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.
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
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.
Ashhar invites you all to test it. if you are not familiar with C code or Arduino programming, it is suggested you wait for a few days until we get all the bugs sorted out. This is only for Arduino regulars.
Send any bug reports directly to Ashhar’s email box at firstname.lastname@example.org. In the subject line use the word “#ubitx40”. I will try to answer all emails but I can’t promise responses to all.
Ashhar has measured the mood of the BITX20 list and has made the call to substantially borrow from Ian KD8CEC’s code and back port it to the original ubitx code. The new code has about 10% more lines but it is substantially more robust and useful.
The main features that Ashhar has cherry picked from Ian’s code are:
1. Keyer. You have to choose which keyer type to use, but the keying is much better and robust now. This code is a total copy/paste of Ian’s keyer. The auto-keyer (that sends out preset phrases in CW) is left out. The Iamabic A, Iambic B and the handkey sending works very well.
2. CAT control. Given the popularity of FT8, Ashhar has rewritten the CAT control by following Ian’s code but follows the ubitx coding conventions. It represents a miminal set of controls.
3. Split operation has been included.
4. Rationalised menus: the menu system is now more consistent.
5. Tuning mechanism: The accelerated tuning works, and it doesn’t jump randomly like before, nor does it work at the same speed. For very long band changes, it is recommended to use the menu option to change band.
Among the things left out was support for different types of displays, WSPR, and many other goodies. The EEPROM memory map has been kept consistent with KD8CEC’s plan. You can switch between both code bases easily.
Ashhar has kept usage of English words at a minimum.
Doug AC9RZ has alerted constructors to the big news of the day – Ashhar Farhan VU2ESE, designer of the BITx series of kitset transceivers has been inducted into the CQ Amateur Radio Hall of Fame! The announcement from CQ Magazine follows:
(Xenia, Ohio – May 18, 2018) – The CQ Amateur Radio Hall of Fame has 11 new members for 2018, CQ magazine announced today. This brings to 321 the total number of members inducted since the hall’s establishment in 2001.
The CQ Amateur Radio Hall of Fame honors those individuals, whether licensed hams or not, who have made significant contributions to amateur radio; and those amateurs who have made significant contributions either to amateur radio, to their professional careers or to some other aspect of life on our planet. The 2018 inductees (listed alphabetically) are:
Marlon Brando, FO5GJ (SK), iconic movie actor
David Brown, KC5ZTC (SK), NASA astronaut killed in 2003 Columbia disaster
Kalpana Chawla, KD5ESI (SK), NASA astronaut killed in 2003 Columbia disaster
Laurel Clark, KC5ZSU (SK), NASA astronaut killed in 2003 Columbia disaster
Ashhar Farhan, VU2ESE, pioneer in popularizing open-source Bit-X “semi-kits” using Arduinos for affordable QRP transceivers
Grady Fox, W4FRM (SK), SSB pioneer; worked on Manhattan Project during World War II and the camera for NASA’s lunar landers
Wendell King, ex-2ADD (SK), African-American pioneer of broadcasting and college radio
Fred Lloyd, AA7BQ, founder of QRZ.com, the most widely-accessed amateur radio website
Mark Pecen, KC9X/VE3QAM, wireless communication and networking pioneer, inventor, cybersecurity expert
Carole Perry, WB2MGP, longtime advocate for youth in amateur radio; moderator of Dayton youth forum for more than 30 years
Ed Westcott, W4UVS, photographer who chronicled the Manhattan Project during World War II and later helped the FBI with its investigation of the Jonestown massacre
Two new members each are also being inducted into the CQ DX and Contest Halls of Fame at the respective Dayton DX and Contest dinners. Their names will be announced separately.
CQ Communications, Inc. / 17 West John St. / Hicksville, NY 11801 USA / 516-681-2922
The world’s premier independent amateur radio publisher.
Publishers of: CQ Amateur Radio, CQ Books, the CQ Video Library
Ashhar Farhan VU2ESE notes that 28 Mhz has, unusually, been open for the last week. As a result, he realised that the ubitx output was woefully low on 10m. Hence the experiment below:
Ashhar suggests the following final fix, and asks for others to try it out and see if it is replicable:
Replace C81 from 0.1uf to 470pf
Replace R83 from 10 ohms to 2.2 ohm
(you can short R83 as well)
Replace 97, R98 from 47 ohms to 220 ohms
Remove C261, C262.
So, what happens is that removing the C261 and C262 increases the gain of the finals. They are run open. Hence greater gain at 28 mhz.
However, the gain is very high at lower frequencies. So, in order to reduce the gain at the lower frequencies, the 0.1 uf cap is replaced by the 470 pf. As the frequency of the signal drops, less and less RF flows through the 470 pf, decreasing the gain of the predriver. 470 pf is not a magical value, 220 pf works almost as well.
Step #1 Increase the predriver gain towards the higher frequencies
The predriver Q90 has a emitter degeneration capacitor C81 (0.1uf) and and R83 (10 ohms). Replace C81 with a 470 pf and R83 with 2.2 ohms. Altenatively, short R83.
With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.
Step #2 Take off the feedback from the IRF510s.
a) Replace the existing R97 and R98 with a value of 47 ohms with 220 ohm resistors.