Quick review of the Kit-Projects AGC board

Jim W0EB, TSW Project Coordinator,  bought two of these kits for his 2 uBITX Version 5 radios. First off, the directions for installing this board are brief, but they are easily followed and the boards are easy to install.

Not wanting to drill a hole for the included switch and run a bunch of wires to it, he just wired the common pad to the “fast” pad for always on, Fast AGC.  A “Via” hole was identified in the trace between R70 and “Vol HI” and Jim ran a wire from there to the “VOL” pad on the AGC board. This worked great.

Even though the “S-meter” output of this AGC system was designed to work with the CEC software, we found it worked with TSW’s BITeensio board as well.

The BITeensio uses the A19 analog input on the Teensy 3.6 for the S-meter. This little AGC system drives the S-meter routines just fine on the BITeensio.

A 50 microvolt (-73 dBm) signal was fed into the antenna connector and Jim adjusted the software’s S-meter routine’s division ratio so the touch screen’s display read S9 as it should with a 50µV input. The rest of the S units were so close to correct that no further tweaking was deemed necessary.

He also found that adjusting the on board RF gain control for max recieved signal was the best way to adjust the level control. As you turn the control counter clockwise, the gain increases and there is a point past which saturation occurs. This is obvious when listening to a weak signal and you can hear the gain drop past this point. Simply adjust the gain to that point and turn it back to where the signal is just peaking.  It is best to just leave it there if you want your S-meter to work right.

Once adjusted, this little AGC board keeps the RF input nicely within bounds on strong signals quite well.

Jim calls out ND6T and N8DAH and says “Well done guys, well done! The kit is certainly worth the price IMO”.

Reference

The experience of cleaning up spurs and harmonics in a V3 uBITx

v4 board at 10w on 21 MHz (overdriven)

Mike KU4QO has shared his results in trying to clean up the output of his V3 uBitx.

He thanks all the folks who spent their time on tracking down the causes and cures and shared his results with the group for others to reflect on.

His setup comprised a signal generator at 1kHz feeding the microphone input, and the radio set to SSB mode.  He applied 12.0 VDC for power. Output of the radio was fed through a 30db attenuator into a borrowed spectrum analyzer. He adjusted the signal generator’s output to vary output power in the transmitter.

Power output of the fundamental was just below the point where more microphone signal did not appreciably increase the output power:

80m  10 watts

40m    8 watts

20m    5 watts

17m    2 watts

15m    2 watts

12m    1.5 watts

10m    1 watt

As built, his V3 uBitx was marginally compliant to US FCC rules on 80, 40 and 20 meters.  Above 20m the output was messy as a result of lower frequency spurs. For example, on 15 meters there was a spur at 2.6 MHz that was down only 23dB from the fundamental.

He first completed the L5/L7 mod using surface mount 0.68uH inductors (from the Ebay seller in Poland), marked “T R68K”.  He re-ran the tests and saw no appreciable change in any of the results.

Next, he replaced the 5 relays with Axicom relays from Arrow.com. This change showed a significant change in the harmonics on the low bands. 80m, 40m and 20m went from marginally acceptable (barely -43dB from fundamental) to easily meeting FCC specs on those bands, with unwanted harmonic outputs dropping an additional 10dB or so. However (as expected), the relay change did nothing for the spurs resulting from operation on the higher bands.

Most interesting was watching the output spectrum while adjusting the microphone drive. For example, the nasty spur on 15 meters at 2.6 MHz went from 23dB down at a fundamental power of 2 watts, to below the noise at a fundamental power of 1 watt. At significantly reduced drive (power) levels, the high bands are all in compliance with FCC specs. Here are the reduced powers that provide a clean output signal:

17m   1.5 watts

15m   1 watt

12m   0.6 watts

10m   0.25 watts

Mike followed Raj’s directions for the L5/L7 mod. What should the affect of that mod have been? Should it have impacted those spurs on the high bands? He is questioning whether the inductors were actually not the correct type.

Jerry KE7ER thinks the consensus is that replacing L5,L7 should reduce coupling from the power amp back into the 45mhz IF amp where it takes another trip through the D1,D2 mixer.

Raj VU2ZAP, the originator of this mode, had a specific manufacturer in mind.  They have to be well shielded, commonly available surface mount inductors did not help at all.

Curt WB8YYY is pleased that the results show that the Axicom relay replacement provided around 10 dB improvement.   He has Gordon’s relay board installed, and he imagined his set of new relays will do at least that good.

He notes that some form of added 45 MHz filtering should help in most rigs. He first tried a xtal filter stage – but it disrupted his 40m CW output (He could restore output only by touching L5 – hence the reason for considering shielded inductors). He instead added an LC filter similar to what now appears in the stock v5 board, but he is not sure of the inductor’s Q.  He thinks it may be around 70.  According to simulation it provides more than 10 dB.  He is still waiting for verification in real life.

Curt notes that in the club’s initial measurement of many ubitx rigs, there is appreciable variation even though these are all the same board design (v4) and they were all received at the same time.

John Sutter W1JDS uploaded a spreadsheet of measurements for his µBITx for 80, 40, 30 ,20, and 10m:

1. Stock
2. Replaced L5/L7 with SMD inductors I found locally, kept C211 in circuit
3. Replaced the relays
4. Replaced the inductors with the specified Murata parts
5. Bypassed C211 per the mod

The spreadsheet can be accessed here: https://docs.google.com/spreadsheets/d/1eUFuHcprJhk_HuINxXJLhBuZ7-0DHPfaGPV-RoN8wBY/edit?usp=sharing

The mods he used are those from this website:  http://ubitx.net/spectral-purity/

Both mods helped.

To desolder the relays, he used a piece of desoldering stick,
a low temp alloy along with solder wick and a hot tip on the iron,  and this made easy work of it.  Cleanup was the worst bit.

Probably about the same as: http://www.zeph.com/lowmelt.htm

He’ll probably put the cap back in the circuit in order to be able to use 30m.

He is still needing to find a solution for 80m in order to ensure compliance.

His testing was done in CW mode with the following configuration:

  • 12V LiPo Battery pack  (link)
  • 40dB homebrew attenuator which I’ve characterized as being pretty flat
  • Agilent E4404B SA & HP 8595E SA depending on which one I could use at the time
  • 3D printed key
  • Misc cables…

    This is a v3 radio.  Right now it’s mounted in the plastic box it came in until it decides what it wants to be when it grows up 🙂   John has a JackAl board ready to go when he is happy with the harmonics.

uBITx v5 Board compatibility with JackAl

 

Jack, W8TEE and Al AC8GY have spent considerable time testing a couple of V5 µBITX boards for use with their JackAl board.  No hardware modifications are required when using their board with the v5 µBITx.

However, Jack is making some minor changes in the software because of the new IF frequency.  They expect to release a new software release for the V5 board in a few days.  They are also expanding the assembly manual with additional photos and narrative.

The JackAl boards are being sold by QRPGuys.com.

Reference

Alison’s audio pop mod prettied up


Mark N7EKU has posted a nice clean schematic and a picture for the audio pop fix he has just completed on his uBITX V3. It works great and saves the ears a lot.

If you check the top part of the photo, you can see that I have done Allison’s fix for harmonics by re-arranging the filters a bit and moving the original relays to the bottom of the board. This worked great too and cleaned up harmonics a lot.

Reference

TSW introduces adapter to protect DC input

Jim W0EB, TSW Project Coordinator,  has announced he has just got the boards and all the kit parts for this little adapter that allows the DC input for your µBITx (or any other kit or homebrew rig) to be protected.

The board allows connections for optional Reverse Polarity (series Schottky diode), a switch on the volume control (or separate power switch) and the switched DC output to whatever item is being powered.  This is achieved t through a small PC board using MOLEX or MOLEX Style male/female connectors. Everything can be plugged in and unplugged for ease of troubleshooting without having to solder or unsolder wires from the rig’s terminals.

TSW is making either the bare board or a full kit of parts available.  The picture shows everything that’s included in the kit.

Full details are on the TSW website

The manual for the SwitchedPowerAdapter is available in PDF form so you can see what it’s all about.

Reference

Q90 blown by strong RF input on antenna line

Further to an earlier post about the vulnerability of Q90 in the µBITx, Gary
W6RAG wrote to uBITx.net today noting that the potential danger to Q90 from nearby strong RF signals is well founded.

Recently Gary was attempting an FT8 contact on 80 meters with his son about 400 miles distant.  Being unsuccessful with the µBITx, he turned off the rig and turned on his TS430 and made contact at about 60 watts output power. The ubitx remained connected to an end fed wire about 10 ft from his dipole that was in use with the TS430.

A few minutes later he tried the ubitx again and it had no output. He traced the failure to Q90. The emitter was shorted to the base just as reported by others.
The diodes in the balanced mixer will not save Q90.

Gary recommends that µBITx users disconnect the antenna if they are going to operate other transmitters with nearby antennas.

A serial connection from your uBITx

Sascha DL5SMB has soldered some wires to D0 and D1 (RX + TX) and ground on the Raduino nano and then installed a MINI-DIN 8 socket to the uBITX like in an FT817.

“Why?” you may ask!   He is  building a  TF3LJs Magnetic Loop Controller at the moment, which communicates with the transceiver via CAT for tuning the antenna. This interface uses just these three lines. So he needed to make them talk 🙂

After a few hiccups, and a helpful suggestion from John VK2ETA, this solution has now been made to work.   The idea of using D0 and D1 connections on the Nano is to provide a separate serial feed to the Magnetic Loop Controller that is independent of the CAT signals coming out of the USB socket.   This may be a good solution for other potential use cases as well (e.g. driving automatic tuners).

Reference

Warming effect of driver and pre-driver affects spurs

Lawrence Oberman mentioned an issue he has on a v4 board that looks like RF coupling or leakage when testing CW on 20m for FCC compliance.

He doesn’t see it with SSB but if the rig is placed on transmit for long enough spurs will start to show up .  He has only just got Axicom relays to replace the stock relays. Strangely enough he has not seen this behaviour on 15m, 17m or the higher frequency bands, or on the bands below  14MHz.

Others have seen similar phenomena on different bands, some with the spurs appearing on starting TX and then disappearing.

Allison KB1GMX says “Both cases come from the same source, heating in the amplifier usually the pre-driver and driver stages but it can include the finals. These stages are not thermally stable and parts heat up and their bias shifts.”

Reference

v5 board more sensitive

Ashhar Farhan VU2ESE tells us that the µBITx v5 board has more gain on transmit, as well as on receive, than the previous boards.  This is due to the BFR93w transistors in the 45 MHZ IF in place of the 2N3904s.

The carrier and spurs will be appear to be higher if the TX gain (controlled by RV1) is running flat out. it is best to reduce this to about 60% of the full gain. One way of achieving this is to connect a 50 mv audio signal to the mic, and transmit SSB on 7 MHz while reducing the power out until it is around 7 watts.

Reference

Using a 100 PPR encoder with the uBITx

Allison KB1GMX gives details on how to wire up the commonly available 100prr-6 encoder.  These have a nice vernier style dial and cover 100 pulses per 360 degree rotation.  The calibration points line up with these pulses.  They come with either 4 or 6 wire terminals on the back.

The A and B terminals have transistor outputs to ground and are used in connecting to the raduino:

  • Ground is ground (0V) terminal
  • Black and brown to the A and B terminals (reverse if the rotation direction is backward)
  • 5V from raduino 5V reg to the internal LEDs used in the optical encoder  (Vcc terminal)

Allison uses the V4.3 code in her µBITx with here own mods and this encoder worked well. Some firmware for the Raduino will not be sufficiently fast to keep up with the signals being sent from the encoder if you rotate the encoder quickly.

The uBITx.net editor likes the black version better.  These are a classy unit.

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