While there are different ways to distribute high-quality reference-clocks to multiple receivers and transmitters, or to general lab equipment, perhaps the best and easiest is with a Clock Distribution Buffer. The TIS-126 has been designed for this job:
This unit can send a square-wave clock to six output ports. Input and output are 50 Ohm impedance, and the frequency range is from 100 KHz to 100 MHz (and down to under 1 Hz in many cases). The input level can range from -20 dBm to +20 dBm.
If the Beacon Blaster 6 is to be used with a receiver on a shared antenna, an external Transmit/Receive switch will be required. Some T/R switches provide automatic sensing that switches modes when the transmitter starts. This can work well, especially given the relatively low-power output of the BB-6.
But there are advantages to an externally-controlled switch, usually driven by the “PTT” output of the transmitter (this is often an “RCA” or phono jack.) This output is generally a contact-closure (relay or transistor) to ground, which connects during transmit.
The BB-6 T/R Switch Interface provides this control signal. Installation will require that a 10mm (or 3/8”) hole be drilled in the BB-6 rear panel, and the attached control wires be plugged onto an existing header on the BB-6 control board.
BB-6 firmware version 2.4.0 or later is required.
The T/R Switch Interface is offered free of charge to all owners of the BB-6, so please let me know if you want one.
(the image above shows a 1/4W resistor where a small surface-mount resistor should be. Turns out I didn’t have the proper value on-hand when building the test unit.)
Announcing the next stage in Beacon Blaster evolution: The WSPRSONDE 8!
This is still in development, but the initial prototypes look very promising. Similar in many ways to the BB-6, this has the same type of frequency-flexible 1W Digital (square wave) outputs, the same GPS and 10 MHz clock reference inputs, 9-24VDC input, and USB configuration port. The WSPRSONDE-8 supports both WSPR and FST4W-120 modes.
Spurious output levels greatly reduced. The BB-6 was designed to meet a spurious output level of -40dBc or better. The WSPRSONDE-8 uses a different modulation technique to provide close-in spurs typically better than -90dBc
A single-board design that eliminates the many subassemblies and interconnections inside the BB-6. This is a rugged system.
The WSPRSOND-8 provides ultimate frequency accuracy and stability limited only by the external 10 MHz reference.
WSPRSONDE-8 shown with 6-band filter/combiner and Bodnar GPSDOClose-in spurious typically better than -90dBc (20-meters shown here, spurs -96dBc)Using the 6-band filter/combiner, the harmonic outputs are typically better than -60 dBc (20 meter output shown here)
I’ve been running this code (or the previous version) for a few weeks now, and it runs well. The latest version of wsprdaemon is able to calculate spectral spreading for WSPR as well as FST4W, so there’s little reason not to switch over to WSPR.
There are some upgrades to the configuration commands and I will be soon editing the user’s guide to show this, but the old commands still work: just add “MODE WSPR” to the start of your config.txt file. Then reload the file (or restart the BB-6) and you’re now running WSPR!
The assembled Shelving/LPF boards were delivered yesterday, and the “missing” inductor was delivered today (one inductor was not in stock at JLCPCB so I had then assemble it without that one part; I ordered that inductor from DigiKey). Amazingly fast service from JLCPCB! So I soldered on that inductor and the SMA connectors and did some testing:
This board has options where the shelving filter can be bypassed, so I tried that first. The results were much better than I had expected:
This essentially matches the performance of the LTSpice simulation (which used the Coilcraft inductor models, not the similar Murata inductors on the board. This shows a reasonably sharp rolloff with better than 60dB stopband attenuation. I am seeing no obvious inter-stage coupling (which can be a concern when using unshielded solenoid inductors.)
Here is a closer look at the roll-off performance. The loss at 1MHz is essentially zero, while the -3dB point is about 28 MHz. We may want to push this corner frequency a bit higher…
There are some spurious responses above 240 MHz. I suspect that these won’t be a problem because the SDRs that will be using these filters will already have an anti-aliasing filter that should adequately knock down these frequencies. I could guess that these come from inter-stage trace inductance, but that’s just a guess:
With the shelfing filters in place, the low-frequency attenuation is about 20dB:
There is also extra attenuation above 25 MHz, which suggests that we shift the shelving mid-point frequency slightly lower to reduce higher-frequency attenuation.
In conclusion, I am quite pleased with the overall design and layout of this board. Tweaking the frequencies (if necessary) will be trivial.
When writing software loses its appeal I take a break and do something concrete — hardware design. Here are some recent filter designs:
Four-Band Filter/Combiner:
This is used to combine the 1W square-wave outputs of the BeaconBlaster, filtering out the bad harmonics and combining all four inputs into one output which will feed a multiband antenna. We wanted to make this unit with off-the-shelf surface-mount components, so we needed to design fairly wide filters, no closer than one octave apart or there would be excessive interaction between sections. The filter topology also needed to show a high off-channel impedance, again to avoid interactions. Available inductors limited the design to 1W power on each port, with less than 1dB of loss.
The image below shows a simulation of the 60/30/15/6-meter version of this filter. Note that each filter has a deep 3rd harmonic notch.
And here is the spectrum of the 80/40/20/10-meter filter, being driven by the BeaconBlaster (using a 20 dB attenuator, actual power levels about 1W per channel). You can see the 80 meter 3rd harmonic at -40dBc:
A Shelving / Low-Pass Filter
This board combines a two-stage 10 MHz shelving filter, and a steep four-section 30 MHz elliptic low-pass filter. This is intended to go between an antenna and a SDR having a sample rate of 66 MHz. The shelving filter attenuates the low-frequency signals (where SDR overload is typically a problem), and the LPF is for additional anti-aliasing. The “ideal” design promises 70dB stopband attenuation, simulations show better than 60dB with available surface-mount inductors, and if I get at least 50dB then I will consider it a success.
Boards should be ready for testing in a couple of weeks.
This is being developed in collaboration with some folks who are running multiband receiver sites, including WebSDR and ionosphere research installations. In particular I should acknowledge the advice and contributions of Clint Turner (KA7OEI). The shelving filter was stolen directly from his website: https://ka7oei.blogspot.com/2020/08/revisiting-limited-attenuation-high.html
While waiting for more chassis to arrive I’ve been testing the first two prototypes and the results are excellent. Prototype #1 has been running in Friday Harbor for a few weeks, using the filter/combiners to feed two antennas. Both antennas are Off-Center-Feed dipoles (which provide good matching on octave-spaced bands, rather than the third-harmonic spacing of a center-fed dipole). One antenna is for 80/40/20/10 meters, and the other is for 30/15 meters. The one Watt signals on the six transmit frequencies have been received by every known FST4W-120 receiver on the planet:
Prototype #2 has also been running, using my site near Bodega Bay, California:
This one is driving a single OCF Dipole designed for 40/20/10 meters. I am also connecting an 80 meter transmit channel to this antenna — a *very* lousy match, but it is still being received far and wide. You may be wondering about the module sitting on the saucer; rather than use a GPSDO at this location I decided to use a cheap surplus 10 MHz OCXO for the reference clock. You can also see the four-band filter/combiner connected between the BeaconBlaster and the SWR meter (and then the antenna). Here’s a typical day’s propagation for this one:
These propagation charts may not look very impressive when you compare them do typical WSPR reports, but remember, there are actually very few FST4W receiver sites in operation. Work is being done to upgrade WSPR signal monitoring software, but for now FST4W is really the only game in town for collecting accurate Doppler spreading data needed for atmospheric propagation research. Anticipating upcoming WSPR upgrades, the BeaconBlaster will be WSPR-capable very soon.
So progress is being made. Enough subassembly boards are available to build a few more units, and more boards are on the way. The new chassis should look a bit more professional as well. Full details will be coming soon (I promise!)
