The WSPR Beacon Project

The multi-band WSPR beacon project has the following objectives:

• To use the smallest available micro controller
• To support 10,15,20,40,and 80 meter bands
• To have a LCD display to show what the WSPR beacon is doing
• To keep Time synced using GPS information
• Runs using a 5v USB charger

High Level Architecture:

• The Arduino Nano is a small micro controller (32k program flash, 2k of RAM) which is the “brains” of the entire beacon.
• The SI5351A is the primary WSPR signal generator. It generates a square wave at the frequency requested over the I²C bus
• The GPS chip is used exclusively to provide a time reference since the Nano can’t connect to the internet. At some point in the future it may be able to provide the maidenhead grid reference but not in this version.
• The SH-D1602 16×2 LCD will continuously report the call sign, band, and current time
• The Chevyshev4^th order filter selector is a I²C controlled filter selector that will insert the right low pass filter for the band being transmitted.
• Finally the amplifier is a broadband RF amplifier that will boost the low power filtered signal.

The 4^th Order Chebyshev Filter Selector

The filter selector is controlled using the I2C bus through a PC8754 8 bit bus extender IC. Each of the filters can be individually switched in or out of the circuit from the SI5351A clock driver and the amplifier. The relay drivers are simple N channel MOSFET (2N7000) driver circuits as shown below.

This circuit is designed with readily available parts (amazon). The 100K resistor is not strictly necessary as long as your control line to the gate never floats. If there is a risk of the control line being floated, then add the resistor to avoid any issues. The 1N4007 diode (flyback, flywheel) prevents back EMF from the coil, when the voltage is cut off, from damaging the MOSFET.

The relays are also readily available (amazon). They’re DPDT which enables the start and end of each filter to be switched in or out. A SPDT can also be used by connecting the clock to the input of all filters and using the SPDT relay to switch the appropriate filter into the amplifier. Or conversely connecting all the filter outputs to the amplifier and using the the SPDT relay to switch the clock to the input of each filter. This could cause problems since the filters are shunt/series types where the shunts for lower frequency filters could cause issues for the higher frequencies which is why the DPDT relays are preferred.

The LCD Display

The LCD display will provide a status of the beacon. Currently the plan is to show the call sign, the band, the power, and the current time. The top left where WSPR is shown on the image will be a progress bar on the current transmission.

The image shows how this will be laid out on the display which is a small 16 character by 2 row display. The LCD display uses the following ICs:

• HD44780U LCD driver controller
• PCF8574 I²C extender controller

How these two interact will be covered later in the software section.

The GPS chip

The GPS chip is used to get an accurate time reference. With a device that can connect to the internet NTP can be used for this. This minimal implementation doesn’t have the luxury of internet connectivity. The advantage of a GPS chip is that this WSPR beacon can run in the field where no internet connectivity exists. The GPS chip uses the NMEA protocol over the serial port which is very chatty. During a WSPR transmission it would be very difficult to service the serial interrupts without introducing unnecessary jitter in the WSPR protocol transmission. To avoid this the GPS chip is monitored continuously for time updates between WSPR transmissions but once a WSPR transmission begins, an internal clock free-runs until the transmission is completed. Given the master clock for the micro-controller is crystal controlled free-running during transmissions shouldn’t introduce any noticeable error in the transmission. Furthermore, the critical timing of the start of transmission will be closest to the most recent time synchronization with the GPS chip.

The SI5351A clock generator

This clock generator takes commands from the I²C bus and outputs the desired frequency as a square wave. There are 2 clocks available on the clock generator but only one will be used.

The amplifier

The amplifier is a simple broadband amplifier to boost the signal from the filtered clock generator signal to approximately 200mW. The amplifier is based on the readily available 2N7000 N channel FET also referred to as a FETlington because if its darlington type high gain.

The 4th order low pass filters

Currently the design supports 5 bands 10m, 15m 20m, 40m, 80m the WSPR frequencies for these bands are:

Table 1: WSPR frequencies for the chosen bands

Band	Dial Frequency	Transmit Frequency
10m	28.1246	28.126000 – 28.126200
15m	21.0946	21.096000 – 21.096200
20m	14.0956	14.097000 – 14.097200
40m	07.0386	07.040000 – 07.04020
80m	03.5926	03.594000 – 03.59420

All filters are Chebyshev 4^th order filters and have the same design. The only difference between the filters are the component values. The cutoff frequency for each filter is set to the upper  transmit frequency shown in table 1. There are many on-line filter calculators available to calculate the component values with some using standard values making it much easier to construct the filters. The one I used is here. The graphs that are displayed below were generated by LTSpice which is free circuit simulator software that you can get from analog.com.

Band	Cutoff Frequency	C1	C2	C3	L1	L2
10 m	28.1262 MHz	130 pF	220 pF	130 pF	390 nH	390 nH
15 m	21.0962 MHz	180 pF	300 pF	180 pF	510 nH	510 nH
20 m	14.0972 MHz	270 pF	430 pF	270 pF	750 nH	750 nH
40 m	7.0402 MHz	510 pF	910 pF	510 pF	1.5 uH	1.5 uH
80 m	3.5942 MHz	1.0 nF	1.8 nF	1.0 nF	3.0 uH	3.0 uH

The input and output impedance should be matched to ensure the filter operates as expected. The chosen impedance for both input and output is 50Ω. Since the output impedance of the SI5351A is about 85Ω it needs to be match to the expected 50Ω impedance of the load (antenna). A 85Ω to 50Ω difference in impedance isn’t really a big deal and matching is not critical. But to maximize power transfer, the easiest way to do this is to use an L pad matching network:

With the formulas above, R1 = 54.5Ω and R2=77.9Ω.

Programming the SI5351A clock generator

The SI5351A clock generator is controlled using the I²C bus.