Digital VFO & RF Oscillator Module
"For The G6LBQ Irwell HF Transceiver"
In
this blog post I present my digital VFO and RF Oscillator module for the Irwell HF Transceiver project which is based around the Silicon Labs CMOS SI5351 Clock Generator.
Before we begin I would like to pay homage to an era in my life which has flourished, brought great happiness, prosperity and achievement.
I make the aforementioned statement with a sense of humbleness and I also feel privileged to have experienced great wealth in the recent flourishing times.
If all this has got you thinking I have won the national lottery or been a beneficiary to a nice windfall you are excused and forgiven :) Truth is I have only ever been good at making other people wealthy and said people choose to forget what the catalyst was to there wealth status!
I digress a little as I sailed off course for a moment there but suffice to say I am very happy with my modest lifestyle.
The era for which I pay homage to is no less than the golden age of ham radio and home-brew construction. If you have been licensed as a radio amateur in the last decade congratulations I award you the "Golden Ticket" for ham radio.
So now I bet you are all wondering what the "Golden Ticket" is all about and what the benefits of being awarded one are?
In short the "Golden Ticket" is mere worthless nonsense but it sure looks good and hopefully the concept held your attention for a while!
Whilst the "Golden Ticket" is fantasy the "Golden Age of Ham Radio" is not, the last decade has delivered an explosion of technology and an endless wealth of knowledge and information that we can all utilize to prosper in our wonderful hobby.
The internet has transformed amateur radio with digital technologies that link the world together and provide new options in the way we communicate. The evolving marvel of the world wide web provides a platform in which we can search out information and share data with all corners of the globe. Gone are the days in which I made frequent trips to the local library in search of inspiration and ideas for my next project!
The internet opened up the flood gates for online shopping whilst inadvertently contributing to the demise of the high street shops. Retail outlets have experienced a steady decline in the last decade with sales dropping and retail outlets closing, meanwhile online sales continue to grow and reports show a staggering 36% increase in sales for the year 2020. I personally welcome online shopping and with the outbreak of Covid it has been a godsend.
For the radio amateur online shopping has never been better and we have the Chinese and the likes of Ebay and Ali-Express to thank for that. The auction sites sell components and modules at ridiculously low prices and often at a fraction of the cost of the major component distributors.
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| Modules purchased off Ebay & AliExpress |
A lot of the cheap modules that are available seem to have been designed and manufactured with the radio ham in mind, you can purchase modules for RF amplifiers, IF amplifiers, mixers, frequency generators, attenuators and audio amplifiers, you can literally build a radio from a bunch of these modules.
Some of the modules require a micro-controller and software to operate but fear not you do not have to be a software engineer with a degree in computer science to use them. Most modules can be controlled using cheap Arduino micro boards that are readily available from the auction websites for as little as £2 or less.
For the software required to program the micro-controller and operate the modules Google is your friend, just perform a relevant search and you will likely find many examples of code created and shared by others.
So to summaries we have a wealth of knowledge at our finger tips, cheap components, ready made modules and the ability to stand on the shoulders of those with greater knowledge.
Now without further ado lets get back to the digital VFO and RF oscillator that will control and generate frequencies for the Irwell HF Transceiver. I am not a software engineer or computer programmer so for this project I will be standing on the shoulders of fellow ham's JA2NKD and JA2GQP.
Whilst searching on the internet for a suitable SI5351 VFO that I could adapt for my own transceiver project I came across the fantastic blog site of Ryuunosuke Matsuura JA2NKD, his blog is a treasure trove of home-brew delights and well worth visiting.
On his blog Ryuunosuke posted details of a Multi-Band VFO he had created using an AD9850 DDS module controlled by an Arduino Nano micro-controller, this was also interfaced to a 2.2 inch colour TFT screen displaying a very nice user interface.
In a follow up blog post details emerged of a newer version of the VFO using an SI5351 instead of the DDS module, a fellow ham JA2GQP helped to make this possible with his work on producing a compact version of the SI5351 library file.
JA2GQP also has his own blog site which contains many interesting articles and projects, these can be viewed at the following web link:
JA2GOP made a post on his own blog which in essence details the porting of JA2NKD's aforementioned VFO onto a much faster STM32 micro-controller board AKA Blue Pill. The board is similar in size to the Arduino Nano but that is where the similarities end, the STM32 packs a serious punch when compared to the Arduino Nano but I will not cover those specifics on the blog.
Here is an image showing the Arduino Nano on the left and the STM32 Blue Pill on the right and as you can see they look similar.
I express my sincere thanks to JA2NKD and JA2GQP for their generosity in sharing the work they have done and their software code, this has given me a sound base to work with and one I can develop further for my own transceiver project.
My digital VFO and RF oscillator is based on not one but three SI5351 clock generator IC's, this may appear somewhat crazy when one SI5351 can generate three clock signals on its own but I can assure you I have not lost the plot :)
The reason why I have chosen to use three of the IC's is basically to avoid crosstalk which is apparent when using a single device to create multiple clock signals.
The SI5351 version that I will be using comes in a 10-Pin MSOP type package that measures 3mm by 3mm so the silicon substrate is very small. With the IC's small package size it stands with reason that crosstalk on the clock outputs is going to be inevitable, there are some other factors that can produce crosstalk such as poor PCB layout, impedance miss-match and inadequate decoupling but these are areas that can be controlled by the end user whilst the actual IC is a defined device set by the manufacturing process.
I have made no attempt to measure the SI5351 crosstalk as it seems pointless when I know using an individual IC per clock will eradicate the problem however reports by others who have carried out tests suggest that crosstalk leakage is in the order of -50 to -60db down. It is difficult to trust these figures as actual measurements will vary wildly depending on how well the clock outputs are isolated from each other when they leave the IC package!
Having decided on using individual Si5351's for each of the clock signals It soon became apparent that there was a problem that had to be dealt with but in order to understand the problem let me first offer you a quick synopsis on the SI5351's mode of communication.
The SI5351 uses the I2C protocol to communicate data, this is a two wire bus that can be connected to many individual devices as illustrated below.
Unfortunately things in life are not always so straight forward, imagine if three houses on the same street all had the same house number the postman would undoubtedly have a problem!
For me this scenario has turned out to be the case with the SI5351 IC, the manufacturers make them all with the same fixed hex address so to coin a phrases "Houston we have a problem"
Fortunately I have an electronics background so I instinctively thought about the possibility of multiplexing or muxing the I2C bus, the technology is commonly used to carry multiple telephone calls on a single line or data channel so I figured it may be the answer to my SI5351 dilemma.
After a short time carrying out some internet searches I soon learned that most of the major semiconductor manufacturers produce dedicated IC's for I2C multiplexing and low and behold the Chinese produce "Multiplexer Breakout Boards" that are available on all of the main auction websites and very cheap to purchase!
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| Multiplexer Breakout Board Purchased Off Ebay |
I purchased the above board for just under 2 UK pounds and as can be seen from the PCB screen print it has a TCA9548A Texas Instruments Multiplexer IC though I suspect the chip may be a clone. The device supports up to 8 same-address slave devices and the IC's host hex address is configurable via three of the chips pins, A0, A1 & A2.
Here is a simplified application diagram showing how the TCA9548A will be used in my SI5351 VFO.
Armed with JA2NKD and JA2GQP VFO controller software and a means to connect multiple SI5351's with the same address on the I2C bus it was now time to turn my attention to making some software changes.
The software changes required include adding support for the TCA9548A IC along with suitable instructions to steer the IC to the relevant SI5351 device. This might appear trivial to a software engineer and programmer but as I am neither it will be a case of blundering my way through with some limited self taught knowledge... Bring on the challenge!
For now I have the VFO bread-boarded and in an experimental state so all that remains for me to do is some code tinkering then I will return with a part 2 follow up and hopefully a functioning VFO!
For part 2 of the Digital VFO click here.
Until next time...
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