SDR: The next phase has arrived!

Mike Richards G4WNC reports on a new approach to SDR and starts a tutorial on FT8 operation.

High-end SDRs almost universally use 16-bit sampling ADCs (Analogue-to-Digital Converters) located as close to the antenna as possible. Most of the popular models directly sample the entire HF and low VHF (50MHz) bandwidth. Sampling such a wide bandwidth at 16bits creates extremely high data rates. For example, using a common sample rate of 122.8Msps and 16-bit depth gives a serial data rate of 122.800000 x 16 = 1.965Gb/s. That was considered way too fast to pass directly to a computer, so it has become standard practice to use an FPGA (Field Programmable Gate Array) to down-sample the data to a more manageable rate. The FPGA device contains extremely fast logic and is able to parallel process the data. The FPGA’s role in an SDR can be likened to the first mixer in an analogue communications receiver, where it translates the incoming RF to a more manageable standard sample rate, or intermediate frequency in the case of an analogue receiver. However, the FPGA introduces two problems. The first is the specialist and expensive programming skills required to write and maintain the FPGA code and the second is the cost of the FPGA itself.

So, what’s the next phase SDR I hear you ask? The past few years have seen a dramatic change in the serial port technology used in popular PCs and the latest PCs with USB3.1, 3.2, USB4 or Thunderbolt 3 can handle bus speeds of up to 40Gb/s! That gives SDR designers the option to ditch the FPGA and process the raw data stream from the ADC using software on the main PC. Not only does this cut the costs associated with the FPGA, but it brings tremendous flexibility. The front-end hardware effectively becomes a spectrum capture device. In addition to the fast serial ports, modern PCs have processors with multiple cores and graphics cards with powerful GPUs (Graphics Processing Units) that can process SDR data.

The reason I raise this now is the availability of the first capture hardware at reasonable prices (circa £150-200). There are several variants out there, but they are all based on the BBRF103 open source design by Oscar IK1XPV. Oscar’s design uses the LTC-2208 ADC combined with a Cypress FX3 USB controller and a clock generator. I’ve shown a block diagram in

Fig. 1. The two main variants out there are the Dragonfly 666 and the SDR-888. Of these, the RX888 is the one to go for as it is based on a later design and includes a pre-amp ahead of the ADC. Without the preamp, the Dragonfly is likely to be a bit deaf. The design also includes an RT820T tuner to extend coverage to 1.8GHz but it’s the 32MHz HF direct sampling that’s the highlight.

As with all SDR hardware, its usefulness depends entirely on software availability. So far, these receivers have been very well supported as there’s modified HDSDR software and Simon Brown G4ELI is including support in SDR Console v3. At the time of writing, the RX888 is available on eBay for just under £150. One important point to note is that these units should be treated as development boards and, as such, you shouldn’t expect much in the way of customer support from the suppliers.

Data Modes Introduction – Continued

Having spent the past couple of issues introducing some of the fundamentals of data modes operation, this month I’ll dive straight into the most popular mode, FT8.

FT8

FT8 and its sibling FT4, form part of the WSJT-X suite of weak signal programs. The name WSJT comes from the primary author’s name (Weak Signals Joe Taylor) and the operating procedures have their origins in the techniques developed for EME (Earth Moon Earth). When working EME, the exchange required for a completed QSO comprises the callsign, report and other information such as the location. In addition to a fixed format for the messages, EME operators use timed transmit and receive slots to improve their chances of success. FT8 follows similar principles by using 15 second timeslots and structured messages. Other than

a 13 character (including spaces) final message, there is no facility in FT8 for free text messages. I’ve shown the structured message format used for a typical FT8 QSO in Table 1. Here you will see that the location is restricted to the first four characters of the Maidenhead locator and the signal report is given in dB (−21 and −19 in Table 1). Rather than use the usual RST report, the FT8 software automatically calculates the signal to noise ratio based on a 2.5kHz bandwidth. This is far more helpful than the dubious 599 exchanges that are all too common!

FT8 Modulation

FT8 transmissions are generated using audio tones, as described last month. In the case of FT8, eight-tone Gaussian Frequency Shift Keying (8-FSK) is used with a slow baud rate of just 6.25 bauds. This modulation systems employs eight, closely spaced, frequencies and the gaussian filtering ensures a smooth transition between tones and reduces the key-clicks that would otherwise be generated by abrupt frequency changes. By using such a slow baud rate, FT8 signals occupy a very narrow bandwidth of just 50Hz. The use of such a narrow bandwidth means the entire FT8 activity for each band can normally be accommodated in a single 2.5kHz speech channel.

Starting with FT8

Before starting to use any new radio system, it’s important to set aside some time to monitor and note how the mode is being used. With today’s softwarebased data modes, it’s all too easy to load the software and dive straight into operating. However, you will get much better results if you familiarise yourself and take time to learn the basics before you start transmitting. As with most data modes, the FT8 software is available as a free download and there are packages available for PCs, Macs and Linux machines, including the Raspberry Pi. The entire WSJT suite of programs is available via a single download that you will find here:

Windows installation is particularly easy as you simply run the downloaded .exe file. If you’re planning to use Linux or macOS, you find links to the installation instructions on the download page.

Before starting to use any of the WSJT-X modes it’s worth making sure that your PC clock is being accurately updated. A simple way to check the accuracy is to visit http://time.is In Fig. 2, I’ve shown a screenshot of time.is site, where it displays the current time along with details of your PC clock. An accuracy of half a second or better is generally fine for FT8. If you want to improve your clock accuracy, you will need to install a time synchronisation program and a popular choice is the free Meinberg NTP software that can be downloaded from here:

ConfiguringWSJT-X for FT8

If you’re new to data modes, I suggest you start by using VOX for transmit/receive switching rather than full CAT control. This makes the configuration simpler and you can add CAT control later. Assuming you now have WSJT-X installed, the first task is to enter your station details as follows: Run WSJT-X

Select File − Settings and then the General tab

Enter your callsign and Maidenhead locator, Fig. 3

Select the Radio tab and make sure the Rig is set to None and PTT set to VOX

Select the Audio tab and choose the soundcard you will be using for the audio input and output from your rig, Fig. 4

Click OK to finish and save the configuration

When WSJT-X started you will have noticed that two windows opened, one of which was the Wide Graph, Fig. 5. This displays the audio spectrum currently being received via the selected soundcard. In addition to providing a view of band activity, we can use the spectrum controls to set our transmit and receive frequencies. However, to get the best display for FT8 operations we need to make some minor adjustments as follows:

Connect-up your audio link between the rig and computer

Tune your rig to the 14MHz band and select USB or Data.

Start WSJT-X and set the mode to FT8. If all is well, you should see signs of life in the Wide Graph and the level meter in the bottom left of the main screen.

Tune your rig to an empty RF channel somewhere around 14.1MHz

Adjust the audio input level to the computer to get a reading of approximately −30dB in WSJT-X

Retune to the main FT8 frequency of 14.074MHz

You should see some FT8 activity as this band is rarely quiet

Make sure the input level bargraph remains green

With the receive sensitivity configured, we need to adjust the Wide Graph parameters to optimise the view for FT8 as follows:

Select the Wide Graph window

Set the Bins/Pixels to 5, Start to 100Hz and N Avg to 2

The next step is to adjust the waterfall and spectrum sensitivity sliders. These are located on the lower-right of the Wide Graph and the top two sliders adjust the waterfall while the lower set deal with the spectrum display. The waterfall is generally the most useful display and you should adjust the sliders for a display similar to that shown in Fig. 5.

Having completed the receive setup, we can move on to the transmit configuration. You should be expecting

to run low power most of the time and a few watts is usually sufficient. One common failing is to overdrive the audio stages of the transmitter. This results in the transmission of multiple distorted artefacts of your signal, so you won’t be popular! To configure the transmit path, begin with a dummy load connected in place of the antenna. You can then click the Tune button in WSJT-X to generate a transmit audio tone. In the bottom right of the WSJT-X main panel you will find the transmit drive slider. This slider adjusts the level of the transmit tone and I suggest starting with this set to about 75%. Now you can move to your rig and adjust the mic audio gain, VOX sensitivity and the PA drive to reach the target output power. I suggest starting with around 5 watts. If you have a digital scope to hand, you could put it in spectrum analyser mode and use a tap of the transmit output to check the spectrum quality. When you start making QSOs, you use the transmit slider in WSJT-X to trim the power output.

That completes the basic setup.

Tuning FT8

Because FT8 signals use such a low data rate, the decoding process requires very little processing power from your PC. FT8 software takes advantage of this and can comfortably decode a large number FT8 signals in each 15 second time slot. As a result, it is standard practice to confine FT8 operations to a single, SSB width, channel in each band. This greatly simplifies operating because you can tune your rig to a single SSB frequency and monitor and communicate with any active station without touching the rig’s tuning.

Another important characteristic of FT8 operation is the use of split transmit and receive frequencies. As the decoder can handle all the signals in the 2.5kHz bandwidth, you can reply to a station using any clear frequency.

The first task before making a call is to select a clear transmit frequency. You can find a suitable frequency by watching a few 15 second FT8 cycles on the Wide Graph display. Once you’ve identified a clear frequency, use the mouse to shiftclick on the left-hand edge of the chosen frequency. You will see the red, transmit frequency, marker move to that location. When you’ve set the transmit frequency, make sure you tick the Hold Tx Freq box in the lower centre. This will lock the transmitter on that frequency, leaving you free to move the Rx frequency to the station you want to work.

WSJT-X does a particularly good job of simplifying FT8 operation. The operational heart of the software is the two main panels. The left panel shows all the decoded activity, while the right-hand panel shows the activity on the currently selected receive frequency (green marker on Wide Graph). CQ calls will be highlighted in the band activity panel and you can respond to a CQ by double-clicking on that station’s entry. This will populate the call and locator fields and set the transmitter to start on the opposite cycle to that of the wanted station. Once the link has been established, the software will automatically step through the structured messages and complete the QSO.

Summary

I’ve squeezed a lot into this month’s column so next month I’ll spend a bit more time on some techniques to improve your FT8 success.