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Project Yamhill Front Panel PCB
...and the kitchen sink
Wow, completing the first pass of this board took longer than I thought it would. The routing was more difficult than I anticipated and multiple times after I thought I had finalized the schematic, I realized that I had either made an error or left out something. Also, it didn’t help that I’m out of practice in designing more complex projects like this. Regardless, this is by far the most complex and time-consuming part of this project, so future modules will not take nearly as long.
The Project Yamhill Front Panel board contains a lot of functionality, in order to turn a handful of radio block modules into a complete radio system. In addition to all of the user interface components and the microcontroller brains, the board contains subsystems that are common to nearly every radio project: a stable oscillator system and an audio amplifier system. Integrating these systems into the front panel board allows the experimentation focus to remain on the RF sections, as it is unlikely that one would need to try different oscillator or audio systems if the ones provided on the front panel are good enough.
In the above renderings, there are quite a few of the components missing a 3D model. I could have spent more time trying to fish up or create these models, but that doesn’t do anything more to advance the project, so you’ll have to imagine them. Briefly, the components missing on the front are the 4.0” LCD, rotary encoders to the right of the display, the 8-pin mic jack below the display, and the audio potentiometer to the right of the mic jack.
Images of the schematics are available in this post, but you can visit the official GitHub repository in order to download a PDF of the schematic to peruse in better quality. With that introduction out of the way, let’s briefly look at each of the major systems on the front panel board.
The heart of the front panel PCB is the Raspberry Pi Pico, a system on a small carrier board much like the Arduino Nano or the Teensy. They are very inexpensive and quite powerful for the price, making them hard to beat in this application. The board comes with castellated edge pins, making installation onto the front panel PCB very easy. Application programming can be done in C/C++, MicroPython, or the Arduino IDE. For official applications from Etherkit, coding will be done in Arduino, giving access to many pre-existing code libraries.
The main display is a 4-inch, 480X320 pixel resolution ST7769S LCD module with a touchscreen and fast SPI interface. The resolution will allow for advanced UIs to be created and for things like spectrum and waterfall displays to be rendered. The availability of touchscreen capability reduces the need for a lot of physical buttons if a complex application is created.
The other user interface components are two rotary encoders, two WS2812B RGB LEDs, four tactile pushbuttons, and four mono-color LEDs. Between the LCD display and these components, there should be the capacity to handle just about any radio application.
Radio systems more complex than the most simple projects typically need a variety of I/O lines for interfacing with the microcontroller. All of these lines on the front panel board are broken out into 0.1-inch pin headers, to be configured as projects require.
TRANSMIT and MUTE lines from the Raspberry Pi Pico are broken out into multiple pins for connection to various block modules. A TCAL6416 I/O expander IC is used to give extra I/O for buttons and LEDs to the Raspberry Pi Pico, and all of its unused pins are routed to a pin header for use by the builder.
An external ADC chip (ADS1115IDGS) is also provided for builder use, for relatively slow sample rate/high resolution analog measurements, such as SWR measurement. A total of four channels are available to the builder.
A 3.5 mm TRRS jack is available on the front panel for CW keying use, if needed.
Every radio (with the exception of a simple crystal radio) needs a stable and accurate oscillator system. I’ve chosen the well-known Si5351C clock oscillator chip with a TCXO reference oscillator for this application. Three of the CLK outputs of the Si5351C are brought out to SMA jacks on the rear side of the board for use as a LO or BFO. A couple of additional CLK outputs are available on a pin header. For ultra-stable oscillator applications, the REF_IN pin of the Si5351C is routed to a SMA jack, to allow the builder to connect a reference clock such as a 10 MHz GPS disciplined oscillator.
The two main sections of the audio system are the audio output amplifier and the microphone amplifier. The mic amplifier is designed to accommodate a Yaesu hand mic with an 8-pin modular connector like the MH-31ABJ. The amp has a fixed 20 dB of gain and an adjustable mic gain control. A simple Sallen-Key low-pass filter conditions the output of the mic amp before it is brought out to a pin header. The mic amplifier output is also routed to an ADC input on the Raspberry Pi Pico for future SDR applications.
The audio output amplifier has two stages: a 40 dB preamp with muting and a 20 dB, 750 mW audio power amplifier with speaker and headphones capability. While some additional AF amplification may be necessary for some applications (such as a direct conversion receiver), this should be a sufficient amount of amplification for many radio receivers.
A pin header is also provided for an I/Q input for future SDR applications. The two pins are connected to high-speed ADC pins on the Raspberry Pi Pico.
I’ll need a basic radio project that can exercise a lot of the capabilities of the front panel to use as an inaugural project, so I’m thinking that a classic direct conversion receiver would be a good start. In order to implement one, I’ll need to create at least three more modules: a RF mixer, another AF audio gain stage, and some front-end filtering.
Specifically, I’m thinking that ye olde Double-Balanced Diode Ring Mixer would be a good choice for the mixer, and that I’m going to try a relatively new W7ZOI audio amp stage designed with a 50 ohm input impedance, specifically designed to interface with the diode DBM. The front end will consist of a classic double-tuned circuit bandpass filter. I believe I can get two on a board, with jumpers to select which one is in the signal path.
Stay tuned, further public updates should be happening much more regularly again!
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