Part 2 Of A Digital VFO For The G6LBQ Irwell HF Transceiver

For part 1 of the Digital VFO click here.

Digital VFO & RF Oscillator Module

"For The G6LBQ Irwell HF Transceiver"

Welcome to part 2 of my blog post series in which I present further details of my Digital VFO and RF oscillator module for the “Irwell HF Transceiver" project. This is a significant building block and is solely responsible for frequency generation and the control of other modules in the project.

Since I posted part 1 on the blog, I have made good progress with the software changes that I outlined. I have added additional commands to JA2GQP’s source code for the I2C Multiplexer and I am now at the stage where I can communicate with two SI5351 devices to produce two chip independent clock signals. In order to achieve this, I have had to make changes to both the main code as well as the SI5351 library file.

For now, I only need to generate two clock signals for my transceiver, one for the VFO and the other for the BFO. This will permit the radio to run as a single conversion superhet in its current state. Once further modules are developed, the receiver will be upgraded to a dual conversion superhet, this will require a third clock signal  for the requisite conversion oscillator.

The additional software code required to support a third SI5351 is going to be a bit more difficult for me to implement. I hope to replicate JA2GQP’s BFO oscillator code for this as it includes a routine for calibrating the BFO frequency and saving it to memory. If I can do this, it should also pave the way for implementing an IF shift control.

For transparency and clarity, I have commented on all of the changes and additions that I have made to JA2GQP’s source code. This will be available to download from my Groups.io community group website once I have finished documenting the VFO & RF Oscillator on the blog!

Joining my group is free. Just click on the button below.

JA2GQP's source code can be obtained from his own download site by clicking on the following web link;

https://drive.google.com/drive/folders/1VpsnNi6JbRnYKEr_JuoT2aHemHWDkQxx

The code additions required to implement the TCA9548 are actually not that complicated, though I did struggle to get it working at first.  I made the mistake of adding my changes to the main software sketch then later realized some of the changes needed to be made to the separate SI5351 library file.

In the main sketch, three lines of code were added to the void setup block, as illustrated below;

Next I made the following changes to the SI5351 library;

First the address of the TCA9548 IC must be defined in software to match the address configured by setting the chips pins A0, A1 & A2. I recommend that you view the manufacture data sheet for the full range of programmable hex addresses.

 

Next the TCA9548 IC is setup and initialized to communicate on the I2C bus using  the Arduino wire library.

Finally the function declaration is made to send data to the relevant TCA9548 slave port and subsequent SI5351 device.

The multiplexer on the I2C bus is working well at controlling the SI5351 ICs and to date I have experienced no anomalies with the VFO so I decided to press on with the software development. I wanted to add further code for controlling the transceivers band-pass & low-pass filter modules. The STM32 "Blue Pill" microcontroller does not have enough spare I/O pins to control the filter modules on its own so I looked at using I/O expander devices to do the job.

 

Research was carried out and I settled on using the popular PCF8574 IC and, like the aforementioned multiplexer IC the PCF8574 uses the same I2C bus for data communication. The chip is available in two variants with each having its own unique set of 8 configurable hardware addresses. This basically means you can have up to 16 of the ICs together on the I2C bus which equates to 128 I/O lines... That's a lot!

The PCF8574 is a popular choice of I/O expander amongst the Arduino community so it was no surprise to discover that our friends in China produce ready-made modules delivered to the UK for under 3 UK pounds.

 

Did I mention😏

I ordered a couple of the Chinese modules to experiment with and here's an image of one of the modules that I received.

During my research of the PCF8574 expander IC, I came across various Arduino libraries written to simplify the job of adding the device's functionality to a sketch, however, I chose not to use them, as they appeared over complicated for my particular application and my programming skills. Instead, I modified the main sketch file by adding the following blocks of code;

First, integers are defined for each of the PCF8574 expander ICs to match the hex addresses configured by the chips A0, A1 & A2 pins. Again I recommend that you view the manufacture data sheet for the full range of programmable hex addresses.

Next the PCF8574 IC's are setup and initialized to communicate on the I2C bus using  the Arduino wire library.

Finally a function declaration is made in the Arduino's void loop that scans a cascading block of;  if, else if  or else  statements. Depending on the frequency of the VFO and where that falls in the frequencies being scanned within the PCF8574 code block data gets sent to the IC to switch on one of the eight I/O pins, thus enabling the appropriate band-pass Filter.

This snippet of code shows the first two filters in the cascading block of eight.

And here is the code for the last two filters in the cascading block of eight.

The same code is recycled for the LowPass filter control using a second PCF8574 IC and further ICs can be added for additional I/O expansion. 

I am sure there are other ways in which the multiplexer and I/O expander ICs can be implemented in software but with the methods outlined, everything works and is proving to be reliable.

 

If you have any constructive comments or suggestions on ways in which my software implementations could be improved, please drop me an email with code examples that I can follow and learn from. You can contact me using the form displayed in the top right-hand corner of each blog page. 

 

There are other changes that I have made to the original software code that I won't cover on the blog but for your interest, these include:

• Added additional mode control outputs
• Added additional frequency steps
• Increased the amount of selectable band segments from 6 to 8

It should be noted that the PCF8574 IC has a total package sink capability of 80 mA; this is not sufficient for driving the relays on the filter modules.  To deal with the current switching demands, I will include a darlington array IC between the PCF outputs and the filter modules.

In part 3 I will be providing details of the VFO hardware along with schematic diagrams.

For part 3 of the Digital VFO, click here.

Until next time... 

 73's From Andy G6LBQ

Its all about the Radio Ga Ga...