Mike K5ESS has made a few enhancements to the µBITx schematic, that will definitely be of interest to many constructors
One of the last challenges to be conquered with the µBITx is the unacceptably high intermodulation distortion produced in the IF stages on transmit.
The IMD seems to be sourced in the Termination Insensitive Amplifiers used as bi-directional IF amps at both 45 and 12 MHz.
Warren WA8TOD is experimenting with a prototype MMIC amplifier utilising the ERA-3SM+ (80 cents each on eBay) mounted on an RF prototyping board ($2.50 each from SV1AFN) that is the proper size and form factor to replace both transmit TIAs. The MMIC/board measure flat response from 300 Hz to well over 200 MHz, and will provide around 22dB of gain.
This test quantifies the IMD performance of the combo. The reference point on the SA is shifted +6 dB so that the readings reflect the power level that would be achieved with a single tone for convenience of interpretation.
Measurements were taken at Vcc of 3.3 volts and current draw was the recommended 35 ma.
The blue trace shows performance at 0 dBm out and is an acceptable -42 dBc (-36 dB minus 6 dB for a single tone carrier). Performance at – 10 dBm out, the level needed to drive the uBitx driver/PA chain, is an excellent -51 dBc (-45 dB minus 6 dB for a single tone carrier).
Yet to be determined is how to add the LM1117-3.3 regulator to hold the Vcc at the required level.
Jim Sheldon W0EB has now replaced all stock relays in one of his µBITx transceivers with Axicom D2n 12v relays.
This improved filtering by a wide margin on all bands except 40m. The results:
80 Meters 3.500 MHz
7.000 (2nd harmonic) – 65.7 dBC
14.000 (3rd harmonic) -76.4 dBC
40 Meters 7.000 MHz
14.000 (2nd) -58.0 dBC
21.000 (3rd) – 47.2 dBC
30 meters 10.000 MHz
20.000 (2nd) -76.9 dBC
30.000 (3rd) -75.8 dBC
20 meters 14.000 MHz
28.000 (2nd) -65.2 dBC
42.000 (3rd) -64.8 dBC
Jim didn’t test 17, 12 and 10 meters, but he suspects their harmonic suppression is in the same boat as the rest.
The graph above provided by Gary AG5TX shows the resultant effects of using Axicom relays (using Jim’s data points).
“Your shared data maybe useful as you have shown the same board and relay with different measured results. For that particular board and the measurement data you gathered, I would say the results ARE encouraging from this viewpoint:
You have 4.5 dBc more margin to FCC spec on your worst case data point (40m 3rd harmonic) with the supplied data set.”
Sourcing Axicom relays
These relays can be obtained from a number of sources. If you live in the US they can be found at Arrow, Digikey and Mouser for less than US$3 each. They are also available from Aliexpress for under US$2 each in a set of 10 and from Component HK in Hong Kong for under US$1 each.
For those interested in the benefits of Axicom relays see this webpage for a switched Bandpass filter bank:
There are wealth of tips on this page for those building switched filter banks.
It appears that the Axicom relays are the answer, and Ashhar Farhan has already committed to replacing these relays in production µBITx, presumably when the current µBITx stocks have been depleted. Hopefully Raj’s mod for removing spurs will also feature in a v4 board upgrade shortly.
Nick VK4PP has 45MHz filter boards to fix spurs available now to order. He has installed one himself and it works fine.
Nick has added them to his ordering page. The price is a stunning ONE cent only + postage. Nick says
“Feel free to take up to 10 and also maybe sample the other warez …”
VK4PP Order Form
Raj VU2ZAP has found good reduction of spurs with the following mods:
1. Replace L5 with an SMD inductor 680nH or 681nH. Mount at right angles to old toroid
2. Replace L7 as above. Note that C211 is bypassed with this mod.
Don’t be distracted by the missing C216 and C210. These parts were never populated.
An additional 45MHz filter is not required and, therefore, you should get almost the same power out as stock boards. If you ALSO use the second 45MHz filter the spurs get further reduced, but with some loss of output power. Raj used Murata inductors 1210 size. 1206 size will do.
With Mikes W0MNE relay fix and this mod, Raj has the uBitxv3 and v4 boards now operating within normal emission standards.
Raj thinks that a combination of high Q, small size and possibly internal shielding contribute to the reason why. Another hypothesis is that the original inductors caused ringing. Raj has also replaced L1-4 with 330nH SMD and that helped a little on one test board. His interest was to fix the problem of spurs with the simplest solution and without major surgery.
Mike W0MNE has come up with a fix for the harmonics problem with the µBITX that is simpler than other fixes …
He has two Ver 3 and one Ver 4 uBiTX rigs. He simply replaced K3, KT1, KT2, and KT3 relays with Axicom relays.
This has knocked down the 3rd, 5th, and 7th harmonic levels from -35dB to below -50dB on all 3 rigs. No board modifications are required, and no changes to the LPF circuitry is required. This may not be reproducible on all rigs, but with 3 rigs showing the same results it may be something constructors should consider.
Mike purchased the relays from Mouser – part # 655-V23105A5403A201. Cost was US$2.89 per relay plus shipping.
Raj VU2ZAP found a picture of the “innards” of the Axicom relay, and suspects the improvement in isolation is the result of the contacts being closer to the ground plane (as the relay coil is on top in this relay, whereas the standard relays supplied with the main board have the relay coil underneath the contacts).
Before the fix on 40m …
And after the fix on his Ver 4 rig …
Hans G0UPL who operates QRP-Labs has released his 10W HF Linear Power Amplifier. This kit comfortably produces 10W from a 12V supply. It is a compact design with huge heatsink included, which will not overheat even on continuous 100% duty-cycle operation. The amp provides 26dB gain with +/- 1dB gain flatness from 2 to 30MHz.
This 10W HF Linear Power Amplifier kit has no Surface Mount Components (SMD) to solder. There are a number of small transformers to be wound, and assembly requires care and patience.
The push-pull driver stage uses two BS170 transistors in the amplifier design used in the SoftRock transmitter stage. The final uses two IRF510 transistors in push-pull. Yes, this humble low-cost MOSFET really is capable of excellent performance all the way up to 10m band and beyond! Short lead-lengths and PCB layout are extremely important, they are the key to success.
As Hans advises on the BITX20 I/O Groups list:
I am not suggesting that this amplifier will solve all problems and without wishing to be offensive, the “garbage in, garbage out” rule always applies – in particular, if you feed it a signal containing spurs, harmonics, or other problems… then you will get the same thing at the other end, just amplified 26dB. It also still applies that all stages leading up to the final amplifier must have flat gain. If the pre-drivers cannot provide a big enough signal then you won’t get full power output.
But, it could be useful or interesting, in the BITX context, to look at the details of this amplifier kit and see the short lead lengths, symmetric PCB and extensively “stitched” groundplanes.
For further information see the product on Hans website.
Dr Flywheel (aka Ron N7FTZ) has provided some details of the Teensy audio add-on board. This is used in the upcoming JackAl add-on board expected to be available shortly, but can almost certainly be home-brewed by savvy constructors using the Teensy 3.6 and audio add-on board.
Here is the circuit diagram for the PJRC audio module:
The CODEC part is very much the reference design provided by the part manufacturer. The support for the EEPROM and the SD card is optional.
The libraries will be compatible with any SGTL5000 or for that matter a long list of other I2S compatible CODECs (look at the source code). If you are homebrewing an audio processor on the Teensy you should consider integration of the CODEC. The libraries will support the internal A/D-D/A of the Freescale ASIC with very few components (resistors, capacitors) added.
Sample resolution of 12-bits is sufficient for the type of audio DSP that we need, though the higher bit-rate CODECs are much better. It is a matter of cost effectiveness and board space.
Below shows mono circuits for a lower grade Teensy, however the circuit is the same for the Teensy V3.6 with the corresponding pins.
The input circuit (mono):
Here is the output circuit (mono):
Jim W0EB latest µBITX has been built with a 5″ RA8875 type Colour TFT display running on the BITeensio Card.
The system shown can be controlled by touching the screen, with an external USB Keyboard or an external USB mouse. This almost completely eliminates the need for front panel controls and helps make a very clean looking µBITX build while keeping full functionality.
In using a 5″ or larger display, Jim and Ron have run into several problems . The 5″ display draws almost half an amp from the 3.3V rail and the even larger 7″ displays draw even more current. Using the 5″ display requires the BITeensio card to be modified to provide a high current, on-board 3.3 volt regulator as the Teensy 3.6’s on-board regulator would be severely strained trying to power this display.
In doing this we build the new 3.3V 1.5amp regulator by isolating and using the pads for the 5 volt I2C bus on the card. We do lose 5 volt I2C capability by doing this but the 3.3V I2C bus is still available so 3.3V I2C peripherals can still be used.
Another disadvantage of the larger display, other than its higher cost and higher current requirement, is that a fan is needed to cool the regulator’s heat sink. This makes retrofitting the bigger displays very difficult to say the least.
The Triumvirate Skonk Works has no plans at this stage to support any colour displays larger than the 5″ one. Even this display is only for experienced experimenters, because of the mods that need to be done on the BITeensio card.
We’re also recommending that using the 5″ display be considered ONLY with a new BITeensio build rather than modifying a previously constructed one as there is much less chance of messing it up this way.
The modification instruction manual, with pictures is located in the “Documentation” directory of their website under the “Files” section. Like the previous 2.8″ Colour version and our older 4 Line I2C LCD display version, this version is capable of utilising an optional external USB keyboard for rig and CW control. Mouse control is only available with the Color TFT versions.
Rig control now allows use of a mouse for almost everything. Purchasing an inexpensive, wireless keyboard/mouse combination will allow simultaneous keyboard and mouse operation. Using the keyboard or mouse for control, you won’t have to even touch the screen unless you want to. This will reduce wear and tear on the touch panel.
The 5″ display we use is the “ER-TFTM050-3 from Buy Display dot com. If you decide to order one you need to get it with the 4 wire SPI interface, Resistive Touch Screen, 3.3V power option (5 V won’t work) and the ER3304-1 font chip. The SD card option is not necessary as all SD card operation is contained on the Teensy 3.6 itself.
Ordering information for the BITeensio kits is also available on the website and kits are in stock.
Lightning took out Q90 on Mike K5ESS µBitx. It also fried the switching power supply he was using. Others have reported similar frying of Q90 due to lightning.
Doug WA8UWV suspects any strong signal or static pulse on the antenna during receive will damage Q90. He has another ham 2 houses away and DX Engineering is also located near by. He contemplated a reverse connected diode across the base and emitter of Q90 to see if it eliminated failures.
Gordon KX4Z has speculated before that the failure mode is a REVERSE voltage on the base-emitter, as the series capacitor charges up due to rectification by the base-emitter junction. This document on pp18-19:
seems to suggest that indeed, overloaded reverse voltage bipolar junctions fail SHORTED, while forward current would be expected to OPEN them. The reverse diode would eliminate the charging of the series capacitor and protect the junction against reverse voltages, and someone tested this for ill effects and found none.
Gordon has put the reverse diode on his µBITx and similar radios and has had no failures as yet.