Flex Radio Systems – Flex 6500

Recently, I purchased a brand new Flex Radio Systems Flex 6500.  I have been drooling at the prospect of upgrading to the new Flex 6500 for some time now and have had a blast using the new SDR.

After a few months of owning and operating the Flex 6500 I thought I would create a review blog post, highlighting both the good and bad as I see it in my opinion.  I will be going over the Flex 6500 specifically, but where applicable I will differentiate between the other Flex models.  In reference to SmartSDR, I am using version 1.3 of SmartSDR software/firmware, so my perspective will be based off of that revision.



The SmartSDR GUI in general:

SmartSDR is a really sexy streamlined interface compared to PowerSDR, but it just doesn’t work quite as well as PowerSDR in some scenarios.  For example, to change frequency, when you were to click on the frequency in PowerSDR, you could click and highlight a single digit, in SmartSDR when you click, it highlights the whole thing.  And you must know to use the decimal as the display is in MHz and if you type in 7200, it will think that you mean.720 and not what you really want 7.200.000.  Tuning with the mouse scroll wheel is adjustable but by default is set to 100KHz, which for most people is perfectly okay.

I would love the GUI to use docked window modules so that I could move them to different monitors.  Although you can stretch out the SmartSDR program and use it over several monitors, you really cannot take things out of the main program window and move them about which is disappointing.  This would be an improvement in future releases that would be really fantastic so that you could move bits around other software, such as a logging program or something.  It’s pretty clear that the developers really like this to be full screen so that the panadapter is predominantly displayed.

The graphics have been drastically revamped from PowerSDR, and it shows.  The large panadapter, and waterfall are much larger and more useful than what was included in PowerSDR.  The fonts used are very nice and easily read on even the smallest on-screen buttons.

There are some menus that are utilized as opposed to PowerSDR where there really seemed to be a button for just about everything.  In SmartSDR you click on the various buttons and other menu windows open to reveal the functions.  Overall this cleans up the interface greatly, but some items you have to hunt for a bit to find.

The panadapter and waterfall combination is just gorgeous!  Using a good PC with a good video card the panadapter is smooth and the detail that you get from signals is amazing.  Here…see what I mean?



It’s porn for Hams…the sexy SmartSDR interface of the Flex 6000 series (click on image for full-size image)

Ok, now for the most exciting of the features of the Flex 6000 series…Slice Receivers!

Flex has implemented the ability to have multiple slice receivers.  These “slices” are separate receivers that are shown on the same GUI screen in SmartSDR.  Slice receivers are fantastic for you to really get a view of what is happening on different radio bands, as well as to work split or cross band.  The Flex 6500 can utilize 4 slices at once (the new 6300 can use 2 slices, and the big boy Flex 6700 can use 8 slices!).  The slices can have both the panadapter and waterfall and can be scaled so that more of the spectrum is displayed (7 MHz displayed bandwidth for the 6300 and 14 MHz bandwidth for the 6500 and 6700).  Here is a shot of 4 slices on the Flex 6500:


4 simultaneous receivers running on the Flex 6500 (click on image for full -size image)

Using slice receivers you can keep track of activity on various bands.  This is an awesome feature to use in a contest to find activity on various bands.

DAX and SmartCAT:

With PowerSDR you needed to load various 3rd party tools on your PC to really get the most out of the previous generation of Flex SDR transceivers.  Tools like Virtual Serial Port manager (VSP manager), Virtual Audio Cable (VAC), and DDUtil were are utilities that were used to interface of PowerSDR to various 3rd party programs such as Ham Radio Deluxe, FL-Digi, etc.  Now all this is built in with various degrees of success and failure.

Digital Audio eXchange (DAX) is what is now integrated into SmartSDR to replace the venerable Virtual Audio Cable (VAC) software used with PowerSDR and previous generations of Flex radios.  DAX is a somewhat clumsy alternative and in a lot of cases, I just have problems trying to get it to work properly and do what I want it to do.  The interface is nice, but I just can’t always seem to get the audio where I want it without really digging down into the bowels of the Windows 7 sound settings in the control panel.  For example, I tried output the audio of a second receiver slice to Ham Radio Deluxe Log.  In the logging software there is a digital recorder that I like to use to record and catalog various transmissions.  I struggled with this for quite a bit of time before just giving up on it (I haven’t had the patients to go back to it again yet ;).  For whatever reason I can only seem to get audio on the 1st DAX channel.  I think this has something to do with Windows settings in relation to DAX, but when I used VAC, I didn’t have any issues or problems trying to get the audio from PowerSDR to various other software applications.  So DAX in my book should be looked at to be more intuitive (however I don’t know how possible this is given Microsoft’s settings for Windows 7 audio).  There is also virtual audio ports for IQ stream data so that this information could be output to other software which I thought was really awesome (however I have not experimented with it yet).

Overall I think they did a great job of integrating DAX and cheers to them for doing this saving users from installing other 3rd party software.


SmartSDR DAX Control Panel

SmartCat is really the replacement for Virtual Serial Port (VSP) manager.  It works, but the interface is a bit confusing.  One issue that I have with this is that there is no descriptors for what serial ports are in use by the computer.  You have to bounce back and forth between the Windows control panel to see what the hardware is.  Now, if your like me, you have a bunch of stuff connected to your PC using serial ports.  I have a USB to serial adapter that connects to the serial port of my amplifier, a USB port for my Wave Node station monitor (uses an FTDI USB to serial chipset), USB to serial adapters used to program various HTs, etc.  SmartCat is a simple interface that is not as nice as VSP manager.  It is basic, does the job, but just not that well.


SmartSDR CAT interface window (click on image for full-size image)

The Hardware of the Flex 6500

First lets talk about the case, then go inside…

The new case design is sleek and easier to deal with than the Flex 5000A.  Rack mounting handles lets you even easily put the 6000 series Flex SDRs in a rack for space saving, and ergonomics.  The large VFD display currently only shows minor bits of information, but I can see this VFD display getting extra capabilities in future firmware releases.  Currently mine just displays “FLEX-6500” on it.  The black paint is nice with beautiful silver silk screened lettering for the logo and descriptors for front panel jacks.  I really wish that they would have moved the KEY jack, used for a morse key or paddle, to the rear of the radio (or at least duplicated it on the back).  When this thing is rack-mounted you don’t want to plug wires into the front of it.  It just looks too damn good to do that to it.  Connecting a bunch of wires to the front of the Flex 6500 would be akin to drawing a mustache on a supermodel.


On the back the Flex 6500 actually uses Anderson Powerpole plugs for the DC connection to the Flex 6500.  I love that they did this and it’s not some esoteric, hard-to-find, have to order Molex connector and wiring harness.  All the connectors are easily identified and are very accessible.  I really love the balanced microphone input (XLR+TRS combo connector) on the rear.  I use a Heil PR-781 which works really well with Flex SDRs.


With 2 USB 2.0 connectors I do have to wonder about the DB-9 “accessory” connector.  This same connector was used on the Flex 5000A, and it was called “Flexwire” accessory connector.  However I don’t know of any “accessories” that Flex has ever made to utilize this connector.  I guess it’s good to have it on there although the likelihood of ever using it is fairly dubious.

With the latest firmware (version 1.3 released 8/2014 as of time of this article) there is now transverter support.

The FlexControl knob was first incarnated to the last generation of Flex SDRs.  Flex control is basically a USB connected external large knob and a few buttons.  It works very well with both PowerSDR on the previous Flex SDRs as well as SmartSDR and the new 6000 series SDRs.  The best part about the Flex Control knob is that it retains window focus, so that if you click on another window with the mouse to log a contact say in a logging application, you can still tune the receiver using the FlexControl knob.  The  buttons on the FlexControl knob are programmable to an extent and they can be used to change what the main knob does, for example, volume control, AGC-T control, frequency tuning control etc.  The knob itself is a 4th button if you press vertically down on the knob.  Tuning is smooth and the knob is balanced well for tuning just like on a standard more traditional transceiver.

FlexControl_420x356The FlexControl external tuning and adjustment control

The Flex 6500 internal hardware:

The Flex 6500 SDR transceiver utilizes FPGA (Field Programmable Grid Array) hardware to directly generate signals digitally at the RF frequency.  This approach is largely flexible using the massively powerful Xilinx FPGAs deployed in Flex 6000 series SDRs.  Capabilities can be further enhanced with simple firmware updates and updates to the SmartSDR application that interfaces with the Flex 6000 series.  FPGAs have been a game changer in most electronics for the past 5-8 years or so allowing highly flexible hardware designs as well as increased performance, decreased power consumption, and smaller size.

The FPGA is a type of intergrated circuit (chip) that can be customized by software (firmware) upgrades into a specific hardware ASIC (Application Specific Intergrated Circuit).  Before the advent of FPGAs, ASICs were designed and then created for a specific hardware task such as a logic IC, or IO control IC, etc.  FPGAs offer flexibility as they can be reprogrammed in the field to be a CPU (central processing unit – processor), an ADC (analog to digital converter), or other hardware that used used to be a bunch of separate ICs.  In more powerful FPGAs they can be several ASICs and combine roles, such of that of a CPU and and ADC with on board logic.  All in one chip!


Flex 6500 & 6700 Hardware Architecture (www.radiocronache.com)

The Flex 6000 series bristles with this advanced technology internally.

The Receiver:

The Flex 6000 series incorporates Direct Digital-Conversion (DDC) into the transceiver so that the received signal is amplified and filtered digitally at the basesband rather than at some high intermediate frequency.  This means lower current drain in the amplifier and active filters, and a simpler task of image rejection.  Selectivity of the receiver is fantastic with an Image rejection of ≥ 100 dB.  In most commercial ham gear even with IF DSP technology 60-80 dB is the norm.

The receiver performance of a Flex 6000 series is spectacular.  Just as good as the Flex 5000A or even better.  The 0.5 ppm TCXO on the 6300 and 6500, and the .02 ppm OCXO on the 6700 provide rock solid stability.  However you can also attach a GPS TXCO for GPS locked clock and stability.  This would make the device suitable for work in radio astronomy or other high precision sciences that need to utilize a near drift free RF receiver.

The Transmitter:

The transmitted signal is clean, with little to no harmonic distortion and the PA gives you 100 full watts out.  I found testing into a dummy load that the performance of the PA was pretty much spot on:

160m – 102 watts
80m – 102 watts
60m – 100 watts
40m – 104 watts
30m – 100 watts
20m – 104 watts
15m – 104 watts
17m – 104 watts
12m – 100 watts
10m – 100 watts

I used a Palstar DL2K dummy load through a WaveNode WN-2 station monitor using RG-213 type coax from the 6500 to the DL2K.  For the 6m measurement I could not use the Palstar DL2K, so I did not record a measurement for 6m.

The ATU:

The built-in antenna tuning unit (ATU) works quite well, being able to compensate for SWR mismatches that are 10:1 or higher.  The specification for the ATU is that it can match 16.7 Ohms to 150 Ohms across all HF amateur bands and 6m (160m – 6m).  I found the tuner in the 6500 to be excellent.  This is amazing since most radios with built-in tuners struggle to match antennas with mismatches 5:1 or less.

The best features of the Flex 6000 series SDR Transceivers:

Here is a quick list of what I think are some of the best features of the Flex 6000 series:

  • Tracking Notch Filter (TNF) – The tracking notch filter is a programmable notch filter that you can place on the panadapter and it can be stored in memory to remove interference on known frequencies.
  • AGC-T – This works like a traditional RF gain, but even better in Flex SDRs.  Removes the noise, but keeps the signal so that you can see what is going on in the bandwidth you are viewing.
  • Slice Recievers – Slice Receivers allow you to have 2 simultaneous receivers for the 6300, 4 simultaneous receivers for the 6500, and 8 simultaneous receivers for the 6700.
  • The Panadapter and Waterfall – High resolution smooth panadapter display and waterfall.
  • Excellent Receiver Performance – DDC with an image rejection of ≥ 100 dB, and outstanding sensitivity.
  • Nice integration in the digital domain with a PC and other PC applications – Really nice for digital modes as well as analog voice, logging, and rig control.
  • The new GUI is super nice looking – PowerSDR looks quite a bit more archaic than the newer SmartSDR counterpart.
  • Excellent Transmit Audio manipulation – Just like with PowerSDR, you can control your transmit audio through a 10 band EQ.  Downward Expander, presets for audio (DX settings) are also super awesome.  Excellent compression.  And now with version 1.3 transmit microphone settings profiles!
  • HUGE panadapter bandwidth – You can see 7 Mhz bandwidth at a time (YES 7 MHz!) with the Flex 6300, and 14 MHz (YES, I typed it correctly 14 MHz!) bandwidth with the Flex 6500 and Flex 6700.  With this you can really see what is going on across a huge swath of the radio spectrum.  Very cool!!!
  • Scrollback history in the waterfall – You can scroll back and see different transmissions in the waterfall for a historical perspective of what was going on within the bandwidth you were looking at.  This scroll back buffer can go back and show a full 24 hrs worth of activity!
  • Synchronious AM (SAM) – This was added with version 1.2 of the firmware.  PowerSDR has it, and now SmartSDR does too.  Makes it very nice to listen to shortwave and ham AM signals with out the annoying fading.   It also uses the carrier to tune the AM signal automatically so that when you are in a group with everyone off of the correct frequency by a few Hz, synchronous AM will automatically tune in each signal as they transmit.
  • Quick Record and Playback  – A awesome feature that lets you record audio and play it back.  Quick record records both sides of the transmission and then you can play it back and transmit the audio.  This is helpful when you perhaps when you have a ham buddy that wants to hear the quality of his audio transmission.  He could transmit to you and then you can play it back for him to hear.


The not-so-good:

Here is a quick list of what I think is not so good with the Flex 6000 series:

  • No FM mode (yet) – If you want to operate 6m or 10m FM repeaters, you cannot yet with the Flex 6000 series.  However FM mode will be included in a future software/firmware update.
  • The SmartSDR GUI is a bit confusing coming from PowerSDR – You have to hunt and find different options in little menus.  Sometimes I find that this is cumbersome for certain features.
  • Using DAX and Windows 7 sound controls – I had a helluva time getting DAX setup and sending audio properly at times.  You need to be patient with this, but I think more of the issue has to do with Windows 7 than DAX.  There is a guide that helps specifically with this feature.  I will link it down below at the bottom of this post.
  • Direct frequency entry is different from PowerSDR – Kind of cumbersome to use, but it works.  It would be nice if the interface just knew that when I type 14200000 without decimals, that it would know that I really wanted 14.200.000 and also when I click on the digits of the frequency I could change that individual digit like I could do with PowerSDR.
  • Problems with QSK when sending CW – I am not a big CW op, but I have heard that there were some problems with the radio giving a “pop” sound when sending CW with full break-in QSK.  I do believe that this problem was address though with version 1.2.  If you are a big CW op, then you should look into this issue before committing to a Flex SDR.  Flex may have addressed this and fixed this issue.
  • Dock-able GUI modules and separation from the main GUI window – I outlined this above, but I would really like to have separate modules on separate monitors.  That would be really awesome!  For example, the panafall (panadapter and waterfall) on monitor 1, RF power and SWR measurement on monitor 2, etc.  It would be awesome to reconfigure the GUI some.
  • CWX – A new feature with version 1.3 that will let you send CW with a keyboard, but it won’t decode CW for you.  It would be nice if there was a CW decode feature with it, but there are plenty of applications that will do this for you and you can send using CWX.
  • Paying for major software update releases – For the Flex 6000 series when you buy one, it ships with the latest software/firmware at the time.  Each major version of SmartSDR comes with free updates and maintenance releases for the life of that release.  However for major releases (version 1.x to version 2.x) they will charge a $199 fee.  This simply sucks. Your radio will not stop working or anything like that, but to get the extra features you have to shell out more money.  Hopefully they see how stupid this is and they don’t stay with this model.  Hey Flex…we like your products, we even buy your newest hardware and upgrade, don’t charge us for the software releases.
  • Only OS that is supported is Microsoft Windows OSes – No Linix yet, no Mac OSX yet as well.



Well that is my 2 cents worth for this Flex 6000 series review.  Over all I would recommend a Flex to anyone that was interesting in owning an SDR.  A Flex SDR really takes your radio operating experience to the next level in technology.



Here are some links to various files and information on the Flex 6000 signature series SDRs:


Flex Radio Systems – Main page for the 6000 series SDRs – http://www.flexradio.com/amateur-products/flex-6000-signature-series/

Flex Radio Systems – SmartSDR Release Roadmap  – https://k9xn.files.wordpress.com/2014/08/smartsdr_roadmap.pdf

Flex Radio Systems – SmartSDR v.1.3 release notes – http://www.flexradio.com/downloads/smartsdr_v1-3-0_release_notes-pdf/

Flex Radio Systems – SmartSDR v.1.3 Windows software users guide – http://www.flexradio.com/downloads/smartsdr-software-users-guide-pdf/

Flex Radio Systems – Community Support portal – https://community.flexradio.com/flexradio/categories

Flex Radio Systems – YouTube Channel (lots of good videos here!) – https://www.youtube.com/user/FlexRadioSystems


I hope you enjoyed this post, and as always if you have anything to add, put it in the comments below…


73! and Flex ON!!!!   Nick N9SJA



4 thoughts on “Flex Radio Systems – Flex 6500

  1. Great review Nick. I was an early adopter of SDR receivers (SDR14, Ciao, Funcube Pro+, etc) and have ordered a Flexradio 6500 to complement a FTDX5000 so the review was very helpful especially the critique on DAX and GUI.

    • Peter,

      They have since released SmartSDR 1.3.8 but I keep hearing about problems with DAX not working as it should. SDR is awesome and I still have a Flex 5k, that is just sitting here (not sure if I will sell it or not) and love SDR and the Flex line. If your looking at buying a Flex, then I would suggest subscribing to their mailing listserv for good information from throughout the community. Thanks for the kind words on the review and I know you will enjoy your Flex 6500!

      73 de Nick N9SJA

  2. Thanks for review Nick. I previously owned a 5000, still own a 1500, and just ordered a “preloved” 6500.
    Doug K4LY

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