signal was the best way to adjust the level control. As you turn the control counter clockwise, the gain increases and there is a point past which saturation occurs. This is obvious when listening to a weak signal and you can hear the gain drop past this point. Simply adjust the gain to that point and turn it back to where the signal is just peaking. It is best to just leave it there if you want your S-meter to work right.
Once adjusted, this little AGC board keeps the RF input nicely within bounds on strong signals quite well.
Jim calls out ND6T and N8DAH and says “Well done guys, well done! The kit is certainly worth the price IMO”.
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:
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.
Mark N7EKU has posted a nice clean schematic and a picture for the audio pop fix he has just completed on his uBITX V3. It works great and saves the ears a lot.
If you check the top part of the photo, you can see that I have done Allison’s fix for harmonics by re-arranging the filters a bit and moving the original relays to the bottom of the board. This worked great too and cleaned up harmonics a lot.
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.”
Ashhar Farhan VU2ESE tells us that the µBITx v5 board has more gain on transmit, as well as on receive, than the previous boards. This is due to the BFR93w transistors in the 45 MHZ IF in place of the 2N3904s.
The carrier and spurs will be appear to be higher if the TX gain (controlled by RV1) is running flat out. it is best to reduce this to about 60% of the full gain. One way of achieving this is to connect a 50 mv audio signal to the mic, and transmit SSB on 7 MHz while reducing the power out until it is around 7 watts.
Allison KB1GMX gives details on how to wire up the commonly available 100prr-6 encoder. These have a nice vernier style dial and cover 100 pulses per 360 degree rotation. The calibration points line up with these pulses. They come with either 4 or 6 wire terminals on the back.
The A and B terminals have transistor outputs to ground and are used in connecting to the raduino:
Allison uses the V4.3 code in her µBITx with here own mods and this encoder worked well. Some firmware for the Raduino will not be sufficiently fast to keep up with the signals being sent from the encoder if you rotate the encoder quickly.
The uBITx.net editor likes the black version better. These are a classy unit.
Alison KB1GMX counts 5 different schematics for the µBITx, and that is not including the HF1 or minima (2014) designs that came before the µBITx. The design was evolutionary.
The first production boards were labelled “V3”. Two further iterations have been sold since, the “V4” and now “V5”.
V3 and V4 are probably technically not fully compliant with emissions regulations in most countries (at least in the developed world). However, the low power output of the µBITx probably means that emissions will have resulted in few issues with interference in practice.
The effectiveness of harmonics, IMD and spur modifications incorporated in the V5 board have yet to be established across a selection of production boards. There are no doubt improvements, but the degree to which the product could be considered to be 100% compliant on all bands and all modes has yet to be established.
[Edited by uBITx.net]
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.
