Pesky volume pot

Geoff, G8BMI is cross-fertilising ideas across hobbies…

He has been assembling a uBITX, and found that the volume control has a 5mm shaft rather than the usual 1/4 inch. This means that none of his spare knobs fit.   We’ve all been there.  Most of us threw the supplied pot into the junk box and moved on with another standard potentiometer.

However, a waking inspiration suggested a very short length of 1/4 inch OD copper pipe, as used in model steam engines, could be used to ‘sleeve down’ a 1/4 inch bore knob. And it does!.

Geoff turned down the copper tube in the lathe, using a file just to ease the fit [It could be mounted in a power drill chuck to get the same result].  It only needs a skim. His tubing was 0.210 inches bore, which is just over 5 mm (5.18)


Updated manual for KD8CEC firmware

Rod KM6SN has updated his manual for the KD8CEC firmware for the µBITx that is pretty popular because it requires no hardware mods.

The updates to the manual:

1. incorporate feedback received on the manual, with some new
operating tips

2. enhance the Memory Manager section a bit

3. improve readability.

As before, please send any feedback to the email address shown in the manual.   A new manual for KD8CEC firmware rev 1.07 is forthcoming, as is a manual on the Memory Manager.


TDA2822 now shipping with socket

The TDA2822 audio amplifier chip (now infamous because of the self-combusting WX versions) now comes installed in a socket for easy replacement from HF Signals (the µBITx manufacturer).

There are still regular reports of spectacular end points for the WX chips on the list.   Most are finding that TDA replacements work reliably with no need for a regulator to feed them with a lower voltage.  To be safe and secure, however, you can easily feed them with 5-9v via  a regulator.

In related news, some constructors are substituting the readily available NTE7155 chip which is 100% pin compatible with the TDA2822.


HF Signals catching up with backlog

HF Signals seems to be catching up with their huge backlog of µBITx orders.  The latest posts from members of the BITX20 List shows that orders up to 17 March are being shipped at present.  This suggests that the manufacturer is just a month behind on shipments now, compared to two months behind in March.  This will be good news for constructors eagerly awaiting their kits.  Note that shipping time is additional (around a week for DHL shipping worldwide and 2-5 weeks for IndiaPost depending on destination).


SSB Crystal filter response

Michael VE1LEB has been scratch-building a uBITx.   This is the first time he has built a crystal filter.  The photo below shows the response curve of the 12MHz SSB filter output by his PHSNA:

To select the crystals, Michael used the K8IQY crystal test fixture and chose the closest ones to 12MHz from a bag of 50 inexpensive computer crystals.

He is not sure whether his PHSNA is accurately calibrated or how much error there is in the frequency readings. However, the -20dBm passband is less than 2.5khz.   He received a new bag of 100 crystals, and was wondering if he should take the time to  

Rod KS6SM commented,

“In the ubitx, the crystal filter is driven/terminated in a 200 ohm impedance. It is likely your PHSNA is 50 ohms.  The passband ripple you are seeing can be severely influenced by driving/terminating impedances.

“Are you measuring the filter with the transformers at each end, or are you going direct to the crystal connections?

“It is normal for the passband to be below 12 MHz. On my ubitx, the BFO is at 11,997,117 Hz, so that will give you a sense of how far below 12 MHz the passband is.”

It turned out that Michael was measuring the filter ‘naked’, without the transformers.  He measured it again but through the transformers, and the result is  much “softer”:

Satish VU2SNK said, “In both the curves without and with transformer your filters loss appears to be around -7 db.   In my opinion this is bit too high -3db is just right up to -5db is acceptable according to the experts, what type of capacitors are used in this filter? The multi layer ceramic capacitors
are really of low loss. If you use the modern SMD ones in my opinion that will reduce the loss in the filter.”

Allison KB1GMX also suggested, “One warning when measuring narrow filter use a very slow sweep. At narrow resolutions if the sweep is too fast the curve will not match the actual. It is as much detector response time as its filter delays. When too fast the it will appear tilted, and when you slow down you will see more accurate result.

The filter insertion loss in the 4 to 7 db range sounds correct. Though I might be better if higher Q capacitors are used we are not talking a 3DB difference. Also dId you calibrate out the transformers first? That can add a DB of loss sometimes more.”

In responding to the suggestions from Allison and Satish,  Michael said, “I’m using chinesium SMD 1206 capacitors.

“I realize now that I had not accounted for loss in my cables, and perhaps I’m not appreciating fully the implications of my test equipment. I’m a dopey artist, not an engineer! When shorted, the cables out-of and into the PHSNA present a 3.5 dBm loss. Each transformer adds 1.9 dBm loss, so double that and add the loss in the cables will represent a total loss of 7.3 dBm. When I bias the PHSNA for the 7.3 dBm loss and attach the filter (including the transformers on each end), I get this result, which puts me well inside the 4-7 dBm loss that you mention.”


RTL-SDR is mated to uBITx

Ian KD8CEC is working on a new release of his CEC firmware for the uBITx that will be more modular in approach (picking up on John VK2ETA’s mods to Ian’s firmware), interface with a range of displays (I2C versions of the 1602 and 2004 displays as well as the display that comes with the Raduino).  More importantly, however, this new version will enable the µBITx front-end of the receiver to be connected to an RTL-SDR USB device.  The RTL-SDR device will draw on the  HF receive sensitivity of the µBITx  along with the front end bandpass filter and first IF roofing filter and associated IF stage amplifier (normally at 12MHz) to produce a quality SDR receiver (for just a few $s invested in the RTL-SDR tuner).

This version is available to download (1.071 Beta) from Ian’s website now.

W0EB/W2CTX firmware gets another update

Jim Sheldon – W0EB has released W0EB/W2CTX I2C software Version 4.00R for I2C enabled uBITX Raduino cards and the W0EB/W2CTX/N5IB RadI2CIno cards.

This software will NOT run on a stock standard Raduino card unless it has been modified to operate with an I2C display This version is for an I2C enabled 2 X 16 LCD display. (The 4 line x 20 character version will be released later).


Flatten the power curve and set your power out in software

John VK2ETA  has some pretty good ideas. A few days ago he worked on an AGC system using the first IF stage (at 45MHz) to control gain at the front end of the system.   He has now got some pretty good results using a limited amount of memory in the firmware to flatten out the power curve on transmit.  At this stage it only works in voice and digital modes (LSB and USB). Because CW is achieved by unbalancing the first mixer after the IF filter, this approach will not work on CW.

By shifting the IF frequency on the filter, John is able to produce a fixed attenuation that keeps the output power within the required limits.

The process is two fold:

  • a one off calibration exercise.
  •  a menu item that selects Low, High or Max power.

John has used 5W and 10W as targets for the Low and High values.

His results (remembering he has modified the final stage with RD16s and a number of other changes have been made to his transceiver):

  • 80m to 10m in low power settings have variations between 4 and 6 watts.
  • 80m to 10m In the high power setting has a range between 9 and 11 watts.
  • Only 16 bytes of data points are required (could be stored in EEPROM if desired).

His ATU uses a 2nd Arduino.   He has used a spare digital output to send a 1500Hz tone, low-pass filtered, to the microphone input for calibration purposes.  The audio filtering plus the 2nd IF filter take care of the audio harmonics and the signal is clean. This could be done in the Raduino if digital lines are freed up, such as by using an I2C display.

The tone also provides a tune up facility in low power.

He can transmit in digital modes in low power mode without having to adjust the drive every time he changes bands.  Perfect!

John will update the code in the file section so that others can incorporate this new feature into their firmware if they desire.

John acknowledges Jerry KE7ER’s idea of using fixed tables to perform the High/Low calibration function.

Firmware uploaded to files area

John has uploaded an update of the KD8CEC based software (V1.04 based) for Raduino and ATU Arduino.

Key changes in Version 20180411:

– Made the menu system dynamic so that items can be inserted at compile time or can be context-dependant at run time.
– Added output power attenuation for 80 to 10M using first IF shift.
– Added new menu item “Power Level”: low/high/max for SSB modes.
– Made CW menu items display dependant on having CW modes selected.
– Reduced the level at which the software AGC comes in. Adjusted the correction of S-Meter when software AGC comes into play.
– Changed tone generation for ATU tuning sequence from CW to using SSB with the 2nd Arduino generated tone.
– Change tune sequence to

a)Select the “Low” power setting before performing a tune and
b)Change tune frequency to tune on carrier (offset by audio tone frequency).

Assumes the following hardware change (only if using an ATU): a digital output on the ATU Arduino is used to generate a tone. An audio filter identical to the Raduino CW audio tone filter is used and it’s output is connected to the mic input of the SSM2167 module. If digital outputs are freed-up on the Raduino, for example by using an I2C display, the same can be implemented with only the Raduino.

This allows a low power tune function.