A couple of constructors have been busy pushing the limits …
Digital modes µBITx in a box
Felice IK1ZVJ project stems from his passion for digital modes. He says “The union between µbitx and raspberry works well”. Felice combines a µBITx with a raspberry pi and a large pop-out screen in a single portable package for use with digital modes such as FT8.
The LCD screen is mounted on an old CD mechanism to provide automatic ejection from the enclosure.
The project includes a dual power supply with a built-in power supply and battery power for portable use. He foresees his pr ubitx, only being used with the raspberry pi on digital modes. The raspberry pi is loaded with WSTJ-x for use on the FT8 and FT4 modes, and for FL-DIGI for PSK and RTTY. His enclosure uses a 3U rack (9 cmx40cmx43cm).
Ashhar Farhan is pushing the boundaries
Ashhar Farhan VU2ESE, designer of the µBITx previewed a Raspberry Pi board for HF radios with a high end codec at the FDIM (prior to Dayton Hamfest 2019).
This is for a new radio that he is building to push the limits of what his homelab capability can produce.
The story so far is that there is a very low noise 24 bit codec on a custom board that plugs into the raspberry pi. The display is a 7 inch capacitive touch screen device.
The radio is a single conversion, very low distortion superhet without any amplification before the crystal filter and triple tuned filters for each band.
Ashhar says “Nothing is casual about this radio. Even the power supply had to be included to have low noise rating.”
Ashhar Farhan VU2ESE, the designer of the BITx range of kit transceivers marketed through HF Signals has launched his next design: the Antuino.
The website states that:
“Antuino is an accurate instrument that can be used in the field to measure SWR, field strength, modulation, etc. In the lab, it can be used to sweep filters, measure gain, distortion, frequency response, etc. It works upto 150 Mhz. On the third harmonic, it is usable on 435 Mhz band as well (with reduced sensitivity).
“The Antuino, unlike simpler instruments is based a superhet architecture that measures the response of the antenna or circuit at exactly the tuned frequency. It is based on Analog Devices’ Lograthmic Amplifier, the AD8307 to provide accuracy of 1db in your measurements. It is tuned with a crystal locked PLL based on Si5351 oscillator chip.”
Ashhar Farhan VU2ESE observes that ALC is just one way of controlling output power from a linear amplifier stage and that an easier approach is to do it in software.
This needs software that can control the ‘mic volume’. You could set the value differently for each band. There is another pay-off with software mic gain, it can make a major difference to the transmit IMD. At voice peaks, the tx linear chain compresses. The gain is not constant between low and high levels of modulation. This is the cause of in-channel IMD. Now, if we have a look up table that amplifies the peaks more than the lows, we can ‘correct’ the gain back to being linear. This simple concept goes by the name of ‘pre-distortion’ in the SDR world.
Ashhar Farhan VU2ESE writes that “there seems to be a light at the end of this tunnel now” with respect to improving the µBITx.
Over the last few weeks he has been talking in the group and off the group with a number of people: Raj, Jerry, Allison, Bill (Meara) and the following represents the consolidated wisdom of this group on the failings (and cures) for the v4 board:
1. The spurs were cause by the harmonic distortion in the 45 MHz IF amplifier. This resulted in a 90 MHz harmonic that mixed with the local oscillator to produce a spur. the local oscillator was at 45 Mhz + tuned freq. The spur was as 90 MHz – local oscillator, that is 90 – (45 + tuned freq) = 45 – tuned frequency. Hence for 21 MHz ,we had a spur at 45-21 = 24 MHz, etc.
The cure was to increase the current in the amp by decreasing R26 from 470 ohms to 220 and introducing a low pass filter between the front-end mixer and the 45 MHz IF amplifier. This consists of L31, C205 and L32. A little snick and solder job can do this on existing boards. WIth these mods, the spurs are well under control. This hack was due to the preliminary work that Raj did followed by taking 50 odd readings of how the spurs moved and analyzing them. in the end, a two variable equation out of a 7th grade algebra book could point out this problem; but we never knew.
2. The harmonics at below 14 MHz were due to the bad routing in the LPF relays. Allison provided a simpler and far better option. The unfortunate part is that the existing boards can’t do this. The other tragedy is that the way these relays are wired . The 28 MHz filter permanantely in the tx output path and the other three LPFs switch in, in series with it. So, the Arduino code for these LPFs is going to be different too.
3. Change of 2nd IF frequency. I am moving the 2nd IF to 11.059 MHz from 12 MHz. This will avoid the frequent problem of having the the 16 mhz oscillator generating spurs inside the IF pass band. The new filter is also a little broader, it is about 2.7 KHz. The audio sounds better, at least I personally prefer it so. Hence, the crystal filters’ caps are changed from 100pf to 68 pf.
4. The LM386 returns. Ashhar Farhan says he is regretting it already. The audio amplifier is now the old frenemy of hams, the LM386. We had to do this because it is the only one that is readily available to manufacture as well as to those who are going to scratch build. This is an open source project that must use easily available components and also strive for a minimum complexity. Using an op-amp with a complementary npn-pnp pair could have worked, but the complexity would have had us to use a bigger board. We wanted to keep the board size same, to allow others to upgrade as well.
5. It is vital that the output above 14 Mhz must be kept to 5 watts. Over-driving this rig will certainly lead to spurs. Check out the captures of 21 MHz. One is with a 5 watts output, the other has it over-driven to 10 watts. It will stay within the legal limits with the supplied electret mic.
Check out the pictures. The green line across the screen is drawn at -43 dbc, the legal limit in the United States for harmonics and spurs. Ashhar adjusted the RF attenuator so that the transmitted power stayed at -20dbc. See how the distortion adds to the spurs in the two 21 Mhz captures.
Ashhar says “If things look good, we will roll with this as v5 board”. We can expect an update in 2019.
The v5 circuit diagram is attached here: ubitxv5.
Ashhar Farhan VU2ESE and designer of the µBITx has announced that he is working on a new kit – the Antuino.
The PCB version of the Antuino (The Arduino based Antenna Analyzer) is still under development, but we will all be looking forward to its release.
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.
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
- T2 – desolder the transformer wires that go to pin 3 and 5. Pin 1 has a square pad.
- Bring out the two wires above board and join them together and solder.
- 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.
- 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
- Remove R27
- Solder the 45Mhz filter two extreme ends to the pads of the resistor.
- 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.
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.
HF Signals has rolled out the new (r4) version of the PCBs.
Changes to board design
Ashhar Farhan VU2ESE has indicated the main changes to the circuit board:
1. A new, low distortion audio amplifier made from discrete transistors. This is one is optimised for head-phones and connecting to PCs for digital modes. It will continue to work with speakers as well. Builders with the earlier versions can try this audio amplifier as an outboard amplifier.
2. The audio thump issue during T/R switching has been fixed.
3. At 28 MHz, the output is about 4 watts. (this involves a single capacitor change from the earlier PCB – C81 was changed to 470pf – this is located on the base of Q90 being the first driver stage).
4. ‘Jumper’ points have been included on the board to make it easier to add interesting stuff like CW filters or S meters to the board. There are a large number of test points now to help you debug and understand the board.
All in all, the changes are ‘backward compatible’ . That is, you can hack these changes to the previous boards to get in the new functionality. Ashhar Farhan (the designer of the µBITx) has updated the circuit diargrams on www.hfsignals.com.
Note that the earlier production board’s circuit diagram has moved to:
In a separate post VU2ESE has indicated that the Nano will be socketed (on the reverse side of the Raduino board). This is a bonus, as many constructors have brought their nano to an abrupt end by accident, and it can be difficult to remove the nano from the Raduino.
The bad news is that HF Signals has had to bump up the price of the board by US$20. Ashhar’s personal ambition was to keep the price inside of US$100. As constructors will be aware, the release price was in fact US$109 (including standard shipping).
The new boards cost $129 USD with shipping. $139 USB with the DHL shipping option.
This price rise is the result of :
- recent changes in the Indian import duty and sales tax
- Increasing costs to preorder parts.
- An increased payout to the women’s collective who wind the coils, and assemble and test the boards.
The good news is that from now on, these boards will be available within a few days of order. The backlog of orders has now been dealt to.