Control CW TX power by unbalancing the second mixer

John (VK2ETA)  has been working on a successful modification to get the clock #1 mixer to unbalance on CW key down.  This involves the following steps:

1. Re-wire the T4 transformer input and output as per T2. That means as Jerry said to “… cut traces to T4 pins 3,5 and 6.  Swap them so T4 pins 3,5 are in from R47, pin 6 is out to C211.”

2. Disconnect (cut trace) from R105 to the common connection of C10, R27 and T2 (3,5).

3. Connect via a wire the disconnected side of R105 to the common connection of T4 (3,5), C211.

John used an audio shielded cable, and passed the wire under the board by drilling two small holes next the two connection points to ensure minimal pick-up of RF.

Pictures of R105 trace cut and wire to T4. (partially obscured by the hot melt glue on the toroids as I use my rig /P and /PM).

Picture of T4 traces swapped and cable from R105. (The line from the yellow toroid is a hot melt glue filament, not a wandering wire..hihi)

4. Change the software.  When in TX CW mode, disable clock #0, generate a clock #1 at “SECOND_OSC_USB” – “usbCarrier” = 45Mhz (+ or -) and clock #2 at (that same 45Mhz signal + target Tx frequency).

The difference in signal strength between key-up and key-down as seen on a control receiver is from way below S0 to S9+20dB or so, giving a dynamic range of over 70dB (indicative value in light of the basic test method).

The output power in CW mode can now be controlled finely by shifting the 45MHz clock slightly along the slope of the Xtal filter. For example I go from 14W max to between 1.5 and 2 watts by shifting the clock #1 frequency by 30Khz on the 20M band. On the 10M band, I need a shift of about 10Khz to reduce the 8 watts out to the same level.

Now, thanks to a bit of programming, John has full control of SSB as well as CW power across all bands. Great for his built-in ATU.

If someone with a spectrum analyzer could check the implications for the harmonics and spurs that would be a plus. John would expect in CW mode that the harmonic for 80, 40 and 30M bands would reduce, but some spurs to appear since we have the beat of two clocks now.  If there is interest John could modify Farhan’s code to match that modification (or publish some code snippets).

By the way, before he  did the mod above, he also tried to put a trim-pot between R105 / Ground / slider to T4 (3,5) and even when turned all the way to zero (slider to ground) it would still constantly unbalance the clock #1 mixer. So the change in topology as described above is the only way he could get it to work as intended.


SSM2167 install details

John VK2ETA some time ago provided information about his experiments with the SSM2167 module available on and for a few dollars.   The following shows the wire up diagram:

Picture of 4 pin molex connector added to the V3 Raduino for pickup of T/R (D7), +5V, I2C SDA, I2C SLD.

Note that the Arduino was replaced and put on headers as John uses a remote LCD display.


Attenuation on TX to flatten the curve (Phone only)

John VK2ETA has uploaded his latest version of a variation on Ian KD8CEC’s software with a default of “stock standard” which means it should run on units “factory” wired.

It includes the “OPTION_ALC” which use a table and interpolation to set the attenuation for 3 levels (Low, High, Max) for SSB power output.

Not a complex code at all (just search for the “option_alc” keywork). This should give up to 50dB of attenuation. This should be plenty to control the power even for WSPR low power users.

You will probably need to attenuate the signal for “Max” power on 40 and 80m otherwise you will get well past the targeted 16-17W on the lower frequencies.

Note that on 10m, you will not get 10w out.   You will need to follow Allison’s work on the power chain to see if a solution can be found to the fall-off in power out on 10m.

John’s software can be found here.


VK2ETA variation updated (based on KD8CEC firmware)

John VK2ETA has uploaded two updates of his Raduino firmware and ATU firmware in the folder: Variations on KD8CEC Software (by VK2ETA) + ATU sketch .

This firmware is targeted at portable and /PM operations. John doesn’t plan to add much in the way of further features unless they are of value to portable operations.

If you want to use sections of code in your projects, follow the #ifdef/#endif segments that mostly enclose the sections of code of interest. John is happy to help with extraction of code if requested.

The software is organised in options that can be enabled/disabled (mostly) independently.

The key features added in this version are:
1. Reduction in libraries size by 1K byte (this allows for more features to be added in the limited memory of the Arduino).
2. Added a “Stock Standard” option for unmodified rigs wired as per HF Signals instructions.
3. Added a two adjustable levels supply voltage monitor with visual and audio indication. Level two will disable TX to protect the battery.
4. Added the WSPR beacon option from KD8CEC software version 1.061.
5. Changed logic for CW modes: frequency displayed is carrier frequency, with Rx frequency shifted up or down (CWL / CWU).
6. In split modes, the VFO A/B label and frequency change when going from Rx to Tx.
7. RIT display:  frequency is kept fixed, RIT is now shown as +/- frequency difference.
8. Added ALC levels for MAX level too. Used to be no attenuation.

The second update is part of his approach to continuously look for program size reduction to have as many features as possible concurrently in the Nano’s memory.   Inspired by Ian Lee’s KD8CEC version 1.701, he has included a modified version of his TinyLCD library.  Compared to the complete default LiquidCrystal library this cut down version saves 452 bytes of program flash memory.  He has made this code into an object oriented class so it can be easily retrofitted into other projects as there were only two lines of code to change.


Using 2nd channel of TDA2822 for S-meter

John VK2ETA suggests using a section of the original AGC circuit of the µBITx (design by Ashhar Farhan VU2ESE) for generating a signal for an S-meter so that this can be used by his modified software.

This was part of the pre-production uBitx diagram but was not implemented in the production version.

The 2N7002 is used as an automatic gain control and can be used or not for that application.   The circuit has limitations since it was not included in the production version.

You would need to insert a trim-potentiometer (10K ohms is good) between pin 6 of the TDA2822 and the VOL-H connection to adjust the sensitivity, plus (VERY IMPORTANT) a voltage divider, between the cathode of the diode and the ground, to limit the voltage to under 5VDC for the analogue input of the Raduino.

John would use 330K ohm in series with 100K ohm to the ground, and connect A7 to the junction of the two resistors.

Further adjustments are available in the software if required as we define the 9 stages of the S-meter display (first stage is zero, then 6 stages for growing bars, 1 stage showing “+” and one stage showing the custom “++” symbol). In ubitx_20.ino it shows as:

int sMeterLevels[] = {0, 5, 17, 41, 74, 140, 255, 365, 470};

The values in the array are the measured values on the analogue input (defined as A7 above) at which we step into a higher “stage” and can go from zero for zero volts to 1023 for a 5V DC value.


SSM2167 Mic Compressor: Avoiding feedback

John VK2ETA notes that Simon, VK3ELH, pointed out  an issue that when inserting an SSM2167 mic compressor circuit between the microphone and the uBitx mic-preamp, it can create feedback when the microphone was placed near the speaker while in RX.  This is because the SSM2167 module is always on.

The solution John has applied is to connect the shutdown pin of the SSM2167 (pin 3) to the Raduino T/R digital output (D7) through a 2.7K ohm resistor. This disables the chip while in RX and removed the mic feedback issue.

Pictured above is an indication of where he picked up pin 3 on the SSM2167 on his module. The purple wire is connected to what is the right hand side of resistor R4. The 4.7K resistor on the RHS is for the mic-bias and the 51K resistor on the top-left is for bringing the compression ratio towards 4.

John feeds the Vcc pin on the board from the regulated 5V of his Raduino. Measured consumption at 2mA is a very small extra load on the Raduino regulator.

There is a DC blocking cap on the input and output circuits of the board already, so no external blocking capacitors are needed.  However, a bias resistor does need to be added for the microphone.

The 2.7K resistor is not mounted on the module, so is not shown in the picture.

Also not shown on the picture are an axial choke of 100uH between the “in” connection and the Mic, plus a 1nF capacitor between the “in” connection and ground to block RF feedback when Txing on higher frequencies. For John, RF feedback was noticeable from 15m through 10m. Others may not have this issue.

John also has a 10K adjustable potentiometer between the “out” connection on the module and the original Mic input to the uBitx.  His is turned to about 80% through its range.

John mounted his board on header pins so he can remove it as required.  He extended the header pin on the “out” side (bottom LHS on picture) past the board to provide an extra connection for the shutdown wire.

John finds the compression and noise gate work quite well on the module. When he is silent the background noise does not trigger any movement of the power needle, but it goes up as soon as he speaks into the Mic. Also despite showing quite an increase in average power, he hasn’t had any negative comments on his  audio. I was told that it was noticeable, but not unpleasant, “good for DX”.   And this was with a change in the standard resistor value for compression to give around 4: 1 compression.



Extensive VK2ETA mods to KD8CEC firmware

John, VK2ETA, has implemented a range of changes in Ian KD8CEC’s software targeted at portable operations (the software can be downloaded here in the files section of the BITX20 IO Group).

VK2ETA Software modifications to KD8CEC firmware

The scope of these modifications is described below:

Options for various features – These can be turned on or off. Key objective is to be able to customise the rig based on your needs and unfortunately on the restricted memory size of the Nano. So not all features can be selected at once. Choices, choices…

ATU control – A servo-based L-Network ATU. The communication between the Raduino and the ATU Arduino is via I2C. There is a separate sketch for the ATU Arduino (Nano or Pro-mini).   ATU operating mode can be set to OFF, Manual as in on-demand, or auto-RX meaning that it pre-tunes based on historical data on a change of band and after first change of dial frequency (for a quick scan of the bands). It uses the EEPROM data of the closest stored frequency for pre-tune or tune on-demand to accelerate the tuning process.

Handsfree microphone/headphone – Using an Android style 3 rings (TTRS) handsfree earpieces/mic combination, with 1 or 3 buttons (Play/Pause, +, -), the PTT is controlled by Play/Pause as toggle, and I use long presses on + and – as respectively pre-tune and smart-tune of the ATU. Short + or – presses could be used for frequency up and down. Requires a very simple hardware mod to free-up A6 (see below).

S-meter measure and display – using analogue input A7 from an 2N7002 based AGC or a MAX9814 circuit or any other for that matter.

Software based AGC range extender – to augment (as in double or triple) the dynamic range of an audio AGC. This uses the slope of the 1st If filter at 45Mhz to attenuate the Rx signal when the audio AGC reaches its limit. Adds over 50dB of dynamic range.

Forward power and SWR measure and display – Currently assumes that the ATU is providing that info over I2C. Otherwise could be adapted with a pair of analogue inputs for measure. See the excellent NT6D design on the wiki.

Options for displaying the S-Meter, SWR and forward power –  in either easy to see “fat” bars with no number, or “skinny” bars with more text and numbers.

Enable a “Memory mode” – selectable by menu, which cycles through all the populated memories (channels). Dial lock also locks the change of channels.

Made some rarely used or once-off functions as options  – to recover program memory after initial tuning and allow for more options to be selected.

Fixed some issues with the IF-shift option – Ian has resolved these in his new V1.06 and later releases. Two issues were present: IF-shift in USB would change the receive frequency and it was applied to TX as well. Now applies to Rx only.

Hardware modifications required to use VK2ETA software mod

The only required hardware mod is to connect the CW key input to the PTT. Since in Ian’s software we select the mode by menu, there is no need to have a separate analogue input tied-up for the CW key. This frees-up analogue input 6 for use by other functions like the handsfree option above.

Still to come

John plans to apply Ian’s improvements in v1.06, especially the CW transmit frequency option and if possible the WSPR beacon mode (as a further add-in option).

How to use VK2ETA software

Download the zip files, and unzip these in your Arduino sketches folder.  Edit the ubitx_20 options sections, using #define for enabled and #undef for disabled.

Perform a CTRL-R to compile and check how much memory is used. If you go over the limit, a warning is issued.  Providing you have enough memory to run the software, upload the sketch to the Arduino.

John has uploaded both the Raduino as well as the Arduino sketch for the ATU and SWR measurement. They can be found in the folder “Variations on Ian Lee’s Software (by VK2ETA) + ATU sketch”. 

All software is released under GPL V3.

Use the 45MHz Roofing Filter for an RF AGC?


John, VK2ETA, came across an idea in the search for a greater range for his MAX9814 AGC circuit.

Stations, above S9+10 would produce distortion in the audio circuit with the MAX9814 AGC in circuit.   He isolated this to the MAX circuit as the distortion would disappear when it was bypassed.

John was curious as to what the first 45Mhz filter (Roofing Filter) shape was like and if there was some plateau to be used somewhere for attenuating the strong signals.

He modified Ashhar Farhan’s original software to include an “Adjust First IF” menu item, in steps of [1000Hz].

By using a local station’s carrier aligned on 1,500Hz audio as a reference and an Android audio spectrum display he plotted the response of the single crystal roofing filter. This also gave an idea of the effect on the audio of shifting the filter up and down. The “noise” in the graph below near the peak is the effect of changing from a measure every [10,000Hz] to a measure every [1000Hz], plus the inaccuracy of John’s rudimentary instrumentation.

As you can see, there are rather slow slopes on each side of the peak (which is off-center by [7,000Hz] approximately when compared to 1,500Hz – the centre frequency of an SSB signal).

So John has proceeded with changing Ian’s software (based on v1.04) to incorporate an automatic AGC step-down when the signal reaches S9+10 and and automatic step-up when it reached S0. In the middle range, the MAX audio circuit does the AGC job.

John used the up side of the filter as he got some birdies on some of the shifts on the down side.

Now the uBitx can handle S9++++ stations with ease, that is until the first amplifier stage before the filter saturates which John suspects is unlikely in “normal” conditions.

The only concern would be for another, possibly even stronger, station which would be placed at the peak of the filter (possibly several KHz away). This could produce intermod distortion. But the chances of that happening are pretty remote.

So, this approach works quite well and is surprisingly effective for AGC control at an early stage in the receiver.

It also works in reverse, with the transmit SSB signal being attenuated by the same amount, thereby leading to a possible ALC software control for the units which measure the power out (or possibly just the current).   It could also be a simple solution to set attenuation (e.g. on digital modes).  Again, check the effect of the slope on the voice tone.

John has attached snippets of his code.  He has also uploaded the modified uBitx software for testing the filter both in RX and TX in the  “Software based IF attenuation” folder in the BITX20 IO Group files.

John will soon publish the complete set of Ian’s modified software including mods to control his ATU unit.   However, in seeing discussions on an IF AGC in the group, he thought this update would be of interest to constructors.

Code Snippets


void doSoftwareAGC() {

int newSMeter;

//VK2ETA S-Meter from MAX9814 TC pin
newSMeter = analogRead(ANALOG_SMETER);
//Serial.print(“newSMeter:”); Serial.println(newSMeter);

//Faster attack, Slower release
currentSMeter = (newSMeter > currentSMeter ? ((currentSMeter * 3 + newSMeter * 7) + 5) / 10 : ((currentSMeter * 7 + newSMeter * 3) + 5) / 10);

//Serial.print(“currentSMeter:”); Serial.println(currentSMeter);
//Scale it
scaledSMeter = 0;
for (byte s = 8; s >= 1; s–) {
if (currentSMeter > sMeterLevels[s]) {
scaledSMeter = s;
//Serial.print(“scaledSMeter, un-adjusted:”); Serial.println(scaledSMeter);
//Apply auto-shift of first IF to increase the dynamic range of the Audio AGC circuit
long previousShift = firstIfShift;
if (scaledSMeter >= 7) {
//Reduce gain by shifting the first and second If by the same value, thereby
// leaving the RX frequency the same but using the slope of the roofing
// filter to deliver progressive attenuation.
// 10kHz or 5kHz per step.
firstIfShift += (scaledSMeter > 7 ? 10000 : 5000);
} else if (firstIfShift > 0 && scaledSMeter < 1) {
//Re-increase the gain if we reduced it earlier
firstIfShift -= 5000;
firstIfShift = firstIfShift < 0 ? 0 : firstIfShift;
if (firstIfShift != previousShift) {
//Serial.print(“firstIfShift:”); Serial.println(firstIfShift);
//Adjust meter by IF attenuation except for the first 10Khz. Approx 6dB per 5KHz.
scaledSMeter += (firstIfShift > 10000 ? (firstIfShift – 10000) / 5000 : 0);
//Serial.print(“scaledSMeter, adjusted:”); Serial.println(scaledSMeter);


//And the setfrequency function becomes:

void setFrequency(unsigned long f) {
f = (f / arTuneStep[tuneStepIndex – 1]) * arTuneStep[tuneStepIndex – 1];


if (cwMode == 0)
if (isUSB) {
//si5351bx_setfreq(2, SECOND_OSC_USB – usbCarrier + f + (isIFShift ? ifShiftValue : 0));
si5351bx_setfreq(2, SECOND_OSC_USB + firstIfShift – usbCarrier + f – ((isIFShift && !inTx) ? ifShiftValue : 0));
si5351bx_setfreq(1, SECOND_OSC_USB + firstIfShift);
else {
//si5351bx_setfreq(2, SECOND_OSC_LSB + usbCarrier + f + (isIFShift ? ifShiftValue : 0));
si5351bx_setfreq(2, SECOND_OSC_LSB + firstIfShift + usbCarrier + f + ((isIFShift && !inTx) ? ifShiftValue : 0));
si5351bx_setfreq(1, SECOND_OSC_LSB + firstIfShift);
//VK2ETA Bring back the BFO to default if using IF Shift and we are TXing
si5351bx_setfreq(0, usbCarrier + ((isIFShift && !inTx) ? ifShiftValue : 0));

if (cwMode == 1) { //CWL
//si5351bx_setfreq(2, SECOND_OSC_LSB + cwmCarrier + f + (isIFShift ? ifShiftValue : 0));
si5351bx_setfreq(2, SECOND_OSC_LSB + firstIfShift + cwmCarrier + f + ((isIFShift && !inTx) ? ifShiftValue : 0));
si5351bx_setfreq(1, SECOND_OSC_LSB + firstIfShift);
else { //CWU
//si5351bx_setfreq(2, SECOND_OSC_USB – cwmCarrier + f + (isIFShift ? ifShiftValue : 0));
si5351bx_setfreq(2, SECOND_OSC_USB + firstIfShift – cwmCarrier + f – ((isIFShift && !inTx) ? ifShiftValue : 0));
si5351bx_setfreq(1, SECOND_OSC_USB + firstIfShift);

//VK2ETA Bring back the BFO to default if using IF Shift and we are TXing
si5351bx_setfreq(0, cwmCarrier + ((isIFShift && !inTx) ? ifShiftValue : 0));



An ATU mod from VK2ETA

The news item has flushed out one or two further modders.

John, VK2ETA, writes:

“For my part I don’t know if it’s hard code developer or more mad hacker, but it is fun.   Here is a picture of the latest addition, an L-Network ATU built in the uBitx case, driven by two servos and controlled by an Arduino mini pro linked over I2C to the Raduino.

“It is now working well (80m to 10m) but needs cleaning up, shields (just to make sure) and a nice display of SWR and Power on the LCD.  More details to come.”

Ron, W7HD, was very interested in the servo control code and components and wiring as he would like to adapt this idea to handle a pair of servos for an az-el rotor for his satellite Arrow antenna, which only weighs about 1-2 pounds. Then he just needs to add a bit of code to show the actual antenna position and he’ll be all set!

Ron suggests another adaptation of this project would be to remotely tune a magnetic loop antenna. One of the problems you run into when trying to tune a loop is that your body affects the tuning.



Mic Compression and Noise gate with SSM2167 module

John, VK2ETA, has  used the small circuit board “SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V”available on eBay or Aliexpress as a compression and mic pre-amplifier.

He simply connected the input to the mic, added a 4.7K ohm resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage.

He didn’t modify his uBitx board,  but simply inserted the board prior to the mic input.  The gain of 20dB is reduced back with the output potentiometer. John removed the “R1” resistor and replaced it with a 51K Ohms resistor to get a 4:1 compression factor, up from the 2:1 as delivered, but this change has yet to be tested “on air”.

John hasn’t received any negative feedback about the compressor except when I pushed the output potentiometer too high.



Simon VK3ELH used the same board and a similar scheme for powering the module from the regulated 5v line on the Raduino.  It is also installed separate to the main board and inline with the mic input.

Simon used a 75k ohm resistor for compression and 1k ohm resistor for the noise gate and a 100k pot on output. At full output, his audio was readable but distorted based on an audio check QSO, so the output has been turned down.

He put a larger heatsink on the IRF510 to cater for the higher average output, as the stock one was getting warm!

A side effect of the mic being on all the time is that there is leakage through to the speaker and it causes some feedback if the mic is within 2 inches or so of the speaker.