KK4DAS MB 20 Transceiver Complete

The MB 20 transceiver is now complete.  The rig is a homebrew 10-watt 20-meter SSB transceiver.  The VFO module was an ebay purchase of a salvaged Yeasu FT-401B module. The FT-401B was manufactured around 1970.  I bought the module after a recommendation from Bill Meara, N2CQR and Pete Juliano, N6QW.  The rig was loosely inspired by Bill’s Mythbuster rigs. 

This rig is really a stone-soup build – with bits and pieces of modules lifted from or inspired by many of the homebrew illuminati! 

The Yaesu VFO runs at 9MHz so I set the IF at 5MHz and built a 5MHz crystal ladder filter.   The rest of the IF module consists of 4 Termination Insensitive Amplifiers (TIAs from W7ZOI, Wes Hayward),  2 on either side of the filter for each of transmit and receive.  I also used a TIA to boost the output of the Yaesu VFO by 7dB to drive the IF mixer.  The BFO is a Colpitts crystal oscillator using one of the crystals left over from the ladder filter build.  The IF mixer is a homebrew diode ring mixer.  The balanced modulator / product detector is a 2-diode circuit that I liberally borrowed from Ashhar Farhan’s Bitx-20 module schematic.  The power chain is two 2N3866 pre-amp/driver stages feeding a RD06HHF1 MOSFET final amplifier, very loosely modeled after the Bitx-40 module PA.  The 20-meter band pass filter is a standard double tuned circuit, the Tx LPF is a W3NQN LPF design with values taken from the QRP Labs kit instructions.  The microphone amp is a one transistor NPN amp configured to support a homebrew electret microphone.  The circuit originally came from Farhan’s sBitx that I modified for my electret mic.  The audio amp is a single stage driver and an LM-386, that I first built for Pete’s SimpleSSB. 

The front panel and base are plywood from my wood shop.  I use copper roofer’s foil to create a solderable ground plane.  Almost all the circuit boards are homebrew Manhattan style.  The IF TIAS are on boards that were sent to me by Todd Carny, K7FTC,  of MostlyDIYRF.  The Tx/Rx switching is relay based.

Lastly, the digital frequency display is Ardunio Nano and a surplus TV prescaler chip that divides the IF frequency by 64 so that it can be counted by the nano.  This is my revision of a circuit described by IMSAI guy on his YouTube channel that has also been build by many others.  I rewrote the firmware to provide a rock solid frequency display with 100 Hz precision.  The digital display makes fine tuning the VFO much easier – and its very good to be able to tell at a glance where I am tuned to.  The red 3D printed bezel for the OLED display was designed by my friend Leon, NT8B

It puts out about 10 watts.  I plan to build an external 100 watt amp to accompany it, but I have been making QRP contacts just about every day into an off-center fed dipole up at about 35’.

As Pete always says: “If you know stuff, you can do stuff”

And as Bill says: “There is a lot of soul in this machine.”

73 from Great Falls,

Dean

KK4DAS

KK4DAS MB 20 Transceiver - now Receiving on 20 meters

 

This week's homebrew adventure was to complete the receiver side of what I am now calling the KK4DAS MB 20 transceiver - MB is a call-out to Bill, N2CQR's Mythbuster rigs which this build is both inspried by and patterned after.  

First order of business was choosig the IF freqency which was easy in this case.  The VFO is at 9MHz so if I put the IF at 5MHz, well my 2nd grade math teacher taught me that 9+5=14 - which puts me right in the 20 meter band.   

I ordered a batch of 5MHz crystals from Mouser for the IF but they had not come in yet - and I was impatient to get started so I built the BFO and balanced modulator using a crystal from the junkbox figuring I would swap in a 5MHz crystal when it arrived.  The BFO and two diode balanced modulator is copied directly from Farhan, VU2ESE's original BITX20 schematic. It went together easily.  Despite conventional wisdom I did not attempt to mach the diodes - I just took the first two off the tape.

BFO and balanced modulator

Nulling out the carrier

To null out the carrier I injected a 2MHz signal into the AF port of the modulator and put the scope on the IF port.  Using the FFT mode of the scope I was able to clearly see the carrier and mixing products.  I was able to effectively null out the carrier using the trimmer cap and trimmer pot that are part of the design. The two peaks either side of the middle are the first sum and difference mixing products.  The display is centered on the carrier frequency and you can see that it has been nulled out.

The crystals arrived and the next job was to build the crystal filter.  I used the Dishal crystal ladder filter design software to design a 6 pole Cohn/QER filter.  The advantage of the Cohn filter is that it uses a single value of capacitor between each crystal.  The QER (Quasi Quasi Equi Ripple) modification to the Cohn adds an addtional crystal in parallel at each end of the filter and reduces the amount of ripple.  When building a filter the recommendation is to pick crystals fom your batch that are the closest in frequency to each other and the Dishal software requires you also to know the motional parameters for the crystal. I used what is called the G3UUR method to evaluate the crystals. I had previously built the simple crystal evaluation circuit and quickly sorted the crystals. I then used the Dishal software to determine first the motional parameters and then to calculate the capacitor values and input/output impedance of the filter.  The calculated input and output impedance were spot on0and I calculate the LC values needed for matching to 50 ohms.

   

Sorting the crystals using the G3UUR circuit

The completed 5MHz, 2.5KHz filter

The last board I needed for the receiver was an audio amplifier.  I opened up by box of "boards that could be rigs someday" and found an audio amplifier stage I had built for Pete, N6QW's SimpleSSB a few years ago.  Its a single 2N3904 pre-amp followed by an LM-386.   With all the boards needed for the receiver at hand it was time to connect everything up.  

The completed KK4DAS MB 20 Receiver

I placed the crystal filter at the rear center with the MostlyDIYRF TIA IF amplifier boards on either side. I left space adjacent to the TIAs for the transmit IF amplifiers. The audio amplifier is on the lower right and just above that is the BFO / balanced modulator board.  The receive band pass filter is on the upper left below the first IF amp.  At the far left is the future Tx RF amplifier stage - not yet wired into the circuit.  I mounted the antenna connector on a piece of single sided PCB.  To keep the DC wiring straight I decided to use blue wire for DC on Rx and yellow wire for the DC on Tx.  Red is used for always on DC.   

A big advantage of modular construction is that since all of the modules have been tested individually it increases the liklihood that the build will work well when everything is connected.  To my great pleasure that turned out to be the case and the receiver woked perfectly on first power up.  There was one glitch - the tuner maxed out at 14.260 MHz about 100KHz short of covering the entire 20 meter band.  I thought I was going to have to rebuild the IF with 5.2MHz crystals but Bill pointed out that Yaesu and built in a variable cap to move the VFO passband up or down. I zeroed out the capacitor and that gave me full coverage across 20 meters.

Adjusting the Yaesu VFO for full coverage on 20 meters

Now that the receiver is built I will spend a few nights following Farhan's advice and take pleasure in listening to the receive I have just built.  

All I need to build for the transmitter is a microphone amplifier and the final low  pass filter.  Then add some relays for Tx/Rx switching and then I can work on fit and finish of the cabinet.  

73 from Great Falls,
Dean
KK4DAS

Homebrew sBitx - RTC and Power Meter - ATTINY85 Tale of Woe

 

sBitx RTC and SWR/PWR Meter

I'm getting down to the end of the sBitx build.  The last two functional modules are the RTC and the SWR/PWR meter.  Both are attached to the sBitx bitbang I2C bus.  What is a bitbang bus and why are we using it?

In Farhan, VU2ESE's original build of the sBitx he found that the WM8731 codec chip and the SI5351 clock generator did not play well with each other on the same I2C bus.  (I2C is a ubiquitous communications bus for microcontrollers and peripherals - its how the Raspberry Pi controls thes other modules in the sBitx).  Farhan decided to implement a second I2C bus using alternate GPIO pins on the Pi - and for that he needed a software implementaion of the I2C protrocol - that is the I2C bitbang.  He left the codec on the dedicated hardware bus and put the SI5351 and later the Real Time Clock and SWR/PWR meter modules on the bitbang bus.

The RTC is an Adafruit module that is a battery backed up clock for the Pi.  So if you take your RTC-enabled sBitx to the field with no internet you can be confident that the time will still be correct and your FT8 QSOs will work.  The SWR/PWR meter consists of a Stockton Bridge directional coupler and an ATTINY85 microcontroller to sample the forward and reflected power from the bridge and send it on the the Pi for processing. 

The picture at the top of the post is the board with the RTC and ATTINY95 sitting next to the directional coupler - just prior to wiring it all up.   The RTC meter worked perfectly on first power up  -so far, so good.  

Here is how the PWR and SWR meter sampling works.  On transmit, the main sbitx program running on the Pi sends an I2C command to the ATTINY85 telling it to sample the forward and reflected power pads on the coupler and send the results back to the Pi.  The ADC on the ATTINY85 should return values from 0-1023 which linearly represent 0 - 3.3V DC.  

TALE OF WOE BEGINS HERE

Directional Coupler - PWR/SWR Bridge 

ATTINY85 PWR/
SWR Sensor

To test, I put the sBitx in CW mode at max power on 40 meters which should be approximately 20 watts into a dummy load so the SWR should be 1:1.   I keyed down and the power meter on the sBitx read 0 and the SWR read 1:1.  Not good.  I also had an external analog power meter in the circuit and it dutifully swung up to 20 watts.  So power out is ok, but the sBitx power sensor is not working.

First question was - is the directional coupler working?   The Stockton bridge samples the forward and reflected power and rectifies to a DC voltage from 0 to about 3 volts.  I put a DC power meter on the FWD pad and keyed down - the result was 1.5V DC.  Check!  For good measure the REF pad measured about 120 millivolts - so pretty close to 1:1 as expected.

Next I add print statements to the sBitx code to print out the values received from the ATTINY85.  This would tell  me two things.  First - was the ATTINY85 sending back anything - and what was it sending back.  I keyed down while observing the console window - yes the ATTINY85 was sending back data on transmit, but instead of being 0-1023 the value was always -1 (all 1s binary).  

Next question - is the problem in the ATTINY85 ADC or the I2C exchange.  So to do that I made changes to the code in the ATTINY85 to send hard-coded values.  That takes the ADC out of the equation.

On receipt of a an I2C command from the Pi an interrupt fires and the ATTINY85 samples the FWD and REF pads of the coupler and sends the data back to the Pi.  The code is brief enough that I can include the whole sketch right here:  

#include <Wire.h>

int16_t fwd, ref;

byte message[4];

// function that executes whenever data is requested by master

// this function is registered as an event, see setup()

void requestEvent() {

  fwd = analogRead(A2);

  ref = analogRead(A3);

  message[0] = fwd & 0xff;

  message[1] = fwd >> 8;

  message[2] = ref & 0xff;

  message[3] = ref >> 8;

  Wire.write(message, 4); // 4 bytes message with fwd and ref

}

void setup() {

  Wire.begin(8);                // join i2c bus with address #8

  Wire.onRequest(requestEvent); // register event

}

void loop() {

}

So for my next test I hard-coded vfw and vref, programmed a new ATTINY85 and tried again.  No change - still receiving only -1.

Next step - is the problem on the ATTINY85 side or the Raspberry Pi side.  To figure this out I wanted to look at the I2C packets as they traversed the bus.  Fortuitously my Rigol Oscillocpe will decode digital signals including I2C.  I hooked probe channel one up to the Clock line (SCL) and channel two up to the Data line (SDA) and after some fiddling and a couple of Youtube videos I managed to capture a single message going from the ATTINY85 to the Pi.

I2C Decode on Rigol DS1202 Scope

Just an aside. This is super cool! I can see the whole protocol, the ones and zeros and acks and nacks. And the Rigol even decodes it for me.  Its such a pleasure to work with good test tools - and at such reasonable prices.  Just a few years ago protocol analyzers ran into the thousands of dollars. Moderns digital scopes and tools like the NanoVNA and TinySA open up a whole new world to homebrew radio enthusiasts.

So hardware probe results confirm the software testg - the ATTINY85 is sending back on all 1s is the data frame.  

So that is where the tale of woe stops at the point. I am certain at this time that this is a software problem in one of the ATTINY85 Ardunio libraries,  I've done some internet sleuthing and found that others have had this identical problem going back almost 10 years.   I suspect the issue is with either the board manager I loaded for the ATTINY85 or a bad version of the WIRE (I2C) protocol code for the ATTINY85.  Like much in the open-source Arduino world there are several competing libraries with either identical or very nearly identical names and there is rarely a definitive version. There are multiple board definitions for the ATTINY85 and multiple versions of the WIRE protocol.  Its not clear which one Farhan used in the original build (he's checking), but I am pretty sure I have the wrong one.  

Away from the bench for a few days - but in the meantime if you have any suggestions, leave them in the comments below.

73 from St. Michaels, MD on the Eastern Shore of the Chesapeake Bay,

Dean

KK4DAS

St. Michaels Harbor

Homebrew sBitx Walkthrough

 

Here is a video walkthrough and quick homebrew sBitx update.  I've intergrated the PA into the rig and calibrated the Tx levels.  I am very pleased to get 20 clean watts CW out on all bands from 80-10.  All modes are working well.  Final calibration meant deriving all my own band settings and updating the sBitx hardware configuration file hw_settings.ini.  Since my analog build is unique I had to start from scratch with the configuration.  I tested transmit on each band through the LPF and check the signal quality and strengthh on the scope.

When I first installed the PA it worked great on 80 and 40 but on the higher bands the relays were chattering and buzzing.   I added .1uF caps to the DC lines and that fixed the problem on all bands except 10 meters.  Farhan suggested adding bypass caps to the GPIO lines that drive the transistors that control the relays.  As N2CQR says - "Bob was my uncle!"  That fixed it.  

The radio sounds great and I have been getting good audio reports.   Had a QSO yesterday with PV8AL,Helio in Brazil on 17 meters SSB.  I got a 5:7 signal report plus good audio.  If you've ever worked Helio you know you can sometimes here a rooster crowing in the background.  Not so this time.

Its scary - I'm running out of bugs and problems.  I better be careful what I wish for.

73,

Dean

KK4DAS

Homebrew SBITX - Tx Modules PA, LPF and Mic

Much progress since the last post. After resolving my ground-bounce and hallucinations with wisdom, I moved on to the transmitter which consisted of a 5 watt power amplifier, straight from the UBITX 40 module.  Thanks to Bill, N2CQR for the recommendation and his excellent Manhattan layout.

The three stage PA starts with a 2n3904 pre-driver followed by a 2N2219A driver with a heatsink cap and the fnal is an RD0HHF1 biased at around 6.5V - which puts it squarely in class A.  Bill's build and other bias more for class AB which I may experiment with later.

Next I needed a microphone and microphone amplifier.  I built the microphone amplifier straight from the SBITX schematic and built a homebrew electret mic.

The homebrew electret capsule in a 3D printed case  The PTT is a SPDT microswitch.  When I first hooked it up the mic did not seem very sensitive - almost had to shout to see output on the scope. A little googling and noodling and I figured it out.   In brief - when I finalized the mic amp build I mistakenly powered it from the 5V rail instead of the 12V rail, that accounted for most of the low output. By then I had read through the long groups.io thread from last year on electret bias and the recent thread on SSB audio quality and had ordered a ag of the -25dB capsules that Gordon recommended.  They accept VCC of 2-10 V through a 2.2K resistor.  So replaced the R21 10K biase resistor with 2.2K to the 5V rail.  With that setup the mic is working great.

With the mic amp and PA done I began work on the LPF module. The module consists of 4 filters covering the 80 meter to 10 meter bands.  In the original SBITX it is a diode switched module. I spent some time understanding the diode switching but decided instead to use relays - it takes away a lot of the complexity of the module.

Next lesson learned: when I first built the LPF board I had the relays only on the input and had tied all of the outputs together.  This created all kinds of problems with the filters interferring with each other even with their input switched out of the circuit.  Solution and best practice is to switch both the inputs and outputs and ground the unselected filters.  Problem solved.  But not out of the woods - still too much loss in the filter module.  I put that asisde for now and moved on to fixing the frequency alignment. 

I had previously tried to align the frequency (so that the displayed frequency matches the actual Tx or Rx freqency without success.  The alignment is done by creating a compensation offset for the 25MHz clock of the SI5351.   Its usually straight forward. Inject a known 10MHz signal into the receiver, zero beat and note the frequency display.  The delta between 10MHz and the frequency display can be used to derive the actual crystal value.  Pop that into the code and the the transceiver should be aligned.  The problem was it was aligned at 10MHz but off at 5 and 20.   After much testing and head scratching and with an assist from Farhan himself we discovered the problem was that the MIKROE WM8731 prototype board uses a different clock rate than the Linux driver in the Raspberry Pi.  In brief, the driver expects the codec to use the standard USB clock rate of 12MHz but the MIKROE board uses the more standard audio processing frequency of 12.288MHz.   The fix is to replace the 12.288MHz crystal on the MIKROE board wiht a 12MHz crystal.  Like so:

The surface mount crystal did not come off the board cleanly - in fact it lifed one of the tracks clean off the board.  So I put in a field expedient patch - the red enamled wired to replace the damaged trace.  To my delight it worked straight away.  I used blue painters tape to mask off the adjacent pins and that made the sodering to the chip much easier.

The results is very satisfying -  here is the first loggable QSO I had after completing the rig:

I'm very pleased with the way the rig is working.  I have a few kinks to iron out but we are appoaching the finish line.

73 from Geat Falls

Dean, KK4DAS

Homebrew SBITX Receiver - Ground Bounce, Hallucinations and Wisdom

 

After a long hiatus I am back at the blog.  I have a number of projects that I have been working on that I will share going forward, but today I want to talk about my latest project - a homebrew version of the SBITX transceiver designed by Ashhar Farhan.  The SBITX is a hybrid analogue superhet transceiver / software defined radio.   The analogue portion of my build is based on the Furlough 40 / SimpleSSB that I built in 2020.  The SDR software runs on a Raspberry Pi 3 or 4 with software written by Farhan.    I'll explain the title of the post before we are done today but first lets take a look at the KK4DAS SBITX.

Here is a demonstration made shortly after I completed the receiver:

For a quick overview of how it works, lets look at the block diagram.

Beginning with the antenna on the upper right let's follow the received signal path.  First we pass through a single 30 MHz low pass filter which passes the entire HF band.  We amplify the incoming signal with a broad band RF amplifier and then pass it through and ADE-1 mixer to mix the signal up the the 40MHz IF.  The LO and BFO clocks are provided by an SI-5351 PLL controlled by the Raspberry Pi.  The homebrew 40MHz crystal ladder filter is 25KHz wide which controls how much of the spectrum you can see on the waterfall display at any one time. The bidirectional IF board I am using is the first board I built for the SimpleSSB at the beginning of 2020.  I have replaced the 9MHz commercial filter with my homebrew 40MHz filter.  The second mixer then drops the signal to a 24Khz IF which is well within the range of the ADC in the codec board. From the second mixer we go through a low noise amplifier to boost the signal and pass it in to  the left line-input channel of the codec. The 24KHz signal is digitized in the codec and passed on to the Raspberry Pi where further signal conditioning and filtering occurs, the waterfall display is generated and the digital audio is extracted.  The digital audio is sent back to the codec where the digital to analog conversion occurs and the analog signal is sent out the left line output to headphones or to an amplified speaker.   The SBITX software also supports FT8, RTTY and CW decoding natively - no additional software or computer is needed.  For a detailed description of the SBITX you should read Farhan's SBITX description linked above.  Transmit will work much the same but in reverse.  I'll cover that when I get the transmitter implemented.

I'm very happy with how the receiver is performing. Its fun to listen to and sounds great.  But getting to this point has not been without a few stumbles and sidetracks.  I was honored to be included as a guest on the SolderSmoke Podcast Episode #250 with N2CQR, Bill Meara and N6QW, Pete Juliano where I shared my tales of woe - a few of which I will describe here in more detail and a others which I will save for another day. 

Ground Bounce - shortly after completing the receiver I made the unsettling discovery that signals that were being transmitted on 20 meters were being received on 20 meters but also at exactly half the frequency on 40 meters. This was not good - it seemed that it had to be strange mixing products in the first mixer, but I had tested the entire IF before hooking it up to the digital board - and this very same IF board was pulled from a working receiver. I looked at the output of both mixers and I couldn't see how the the 20 meter signal was leaking in on 40.  After thinking about it for a bit I decided to look at the SI5351 outputs on my TinySA Ultra spectrum analyzer and instead of seeing one clean signal on each of the LO and BFO clocks I saw both signals on both clocks.  This was clearly the source of my problem.  Skipping over a day or two of troubleshooting I sent a note to Farhan and he immediately identified the problem.  It was "ground bounce.  Apparently if the clock outputs are not properly grounded it causes current to rise internal the SI-5351 and signals to bleed between the clocks.   In following a separate piece of advice from Farhan on buildiing the digital board I had very carefully insured that there was one and only one ground connection in the digital board and that was directly back to the main DC input.  I had installed the SI-5351 directly onto the digital board and it shared that common ground.  But that meant that I couldn't also ground both ends of the coax shield between the SI-5351 and the mixers.  That was the cause of the ground bounce.  The solution was to remove the SI-5351 from the digital circuit and put it on the analog circuit - with the only connection between the SI-5351 and the Raspberry Pi were the two I2C control lines.  And also to ground the coax connecting the SI-5351 and the mixers at both ends.  That fixed it - the ground is no longer bouncing!

Hallucination - after curing the ground bounce I spent an evening listening to the rig enjoying the glow you get after fixing a thorny problem.  But my enjoyment was short-lived.  I noticed that from time-to-time that the waterfall display would go a little crazy,.  It appeared as if the the receive signal was being duplicated all up and down the band somewhere internal to the SBITX.   It looked like this:

The signal at the center is the received signal - all of the mirror images are false.  Those are the hallucinations.  Farhan identified that fairly quickly and let me know about a software fix in the SBITX 3.2  which led me to:

Wisdom - I don’t have a complete understanding, but the hallucinations are artifacts created during the Fast Fourier Transform of the received signal under certain circumstances.  The SBITX uses the open-source FFTW (Fastest Fourier Transform in the West) library.  There is an extension to the FFTW library called FFTW-Wisdom that is used tune the FFT algorithm the first time it is used.   The tuning  parameters are saved in what is known as an FFT “Wisdom” file .  The Wisdom file, which only has to be computed one time, contains saved information about how to optimally compute Fourier transforms of various sizes. The FFTW Wisdom File man page has more details.  That was what was implemented in SBITX V3.2 which eliminated the hallucinations.

I'm still chasing a few problems in the receiver.  Top of my list is a tuning problem. When I zero beat WWV on exactly 10 MHz, the displayed frequency on the SBITX is a few hundred Hz off of 10MHz, and when I tune to 15MHz WWV the display is off by a different amount. So, the delta between the displayed frequency and the frequency the radio is receiving changes with frequency – but not in any linear way.  I’ve tried several different ways to align the radio but have not yet been successful  The last thing I did was disconnect the analog receiver entirely from the SDR and used a signal generator to put a fixed 24KHz signal into the audio codec which should result in a signal displayed dead center on the waterfall – but it did not – it is a few hundred Hz off.  the current suspicion is that the crystal on my WM8731 protottype board is out of spec.  Farhan has offered to send me one of the codec boards he produced for the early SBITX prototype.  When that arrives, I will replace my audio codec with the one he sends.  That should resolve this last issue but it still doesn’t explain why the delta moves with HF frequency.  That’s what was puzzling me and what I was referring to on the podcast. 

That's it for now,

73 from Great Falls

Dean

KK4DAS