The ZL1BPU Mini-New Exciter (and the bench-top NEXC model from which it is derived) replace a less capable unit designed 18 years ago, which was for LF/MF use only. (NEXC stands for New EXCiter). This new unit (pictured on the right) is based on the Silicon Labs Si5351 synthesiser chip. It is highly flexible, and will operate from 2.5 kHz to 200MHz. It uses either a TCXO or a special low-power OCXO to stabilise the reference in the synthesiser module. Because the oscillator is external to the module, the stability and phase noise are considerably enhanced.
The design is based on two popular items: the Arduino UNO micro-controller, and the Adafruit Si5351 synthesiser module. These devices are multi-sourced, inexpensive, and the combination make construction relatively simple. The power supply is 5 V DC, either powered by a computer, a phone charger, or an 18650 single-cell LiPo battery bank, as typically used to charge cellular phones. In the Mini-NEXC configuration, the display is a serial type, for minimum wiring.
The picture below shows how the basic modules are assembled. Note that there is no circuit board, just the modules.
The bare bones of the working Mini-NEXC: nothing to it!The Arduino Nano processor board (top left) is powered by USB, and in turn powers the Synthesiser module (top right) and the display (centre right). The collection of parts at the bottom are the various switches. This photo shows the complete assembly (apart from the RF outputs) prior to fitting them in the box.
The design has been conceived in three steps:The picture above shows the 'Advanced' version without the case. The parts list below refers to the 'Advanced' version. The same Arduino code works for all three options.
- 'Minimal' - simple, quick, PC controlled - and only four wires!
- 'Improved' - adds display and knobs, independent of PC.
- 'Advanced' - external reference, significantly improved stability, low noise.
Only the major items are listed. You will also need a knob, possibly a trim pot (to trim the VCXO or TCXO), some recycled ribbon cable and various nuts and bolts etc.Most parts for the prototype were from the junk-box or were purchased from Banggood, Aliexpress or Jaycar. The four pillars were made to fit from brass stock using a lathe, but plastic tubing and long bolts will work just as well. The Si5351 module is housed in a small cardboard box to reduce convection-induced instability.
- Arduino Nano V3 or near clone. Must have the ATmega328 processor and USB port.
- Si5351 module. Adafruit (or a clone) preferred.
- 20 to 26 MHz TCXO or OCXO (if low noise and stability are important).
- 16 x 2 serial (I2C) LCD display with 'piggy-back'.
- Miniature Rotary Encoder with push-button, Jaycar SR1230.
- Two push-button switches.
- Three panel-mount BNC female connectors.
- Blank PCB copper laminate 140 x 70 mm (as chassis).
- Four plastic or brass pillars ~ 40 mm long.
- Plastic case with transparent lid 160 x 90 x 60 mm (Banggood).
- Coloured card for front panel underlay.
- USB type A to B cable.
- USB 5 V 1 A phone charger, The Warehouse item # 9401056118601.
Take a look at the annotated photo above, which clearly shows how to connect things up, using recycled ribbon cable. Most of the leads can be about 150 mm long, although length is unimportant. The drawings below show where everything goes. These are the first steps to assembly, and they are really easy. By the way, these are the same steps that take the project from 'Minimal' to 'Improved', and then 'Advanced', and are numbered the same way.1. Connect the Synthesizer
Yes, just four wires to make a working synthesizer! Use a length of recycled ribbon cable, stripped to provide a four-way cable. You can identify the individual wires with a felt-tip pen. If you've not done so yet, now is the time to connect up the USB cable, fire up the the Arduino IDE, and program the device. If you're not familiar with Arduino programming, ask a friend for help.
2a. Adding the Display
Because the display is a serial type, connection is really easy. In the drawing above, you see the back view of the display, since it is the piggy-back board you will be connecting to. Again, just use a four-way length of ribbon cable. Be aware that the display shares the same pins on the Arduino board as the synthesizer.
2b. Adding the Switches
Look at the drawing above. Things couldn't be simpler. Use individual pairs of ribbon cable wires to each switch. This is clearly illustrated in the annotated photo further up the page. You might like to use the push-button on the Rotary Encoder to perform the RES function, which will save one switch. You can then use a three- or four-way ribbon cable to the Rotary Encoder.
3. Adding a TCXO or OCXO
The crystal and oscillator on the popular Si5351 modules are notoriously noisy (phase noise), and in addition rarely provide better than 5 ppm stability. This can be seen in the graph on the right, which shows the typical behaviour of an Adafruit module over the temperature range 0°C to 90 °C. Over an ambient temperature range, say 10 °C to 25 °C, the oscillator moves down in frequency by 3 ppm (30 Hz at 10 MHz!).These are the compelling reasons for replacing the crystal with something better. With an external OCXO, you can achieve phase noise too low to measure without specialist equipment (well under 1 ppb p-p at 30 dBc), and achieve stability of the order of 1 ppb over the usual ambient temperature range.
This is probably the trickiest bit of the project, as you have to remove a really tiny part from the Si5351 module. The orange arrow in the drawing above identifies the crystal, which will be a small rectangular ceramic unit, perhaps with a metal top, more likely just as shown in the diagram. This is best removed using a hot air tool (or a heat gun with a very small nozzle), but it can be done with care and patience using nothing more than a SMALL soldering iron. Make very sure you don't damage the tracks and pads underneath the crystal, or heat other components so they fall off.
Mount the oscillator on the module (or vice versa) with double-sided foam tape, and keep the RF lead short. Connect the series capacitor to the pad shown, which leads to pin 2 of the chip. For sine-wave devices such as an OCXO or some TCXOs, use just a series capacitor. If your device has 5V TTL or CMOS output, add a pair of 200 Ohm resistors as an attenuator before the capacitor. Connect the ground pin of the oscillator to ground on the module. A 3.3V CMOS device can be connected directly, without the capacitor.
The drawing does not show the power connection to the oscillator, as this depends on the type used. If it is a 3.3 V TCXO or Triton device, find the output of the 3.3 V regulator on the Si5351 module, and connect it to that. In this drawing, the regulator is the 5-pin chip at the bottom of the module.
If the device is a 5 V TCXO or XO, connect the power connection to the VIN pin of the module. If the device is a conventional OCXO, you will need to provide a separate regulator to power the device.
Ideally the TCXO or OCXO should be between 10 MHz and 30 MHz, although the actual frequency is completely unimportant, as the nominal frequency will be be set in the header file before assembly. Further, if the TCXO or OCXO is not adjustable, any offset from the nominal frequency can be corrected using the Calibration command once the project is complete.
You are now in a position to apply power and check out the unit, but check each connection carefully first!
Make yourself familiar with programming devices with the Arduino IDE. The source code and the necessary libraries are provided in this archive in a separate folder. That's about all you need to know if you plan to use an unmodified Si5351 module. If, however, you plan to use an OCXO or TCXO on a different frequency, you will need to patch one line in the library si5351.h definition file. It's the second #define, right at the beginning of the file:The default value is 25 MHz, as the standard module uses a 25 MHz crystal. Enter the nominal value (in Hz) for your device, and save the file. Then compile and program the device. When setting up the Arduino IDE, set the 'board' type to 'Arduino Nano', and the 'processor' to either 'ATMega328P' (if you have a recent genuine Nano) or to 'ATmega328P (Old Bootloader) if using an older Nano or clone device. This is because there are two bootloaders, and they work differently./* Define definitions */ #define SI5351_BUS_BASE_ADDR 0x60 #define SI5351_XTAL_FREQ 25000000 <====== This Line #define SI5351_PLL_FIXED 80000000000ULL #define SI5351_FREQ_MULT 100ULL #define SI5351_DEFAULT_CLK 1000000000ULLNow is the time to program and check out your unit before final assembly.
The picture below gives a good idea of how the unit is assembled. This is a photo of the completed unit, but without the cardboard front panel underlay.
The finished unit without the front panel underlayThe modules are assembled onto the base plate, which can conveniently be a piece of unetched copper laminate. This is pre-drilled so you can mount it on the studs in the bottom of the box. The various modules are wired together as described above, and then secured to the base plate. The Arduino module is right at the back, with its USB port extended so it will just clear the back of the box on assembly. The Nano has very small holes in the corners. I secured it by soldering four small picture framing pins to the base plate, adding short plastic tube spacers, and then after mounting the Nano, adding short lengths of tubing to hold the board down. Old ball-point pen refills are about the right size. You could use heat-shrink tubing (but don't fry the Nano!).
The Si5351 module is mounted on the right, near the BNC connectors. It requires 2.5 mm screws, and if you can't find any, use the same technique used for the Nano. Use very short connections to the BNC connectors. Just use flexible wire - you won't need screened cables. Glue a small cardboard box (matchbox inner) over the synthesiser with hot-melt glue. Secure the frequency trim pot on top of the box the same way.
Once the sub-assembly is complete, carefully measure where the hole for the USB connector (at the back) will be, and drill its hole. Start with 3 mm, and enlarge it with a file by fitting the board and checking the hole. The final hole size will need to be about 9 x 4 mm, preferably oval in shape. Drill all the other holes. Mount the subassembly and secure it to the fixing bosses in the bottom of the box with short self-tapping screws. Connect up the switches, and mount the BNC connectors. Finally, wire the BNC connectors.
The major advantage of a transparent box is that you don't have the problems of making a rectangular hole for the display, which is never easy to do. However, you will have to make pillars of just the right height to lift the display so that it is just inside the lid, and clear by at most 1 mm.
Finally draw (on the computer) the underlay to fit your box and display. Arrange the sides to fold down 15 mm (will depend on your box), with cut-outs in the corners to allow for the corner posts. Measure and draw really accurately. Print the drawing onto coloured card. Cut the outline for the display with a very sharp craft knife, then cut the rest of the card to size. Fold the edges of the card accurately using a rule, and secure the card inside the lid with tiny pieces of double-sided tape on the sides only - not on the top.
Top view of assembled unit
Completed unit, top view