The following list of circuit designs has been compiled by Tom KF5NWC:
Minima
A large group of enthusiasts worked with Farhan to create and test a general coverage receiver and multi-band transmitter called the Minima. Its simplicity, innovations such as its KISS mixer, shuttle tuning using a potentiometer, and overall low build cost inspired hope of an affordable multi-band transceiver (to me anyways). 🙂
c210 and c216 – are listed as 0 pf and are not present on the board used in production by HFSignals.
Q942 and Q952 – are not populated on board sold by HFSignals. As sold by HFSignals, the final drivers for the PA are IRF510. These alternative pads allow experimentation such as replacing the IRF510’s RD15HVF1’s which are a different pinout.
c210 and c216 – are listed as 0 pf and are not present on the board used in production by HFSignals.
Q942 and Q952 – are not populated on board sold by HFSignals. As sold by HFSignals, the final drivers for the PA are IRF510. These alternative pads allow experimentation such as replacing the IRF510’s RD15HVF1’s which are a different pinout.
Ashhar Farhar reported on 6 February 2019 that HF Signals has pushed the uBITx v5 into production. An initial 100 boards had arrived so they may run out quickly (reminding us of the v3 board shortages of 2018).
The new firmware breaks backward compatibility with the previous firmware as the Intermediate Frequency is now 11.059 MHz and the relay switching is different. Probably, is it is possible to patch the new code to switch between the two LPF relay switching schemes as well as the IF with some #define statements.
The new ‘features’ are :
The LM386 is back as the audio amplifier (like in the BITx40. All hail the cross-over distortion sickness!
The IF is now 11.059 MHz. These 68pF crystals are easily available, with a pretty good 2.7 KHz band pass. As their frequency is away from any spur of 16 MHz and 25 MHz (the two crystal oscillators on the Raduino) there will be no spurs.
The relay switching scheme is different. The 30 MHz LPF is now always on, the individual LPFs for lower bands are switched in or bypassed (by default). It is easily understood in the circuit diagram
The CW wave-shaper uses a 1 uf capacitor in place of 0.1u, this reduces the key-clicks.
The circuit now uses BFR93W in the 45 Mhz IF amps, for more overall gain in the TX lineup.
A 470pf in the emitter degeneration of pre-driver instead of a 0.1uf gives more gain at 28 MHz than 3.5 MHz.
Jim AB7VF made some measurements of CLK0 ouput (from the SI3571a chip on the Raduino board) and concluded that it would be helpful to filter the output.
He built a simple LPF on the raduino board to cut some of the ‘cruft’ getting into the mixers.
Pop VU2POP shared pictures of his homemade ubitx cabinets. The design involves a common cabinet for both the standard manufacturer supplied 2 line 1602 LCD display as well as the 2.8″ TFT Nextion display. Pop says “You need to change out the bezel.” This is a really good idea.
He supplies the complete cabinet, fixtures, microphone, microphone and PTT sockets, associated PCBs, and the speaker, fuse holder, fuse, heat sink insulation kit, heat sink compound, tags, screws, usb cable panel mount, knobs, folding feet, glue, and a USB to rear panel connector cable to fit on to Radiuno.
This is similar to his earlier cases, except this case will have common front panel bezels for 2.8 inch TFT & 2 line LCD. Just change out the bezels! Constructors can build the stock µbitx to get going and then upgrade to a 2.8″ TFT display at a later date when they have the basic rig working perfectly.
At present these cases are only available for VU amateur operators.
Complete kit and kaboodle
DX shipments will be available at www.radiokart.in from Feb 2019.
It is better to have a quiet fan that meets all of the requirements without any extra circuitry that can fail. You can make a fan more quiet by adding a series resistor. Experiment to find the right value, but perhaps start with around 500 – 1000 ohms.
Another easy solution is to simply run the fan from the 12V transmit power, assuming that is not what you are already doing. No reason to run the fan while receiving.
If a noisy fan is the only choice, then putting in a controller to minimize the noise while still removing the heat is the next best solution. If properly designed, installed, and tuned, then there would not be a change in the amount of heat that can be removed as the heat sinks get hotter. The response time can get in the way of keeping the finals cool quick enough to not cause an issue. An always on at max fan does not have this issue.
Woody KZ4AK suggests fan controllers are easy to build and minimize noise when not needed.
Something like these… YMMV
Variable analog – Had to add the 3.3 ohm to reduce induced noise in the uBITx from high current fan.
OR
The power output of the µBITx varies according to frequency.
Richard Pushman provided a chart showing the power output of the rig by frequency from 2MHz to 30Mhz for two different supply voltages. The shape of the peaks and troughs is, of course, influenced by the LPF filters in the output stage.
He added a tandem match from eb104.ru, 4 arduino relays for PTT and ATT (-6dB) and an optocoupler board for switching the LPFs.
The Nano is connected to the ubitx via i2c. Mark had to insert some lines of code to send the frequency and PTT info to his amplifier. It was a tight fit in Ian KD8CEC’s software, as there was not much memory left for additional code.
I tend to forget to switch antennas when changing fom 40/80m to higher bands and blew up a few finals in the past, so the code in the amplifier demands a low power tune after band changes.
When transmitting and high SWR occurs, PTT is disabled.
ATT is enabled on 20-80m as Mark’s µBITx puts out more than 5W on these bands.
Mark will gladly share all details if anybody is interested,
Link to the low pass filter used in the linear amplifier by Mark.
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.
v5 board at 21MHz on 10w (overdriven)v5 Board spurs with 5w (not overdriven)
Ashhar says “If things look good, we will roll with this as v5 board”. We can expect an update in 2019.
ND6P came up with this mod after testing his uBITX on the bench to determine how much signal it could handle before distorting at the speaker due to clipping of the audio signal. What he found is that RF input signals greater than S9+5dB (-68 dBm) become distorted due to the audio output clipping on the negative side.
What the mod does is insert enough attenuation at the RF input to prevent the audio from clipping for signals stronger than S9+5dBm. Signals S9+5dBm and weaker are not attenuated. He’s tested with RF input as strong as S9+40dBm (-33 dBm) and gets a clean output.
He says he can now listen to a weak signal without having to reach for the volume control when a strong signal comes on. So effectively this is an automatic RF attenuation circuit to provide AGC functionality for the uBITX.
Points connected to the volume control are in parallel with existing wiring.
The pin diodes (MA27B) are available at RF Parts Co.
Ashhar Farhar reported on 6 February 2019 that HF Signals has pushed the uBITx v5 into production. An initial 100 boards had arrived so they may run out quickly (reminding us of the v3 board shortages of 2018).
