Content provided by: Steve, N5AC CTO

A lot of new owners have joined the FlexRadio family this year, and many more hams are taking a close look at what we build. Whether you’re coming from a traditional superheterodyne transceiver or from another SDR, it’s natural to assume all software defined radios are about the same inside. They aren’t. This month I want to explain how a FlexRadio is built and why we built it that way.

We made these choices with a clean sheet of paper for the FLEX-6000 Signature Series, and everything below also applies to the FLEX-8000 series and Aurora radios. Plenty of current transceivers are direct sampling too, so the first few sections cover what direct sampling gives you over a superhet, and the rest cover where a FlexRadio is different from other SDRs.

Why Direct Sampling

A superhet converts your signal to an intermediate frequency through one or more mixers, each driven by its own local oscillator. Mixers are nonlinear by design. When strong signals are present, they create intermodulation (IMD) products that can land right on top of the weak signal you’re trying to copy. Every local oscillator also adds its own phase noise and spurs.

A FlexRadio digitizes the antenna signal directly at RF, with no analog mixers in the receive path. That gives you:

A cleaner spectrum. There are no mixer-generated IMD products and no images from analog conversion stages.
One oscillator instead of many. The sampling clock is the only physical oscillator in the receive chain. Tuning is done digitally.
Receivers that cost only processing power. In an analog radio, a second receiver means a second complete signal chain. In a FlexRadio, each additional receiver, up to the model’s limit, is more processing inside the radio, not more hardware.
The whole spectrum at once. The converter can see all of HF and 6 meters at the same time, so receivers and panadapters on different bands can share one antenna, with no hardware to retune. When you’re working a single band, the preselector narrows that view to keep strong signals on other bands out.
Filters defined by math. Receive filters are digital, so you can set any bandwidth you like with steep, brick-wall skirts, and there are no crystal filters to add ringing or drift with temperature.
A radio that gets better over time. Because the signal processing is software, new capabilities such as improved noise reduction reach the radio you already own through a firmware update.

The fair question is what happens when the big station down the road fires up during a 40-meter contest. Band-pass preselector filters ahead of the analog-to-digital converter (ADC) keep strong signals on other bands away from it. But that station is on your band, inside the filter. With the mixers gone, the converter and the clock that drives it set the limit on how well the radio handles it, which is why we were so careful choosing both.

Choosing the Right ADC

The ADC is where your antenna signal becomes numbers, and everything after it can only be as good as what it delivers. In a contest station or a DX pileup, you’re trying to copy an S1 signal a few hundred hertz from an S9+40 signal. So we chose the highest-performance converter we could buy: the Analog Devices AD9467, a 16-bit ADC. Every FLEX-6000, FLEX-8000, and Aurora radio uses it.

Each bit adds roughly 6 dB of dynamic range. In theory, a 16-bit converter delivers about 98 dB, compared with 86 dB for 14 bits and 74 dB for 12 bits. Real converters fall short of theory. The AD9467 datasheet specifies a signal-to-noise ratio of about 76 dB relative to full scale at its rated 250 Msps, and it’s essentially the same at our 245.76 Msps. Either way, that figure covers the converter’s entire Nyquist zone, which is everything from DC to half the sample rate. Sampling at 245.76 Msps, that’s 122.88 MHz of spectrum, and the converter’s noise is spread evenly across all of it.

You don’t listen to 122.88 MHz, though. You listen to a few kilohertz. The radio filters the signal down to the band you want and lowers the sample rate to match, a step called decimation, and in doing so it throws away the noise outside that band. The improvement is called processing gain. A receiver whose final output is 24 ksps of complex I/Q covers 24 kHz of spectrum, because each complex sample carries both the upper and lower half of the passband. So:

Processing gain = 10 × log₁₀ (122.88 MHz ÷ 24 kHz) = 10 × log₁₀ (5,120) ≈ 37 dB

That’s the equivalent of about six more bits. The noise floor that sat 76 dB below full scale across the whole Nyquist zone sits about 113 dB below full scale inside a 24 kHz receiver, and lower still as you narrow to an SSB or CW filter. That’s a noise measurement, not the two-tone dynamic range you’ll see in lab reviews, which also counts distortion. But it shows why the processing after the converter matters as much as the converter itself. Models that sample at 122.88 Msps gain about 3 dB less.

It’s tempting to think a faster converter would win, since processing gain grows with sample rate. Our research found that faster converters often produce a worse signal, because higher sample rates tend to cost noise and linearity, exactly what matters at HF. Most new ADC development today is aimed at much faster parts for other markets, which is why the AD9467 is still the right converter for an HF radio.

When the Strong Signal Is Next Door

In a superhet, the roofing filter protects the later stages from signals outside its passband. Once a strong signal gets inside the roofing filter, the IF amplifiers and later mixers see it, and dynamic range drops. Even a narrow roofing filter can’t help with a signal a few hundred hertz away. That’s why it pays to look at dynamic range at close spacing, not just wide spacing.

A direct-sampling radio has no roofing filter to fall inside of, so its third-order dynamic range doesn’t collapse as the strong signal moves in close the way a superhet’s does.

The close-in limit a direct-sampling radio still has is phase noise. Every oscillator has a little noise on either side of its frequency. When a strong signal is nearby, it mixes with that noise and spreads it across the band, burying weak signals. This is reciprocal mixing

In a FlexRadio, the sampling clock does the job of the local oscillator for every receiver at once, so instead of a chain of synthesizers, each adding its own noise, we put our effort into a single, very low-noise clock.

More Than One Kind of Receiver

The payoff you’ll notice first is the view. A FLEX-8600 or an Aurora AU-520 can display more than 14 MHz of spectrum in a single panadapter, and you can open up to four panadapters, each on its own 14 MHz segment.

The built-in band scopes on most transceivers top out at about 1 MHz, and some show only a few hundred kilohertz. You can watch all of 10 meters at once for an opening, or keep 20 through 15 meters in one view while you work a station on one of them.

That’s possible because we built different kinds of receivers into the radio for different jobs. Many SDRs use a single kind: a digital down converter (DDC) that takes a narrow piece of spectrum to audio.

That works well for listening, but it’s the wrong tool for showing you a whole band. In a FlexRadio, Slice Receivers are what you tune and listen to, and Panadapters are wideband displays built to show you as much spectrum as possible without using up your listening receivers.

Two Antennas at Once

The FLEX-8600 and Aurora AU-520 have two Spectral Capture Units (SCUs), each a complete receive path from antenna connector to ADC. Both are driven by the same sampling clock, so they stay phase coherent. That lets you run full-performance receivers on two different antennas at the same time, on the same band or on different bands. It also enables diversity reception: the same signal from two antennas, one in each ear, so your brain can ride through the fading. And because the two paths are coherent, Enhanced Signal Clarity (ESC) can combine them to null out a noise source. Even more enhancements to ESC are coming up in a future release of SmartSDR so stay tuned!

The Radio Does the Work

Many SDR designs send raw samples to a PC and let the PC do all of the processing. A FlexRadio works the other way around. The radio is a server: demodulation, filtering, noise reduction, and the panadapter and waterfall data are all produced inside the radio. The software that controls it is a thin client that sends commands and displays what the radio reports, so your PC doesn’t need to be powerful.

That means every client gets the same results. SmartSDR for Windows and Maestro, our standalone control console, work with the same receiver, and so do newer independent clients built on our open API, such as AetherSDR, RimuSDR, and of course, the long-standing SmartSDR for macOS and iOS by Marcus, DL8MRE. That’s true in the shack or from across the country with SmartLink, our remote operating service. When we improve something like noise reduction in the radio, every client gets it.

All of this travels over Ethernet, which has been part of the design from the very first FLEX-6000 because it’s everywhere and carries data at almost any speed. Digital mode programs and loggers connect over the same network: DAX (Digital Audio eXchange) carries the audio, and CAT handles rig control.

The Transmit Side

Transmit mirrors receive. The radio generates your signal digitally and converts it straight to RF with a digital-to-analog converter, with no analog mixers and no IF stages. Carrier and unwanted sideband suppression are set by math, not by component tolerances, so they stay put over time and temperature.

What’s left is the power amplifier, which adds distortion of its own as it approaches full power. That’s what SmartSignal™ adaptive predistortion addresses. The radio samples its own output, measures how the amplifier is distorting the signal, and pre-corrects what it sends so the signal leaving your antenna is cleaner. It works with an external amplifier in the loop too. SmartSignal is included on every FLEX-8000 and Aurora radio.

The Bottom Line

A FlexRadio is built so you can copy the weak signal next to the strong one, see the whole band while you do it, and put a clean signal back on the air.

If you’re a new owner, open a second Slice Receiver and widen your panadapter to see what’s out there. The getting-started videos SmartSDR 101 and the FlexRadio Community are great next stops. If you’re still shopping, compare the models.

If two antennas and diversity matter to you, look at the FLEX-8600 or Aurora AU-520. If you’d rather have knobs than a mouse, the M models have a built-in front panel, and Maestro works with any of our radios.

Bring your questions to the Community, where plenty of owners are happy to answer them.