The FLEX-6000 HF Software Defined Radio

What do you get when a group of talented hams, computer experts, and radio engineers are confined together with a drawing board? The sum of their dreams: a radio like the FLEX-6000. Not satisfied to coast on their successes with recent software radio designs, they started from scratch and built a radio truly on the bleeding edge of technology. It uses direct sampling and is designed as a client - server system so that multiple users may simultaneously conduct advanced operation techniques and enjoy RF performance second to none. Though this article focuses on its use as an amateur radio system, it clearly has potential in utility / aviation / military applications. Hopefully there will be great interest in the FLEX-6000 series, because the technology is really good and its development advances the radio art.

flex-6000 software defined radio
The Flex 6000 direct sampling SDR.

Greg Jurrens introduces the FLEX-6000 HF Software Defined Radio.

Direct Sampling RF Architecture

The FLEX-6000 diverges from earlier SDR designs in its use of direct sampling hardware. Instead of digitizing the output of a quadrature sampling detector, which provides access to limited chunks of spectrum at a time, this new radio uses "signal capture units." Each unit contains bandpass filtering and an A/D converter to digitize the entire HF spectrum. There are no mixers or roofing filters in the FLEX-6000! It has a very clean and overload resistant signal chain, and the result is a software defined radio with signal purity / dynamic range specifications second to none.

Direct sampling has advantages over quadrature sampling / downconversion SDR designs. First, only one oscillator (at one frequency) is required as a system clock source. Signals are not tuned in a conventional manner; they are digitally extracted from the captured spectrum. Phase noise and jitter are far below levels associated with superheterodyne radios. Mixers are a source of intermodulation distortion, and eliminating them makes for a signal chain much less prone to problems caused by strong signals. The FLEX-6000 design is capable of top notch weak signal reception as well as super clean, spur free transmission.

Significant Internal DSP

After the incoming signals are digitized, substantial internal processing power inside the FLEX-6000 is used to do DSP and provide users with four full featured transceivers per signal capture unit. Multi-core ARM processors, FPGAs, and a fair amount of memory are built in to the FLEX-6000. It uses a multitasking operating system to do its work, so this SDR differs from others in that computing power is more internal. Most other software defined radios, especially quadrature sampling types, offload DSP functions to the client-side computer. Flexradio assures consistent quality of the FLEX-6000 by keeping a great deal of critical computing internal.

Client - Server Software Architecture

The FLEX-6000 series radios are designed for the radio hardware to function as a server, remotely installed, with users connecting via a local area network. It is fast enough to handle gigabit ethernet, so there need not be a bottleneck between end users and the radio. On the client side, users enjoy sophisticated PowerSDR software to operate up to four radio instances per "signal capture unit."

In practical terms, mount the radio in a cool and dry place. Connect an ethernet cable to the radio and run that ethernet line to a personal computer. For a multi-user station, run ethernet to a router. Then, each client can connect via the LAN. With a bit of extra hardware, such as a Rasberry pi, one could use the radio via wireless. Flexradio does not advocate putting the FLEX-6000 directly on the internet for world access due to security issues. They don\'t want the FLEX-6000 to get hacked and possibly bricked by remote attackers. That being said, one could put the FLEX-6000 behind a firewall and implement other measures to keep it safe and also available to distant users on the internet.

As to its transmit capabilities, the FLEX-6000 can transmit 100W at 100% duty cycle in authorized bands and up to 50 mW on any frequency in its operating range. The usual precautions are necessary for digital modes regarding peak versus average power. Don\'t trash a pristine signal by overdriving the transmitter. An included automatic antenna tuner can match a wide range of loads and remember its settings.

Specifications and User Reports

Flexradio is certainly quite proud of their brand new ultra performance radio. How does it measure up and what do people think of it? See below.

	Receiver:  direct sampling digital downconversion and demodulation
	Transmitter:  direct digital upconversion from baseband to RF
	DAC Resolution:  16 bits
	DSP Resolution:  32 bit floating point
	DAC / ADC Sampling Rate:  491.52 Msps

Frequency Range
	FLEX-6500:  300 kHz to 77 MHz.
	FLEX-6700:  300 kHz to 77 MHz and 135 to 165 MHz

Dynamic Range
	3rd order intercept point:  +45 dBm
	3rd order dynamic range 110 dB
	phase noise:  -130 dbc/hz @ 1 kHz offset, -146 dBc/hz @ 10 kHz offset, -152 dBc/hz @100 kHz offset

Spurious and Image Rejection:  greater than 100 dB

Frequency Control
	FLEX-6700: standard ±0.02ppm Oven Controlled Crystal Oscillator
	FLEX-6700: optional ±0.01ppm GPS Disciplined Oscillator
	FLEX-6500: ±0.5ppm Temperature Controlled Crystal Oscillator

CW Operation
	Carrier digitally generated at the operating frequency.
	Full QSK designed in hardware and DSP; switching time below 10 ms

Single Sideband Operation
	Carrier Suppression: < -80dB
	Unwanted Sideband Suppression: < -80dB
	Transmit Audio Bandpass:  Default 300 - 2700 Hz, adjustable 50 - 5000 Hz

Digital Mode Operation
	Max Bandwidth 5000 Hz
	Peak power 100W
	Modes dependent on user software

At the moment, there are no user reports on operating the FLEX-6000. Forums have plenty of speculation on how the radio will sound, how easy it will be to operate, and so forth. A few hams do not like the history of Flex Radios on CW, and a few complain about audio quality of older radios. Interestingly, all of the gripes can be attributed to improper operation of the transceivers. These are very capable radios which are not tolerant of abuse or clumsy adjustment. Overdriving the inputs, not grounding properly, and using bad quality interfaces have caused most of the instances of bad signals from the older FLEX equipment.

Quite a lot of interest exists among club stations, contesters, and extended single sideband enthusiasts. Oddly, these disparate classes of radio operators - with their own specific ideas of a "great radio" - have high hopes for what the FLEX-6000 will bring. It will likely manifest the sum of their dreams.

Tags: FLEX 6000, Flexradio SDR, Direct Sampling SDR

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