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

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

Is a heatsink required on FT8 mode?

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.

Reference

New version of uBITx

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:

http://www.hfsignals.com/index.php/ubitx-r3-archived/

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.

Price increase

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.

Reference

New base firmware for uBITx under testing

Ashhar Farhan VU2ESE have created a next version of the base software after thinking hard about it. You can download it  here:

https://github.com/afarhan/ubitx4.

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 farhanbox@gmail.com. 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.

Reference

Ashhar Farhan VU2ESE inducted into CQ Magazine Hall of Fame

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

Reference

VU2ESE power mod for 10m operation


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.
b) Remove C261, C262.
Reference #1
Reference #2

Updates made to HF Signals website

Ashhar Farhan VU2ESE, designer of the µBITx, has posted to the BITX20 IO Group to say that there have been changes made to the wire up diagram to fix the errors.  This is good news as there will be fewer mistakes made by constructors meaning more happy customers!

While many will have moved on to one or other of the other firmware versions available for the µBITx, those wanting updated factory firmware will be pleased to know that a bug fix release will be coming soon.   uBITx.net will let you know when this happens.

A further comment from Ashhar suggests that he may use the PTT line as one of the keying lines (for the straight key).   This will mean a change to the wire up in line with the approach taken by the W0EB software team.

Reference

Replacement to TDA2822 coming soon!

Sajid, VA3QY asked Ashhar Farhan where he could source a new and reliable TDA2822 audio chip.

Ashhar Farhan has replied, “I am working on an audio amp.  I am testing it live. I always personally use circuits on the air before recommending them. I hope that by Tuesday I will have something you can use.   About 100 out of 4000 boards have had this problem. and less than 20 of them have reported the blow out.”

Reference

Ashhar Farhan tells it as it is … the problem of the TDA2822

Ashhar Farhan VU2ESE, designer of the µBITx tells us the sorry saga of the TDA2822:

“Here is the story. This is going to cause a lot of heartaches. I chose TDA2822 after listening to a lot of bad press about the LM386. Upon looking at its harmonic distortions et al, it was found to be a reasonably good device. In, it went.

“Now, unfortunately, the TDA2822 production has entirely stopped. None of the standard suppliers, including mouser, newark, element14 have no stocks left. We tried to find other source of remaining stock from our reliable supplier who has been supplying to us in the past as well.

“He turned up with a batch. We tried the ICs in burn tests for. Ten of them on ten boards at full volume for a whole day. In retrospect, we should have tested every individual IC. That batch had these WX ICs as well. About 100 to 150 of these must have shipped, there is no way to know. The way it works is like this : Not all the ICs turned out by a factory are good. So work very well, some not so well, some don’t work at all. These wafers make it to the silicon industry underground where they are cut out from the wafers and packaged and sold as low grade versions of the same parts. We got a a hundred or so of these lemons.

“We finally located a source of brand new, high quality TDA2822s that are currently being shipped. They cost almost five times as much as we were paying while this was in production. Well, such is life.

“In the meantime, we have to look for alternatives to the TDA2822. I am highly inclined towards making a discrete device amplifier. We will never run out of discrete devices, the audio would be so much better. Does the gang have any ideas?”

Initial feedback suggests that there will not be a consensus reached on a replacement.  Some want to stick with the TDA2822 (assuming a reliable source of affordable devices can be found), some would prefer the ability to add in a module of the builder’s choice, and others would like to see the device replaced with discrete components.  David N8DAH suggests the drop in replacement –  NJM2073D-ND

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