Last modified 6 years ago Last modified on 03/13/11 22:06:06

Using Fldigi with PowerSDR

These instructions have been tested on Windows XP. Please email us with any changes needed for more recent versions, or add the information to this page.

  1. Installation
  2. VAC configuration
  3. VSPmgr configuration
  4. PowerSDR configuration
  5. Fldigi configuration


  1. Download all the software pieces you need.
    1. PowerSDR.
    2. The Flex firewire driver.
    3. Fldigi, either an alpha version or a release version.
    4. The PowerSDR rig description XML file for fldigi's RigCAT.
    5. VAC v4.09. You must purchase the release version. You MUST NOT run the demo version.
    6. VSPmanager.
  1. Install the Flex firewire driver.

I am not going to address which one because it depends on which version of PowerSDR you are running. For the purposes of these instructions you should assume that you are installing the latest released version of the driver. Follow Flex's instructions. I use a buffer size of 2048 and a sample rate of 96 ksps.

  1. Install PowerSDR.

Again, I assume you are installing the release version and it is compatible with the driver you have installed. Make sure that the sample rate and buffer size (Setup→Audio→Primary) match the driver. (The new driver may automatically track the settings in PowerSDR. Even so, verify they are set the same.)

  1. Make sure that PowerSDR both transmits and receives. It should run normally.
  1. Install VAC.

Follow the instructions provided with VAC. There are some differences between installing with XP, Vista, and Win7 so be sure you read the instructions, most notably in how to deal with unsigned drivers in Vista and Win7.

Note: you MUST use the purchased version of VAC 4.09. You MUST NOT use the demo version! Go ahead and buy it. It will work.

VAC configuration

  1. Open the VAC control program Control Panel and configure VAC.

You get to the VAC control panel from the Windows "Start" button under "All Programs". Look for the "Virtual Audio Cable" menu item and then select Control panel.

In the upper left hand corner of the control panel for VAC you want to set it for 2 cables. I left all the other settings alone. Click the "Set" and "Restart" buttons. If you want to check, here is what I have all the VAC parameters set for:

SR 22050..48000
Max inst20
Ms per int 7
Stream fmt"Cable Range"
Volume control(unchecked)
Connected source lines"line"
Clock corr %100.000

Once you click the Restart button you should see cable 1 and cable 2 appear in the lower section of the Virtual Audio Cable Control Panel (if not sooner).

VSPmgr configuration

  1. Now set up VSPmgr.

For now you will create a single port-pair that will be used for CAT control of fldigi by PowerSDR. Both rig control and PTT will be handled by this single port-pair.

Start the VSPmgr utility. It will show you your physical serial ports and offer to create a virtual port pair. Make sure that your virtual port pair starts above any possible physical serial port numbers. It is possible for virtual ports to conflict with physical ports if the physical ports are added later. In my case I created port pair COM6/COM7 for communications between PowerSDR and fldigi. Most of the examples use COM6/COM16 or COM7/COM17 but I discovered that some programs, e.g. WSJT and WSPR, have problems with port numbers above 9. Therefore, creating a port-pair where both COM numbers are below 10 means that the configuration may be used later on to communicate with these other programs.

Setting up the port-pair is nothing more than selecting the desired values for End 1 and End 2, COM6 and COM7 in this case, and clicking the "Create Pair" button.

PowerSDR configuration

  1. Set up PowerSDR to use VAC.

With PowerSDR running, click on "Setup", select the "Audio", and then the "VAC" tabs to display the VAC setup page. Set the options as listed here:

Enable VAC(checked)
DriverWindows WDM-KS
InputVirtual Cable 2
OutputVirtual Cable 1
Auto Enable(checked)
Buffer Size512
Sample Rate48000
RX gain0dB
TX gain-1dB

Leave the other boxes, i.e., Mono/Stereo?, Latency Manual, and Direct I/Q, unchecked. (The check box for "Allow PTT override VAC for Phone" is a personal preference. I do not have it checked on mine because I don't want to accidentally press PTT and have my mic go live when operating digital modes.)

  1. Set up CAT in PowerSDR.

With the "PowerSDR Setup" window still open from step 8 above, click on the "CAT Control" tab.

In the upper right of the window set "ID as:" to "PowerSDR".

Under "CAT Control" make sure the "Enable CAT" check box is unchecked. You need to do this to be able to change the settings. Set the following values:


After you have set the above values, check the "Enable CAT" box.

Click the "Apply" button at the bottom of the frame.

(Note: with a virtual serial port baud rate, parity, and number of stop bits really has no meaning. I specify them here just to make sure that no one is confused by an omission.)

Do not check the box "Enable PTT". That is for hardware PTT using a separate serial port. Fldigi will use a CAT command to key and unkey the Flex rather than RTS or DTR on a separate serial port.

Also, do not enable either of the RTTY offsets. If you do it will cause the frequency display on the waterfall in fldigi to be incorrect. Since fldigi does not adhere to a strict offset of 2125 Hz for the mark tone in RTTY, the offset will just confuse things. It also is meaningless in other modes, like PSK.

If you are a veteran RTTY op you will find that running digital modes with fldigi is a bit of a departure because it doesn't use a fixed offset. Rather you can select any station on the fldigi waterfall just by clicking on it and you won't have to retune the rig. After a few minutes of operating this way you will see the advantage and how it reduces confusion.

  1. Set the "sweet spot" for PowerSDR's click-tune.

One of the nice features of PowerSDR is the ability to use the mouse to click on a signal that appears in the panadaptor or the waterfall and have the radio tune to that frequency. Normally that would set the VFO to the frequency represented by the cursor bar which works for SSB (where the carrier should go) and CW (already has an offset for the beat note) but does not work for digital modes because you would have to displace the cursor by the proper amount from the signal. To make this easier PowerSDR lets you specify a click-tune offset. I use 1500 Hz and then have fldigi normally use a 1500 Hz offset for normal operation (1500 Hz "sweet spot"). This allows me to center the PowerSDR frequency cursor on the middle of the digital signal I want to receive, and have the signal appear at 1500 Hz in fldigi. In most cases I can get close enough for good copy and the AFC in fldigi takes care of the small error, if any (usually less than 10Hz). This makes hopping around to see different signals very easy, much easier than using the waterfall in fldigi. Here is how you set the digital mode offset in PowerSDR:

From the PowerSDR Setup window click on the "General" tab. At the lower left corner of the window you will see a box labeled "Click Tune Offsets (Hz)". I set them to 1500Hz so that when I click on the center of a digital signal, it will cause that signal to be centered at 1500 Hz in fldigi.

  1. Make sure that PowerSDR is running.

If not, click the PowerSDR "Start" button.

This completes the configuration of PowerSDR to use fldigi. Now we setup fldigi itself.

Fldigi configuration

  1. Install fldigi.

(This document assumes that you are running fldigi on the same computer that is running PowerSDR so that means you have installed the Windows version of fldigi.)

  1. Install the RigCAT XML description file for PowerSDR.

On Windows, fldigi keeps its configuration files in the fldigi.files directory (folder) in the user's home directory, i.e. C:\Documents and Settings\<username>\fldigi.files (assuming XP). In fldigi.files you will find a folder named "rigs". Place the RigCAT description file for PowerSDR, i.e. PowerSDR.xml, into the rigs folder. Note: in Windows 7 and using Fldigi V3.20.28, you must run FLDIGI.exe first to be able to see the rigs folder.

  1. Start fldigi to perform configuration.

You may get an error message because the audio input has not been selected. Just dismiss any error messages. Click on the "Configure" menu item to perform the configuration process for fldigi. It will offer you several options. It doesn't matter where you start but you need only configure "Sound Card" and "Rig Control" to get fldigi working with PowerSDR.

  1. Click "Sound Card" to start the configuration of fldigi to use VAC to communicate with PowerSDR.

The "Fldigi configuration" window will pop up with the "Audio" tab selected. If the "Devices" sub-tab is not selected, select it.

Click the "PortAudio" selection button if it is not already selected. For "Capture:" set "Virtual Cable 1". For "Playback:" set "Virtual Cable 2".

The waterfall should come alive just as soon as you set the proper virtual cables for input and output. In my case that didn't happen. I found that I have had trouble with fldigi getting the sample rate correct when in "Auto" or "Native" mode. If you see the following error message, "PortAudio error: Invalid sample rate," click on the "Settings" sub-tab and set both the Capture and Playback sample rates to 48000 to match the setting for VAC in PowerSDR.

As long as you are at the "Settings" window for "Audio" you can check the following:

  • For the sample rate converter ("Converter") set "Medium Sinc Interpolator" if it isn't already set.
  • Corrections should all be set to zero.
  1. Set up RigCAT to control PowerSDR and vice-versa.

Click the "Rig" tab at the top of the configuration window. Fldigi supports several ways to communicate with your rig. We are going to use RigCAT. (Hamlib works too but I am not going to cover that.)

Click on the "RigCAT" subtab to bring up the RigCAT configuration window.

Click the "Open" button next to the "Rig description file:" box. That will let you select the proper rig description file. Navigate to the rigs folder if not already there and select "PowerSDR.xml".

Below that are some timing settings for communication between fldigi and PowerSDR. These are influenced by the speed of your computer. I use the following:

Retry interval50ms
Write delay25ms

If you get error messages from fldigi about communications failure with the rig, try increasing the Write delay and/or Retries. If you notice that the response time is too long, i.e. frequency changes in PowerSDR or fldigi take noticeably too long to be reflected in the other program you can try reducing the delays.

  • In "Device:" select "COM7".
  • Click the radio button next to "CAT command for PTT".
  • Go back up to the top and click the check-box for "Use RigCAT".
  • Now click the "Initialize" followed by the "Save" buttons.

If rig control is working you should see the VFO frequency for PowerSDR appear in the frequency display of fldigi. If you tune PowerSDR, the frequency display of fldigi should automatically follow. If you click the "T/R" button near the lower right side of the fldigi screen, the Flex radio should key up. These two tests confirm that RigCAT is working.

  1. Set the "Sweet Spot" so that fldigi works properly with the click-tune function of PowerSDR.
  • Click on the "Misc" tab at the top of the configuration window. Click on the "Sweet Spot" sub-tab.
  • Set the CW value to the frequency of the CW note you use. Set "RTTY" and "PSK et al." both to 1500.
  • Click the "Save" button.
  1. Set the operator and QTH info.

Click on the "Operator" tab at the top of the configuration window and fill in the information. When done, click the "Save" and "Close" buttons.

At this point, fldigi and PowerSDR are set up and should be working together. There are quite a few other things that can be set up in fldigi to suit your operating preferences but that is beyond the scope of this document. Refer to the fldigi manual to see what all the other functions do.

Written by Brian Lloyd, WB6RQN/J79BPL (original mailing list thread).