Part 3 Of A Digital VFO For The G6LBQ Irwell HF Transceiver

For part 1 of the Digital VFO click here.

For part 2 of the Digital VFO click here.

Digital VFO & RF Oscillator Module

"For The G6LBQ Irwell HF Transceiver"

Welcome to part 3 of my blog post series in which I present details of the hardware for my Digital VFO and RF oscillator module. This is a significant building block for the “Irwell HF Transceiver project” and we finally get down to component level, PCB design and a functioning module.

My prototype SI5351 buffer & signal splitter circuit!

 

 

When I set out on my journey to develop and build the “Irwell HF Transceiver” a decision was made to adopt a modular construction approach, my rational being that the transceiver could evolve over time with improvements and upgrades. The modular approach also helps to simplify what is actually a complex project and it paves the way in establishing a useful set of modules that can be utilized in other projects.

When designing and developing the modules a degree of foresight is needed to determine how the various modules will interconnect and as I have discovered failure to get this right can be costly and set the project back.

The digital VFO and oscillator is one such module where foresight is needed as it will be used for both the receiver and transmitter as well as providing control for switching filters, signal paths and more. For now I am concentrating all of my efforts on the receiver modules and will add the transmitter modules later.  

"In effect I am building a multi-mode HF communication receiver that can be upgraded to a transceiver at a later date if desired"

My approach has the benefit of providing me with a high performance receiver that I can listen to and enjoy whilst working on the modules that will later form the transmit side. 

I have chosen to use separate RF mixer circuits for the receiver and transmitter stages as this will simplify signal routing and provide possibilities for split frequency or split band operation. The use of independent RX/TX mixers does however mean that signals from the VFO and oscillator module will either have to be switched between modules or buffered and split. 

Splitting the VFO and oscillator signals will require a total of six buffer circuits, two for the VFO, two for the BFO and two for the 2nd conversion oscillator (for when the receiver gets updated to a dual conversion superhet).  This is the option I have chosen as  it provides a convenient way to control the signal level provided to each of the mixer, de-modulator and modulator circuits.

Some may find my approach a bit excessive and a poor use of resources but as it is not a commercial product I say "to hell with the expense".

For the buffer stages I designed a simple common collector amplifier circuit that is biased directly from the Si5351 clock output. The circuit diagram below shows a pair of the buffer circuits that split the source signal into two paths, RF out 1 and RF out 2.

I used high linearity low noise BFR106 RF transistors for the buffers as they are  current and readily available. The devices are supplied in a surface mount SOT23 package and their specifications make them an ideal device for ham radio projects.

From the above circuit it can be seen that PI resistor attenuators are present in the transistor emitter legs, this establishes an impedance match of 50 ohms to the proceeding stage as well as defining the output signals amplitude.  The PI network resistor values are simply re-configured for each of the buffer circuits so that the correct level of signal is provided at the output and proceeding stage. With the PI values shown the output signal measured on my home-brew digital RF power meter is +7dbm  which is ideal for diode ring mixers.

Using the criteria I have outlined my VFO oscillator module contains three Si5351 clock generators and a total of six RF buffer stages, these can be individually configured to provide a signal level sufficient for driving most RF mixers and modulator/de-modulator circuits. 

Now that the salient points of the VFO and oscillator have been covered we can now move onto the schematic diagrams and hardware. The project has been divided into three sections with a schematic diagram drawn up for each one.

Please note that the schematic diagrams presented on the blog have been compressed to improve page loading times, this does reduces the overall image quality but is a necessary compromise for the web. Higher resolution schematics will be made available to downloaded from my Groups.io community group once I have finished documenting the VFO and oscillator section in part 4.

Here is the schematic diagram for the micro-controller board which also contains the function buttons and a TFT display:

Click to enlarge to full size!

Next is the schematic diagram for the I2C multiplexer, the Si5351 oscillators and the splitter buffer circuits:

Click to enlarge to full size!

Note I have used individual 25 MHz clock crystals for each of the Si5351 devices though it may be more beneficial to use a single precision TCXO oscillator.

And finally here is the schematic diagram for the I/O expander:

Click to enlarge to full size!

Three PCB's have been designed for the VFO and oscillator module that conveniently connect and stack together. My intention is to keep the digital electronics together and away from the analogue RF circuits thus avoiding unwanted interference between the two. The digital electronics will be mounted behind the transceivers front panel in a screened cavity away from everything else.

All of the printed circuit boards are double sided with components mounted on both sides, this helps to reduce the size of the PCB's to a practical size.

 

Top view of Controller PCB measuring 110mm by 75mm

 

Top view of Oscillator PCB measuring 110 by 75mm

 

 

Top view of I/O Expander PCB measuring 110mm by 51mm

 

An order has been placed to get a batch of the PCB's manufactured and I hope to take delivery of these in a few days. Once they arrive I will build a set up and present these in Part 4, my final blog post on the VFO oscillator module.

For part 4 of the Digital VFO click here.

Until next time... 

 

73's From Andy G6LBQ

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