SDR# SDR# Carrier Off Center

[CommRX]

Good evening. This is not a performance issue, more of a curiosity. AirSpy mini running SDR# with Frequency Scanner plugin. Active Channel Spectrum activated. Nearly every VHF air channel is slightly off center from the peak signal. Reception is fine, just wonder why its off center (from the blue line), or is this just the way it displays. I have tried "lock carrier" setting both on and off. I haven't noticed this on other bands as traffic is light tonight. If it makes any difference, as you can see, channels have been programmed into Frequency Manager.

 

[Dirk_SDR]

Did you already make a frequency calibration with your Airspy Mini?

 

[dave3825]

Nearly every VHF air channel is slightly off center from the peak signal. Reception is fine, just wonder why its off center (from the blue line), or is this just the way it displays.

Did you already make a frequency calibration with your Airspy Mini?

Whenever I am in airband, I see many of those off on the display. When I go to rail or other vhf, pretty much dead on. If the Airspy ppm is dead on, that's really all that matters.

My Airspy mini is off by 1.2 ppm so using it with Sdr# is useless since there is no ppm adjustment in that software. Fortunately Sdrtrunk and Unitrunker 2 allow ppm adjustment on Airspy so that's where I mainly use it.

 

[CommRX]

I have not. Is this something that can be done via software, such as the "SV PPM" setting under configuration, or do I need external devices? While I have test equipment, including a signal generator, its max output of 1 MHz is well below the lower end of the mini's reception. On further investigation, a local 155 MHz FM law enforcement signal is received at 2.5 KHz over the assigned frequency. I had first assumed the transmitter was off a bit. Again, not enough to affect reception, but I still have interest in making things work to their best performance.

 

[CommRX]

Whenever I am in airband, I see many of those off on the display. When I go to rail or other vhf, pretty much dead on. If the Airspy ppm is dead on, that's really all that matters.

My Airspy mini is off by 1.2 ppm so using it with Sdr# is useless since there is no ppm adjustment in that software. Fortunately Sdrtrunk and Unitrunker 2 allow ppm adjustment on Airspy so that's where I mainly use it.

As my primary use of the SDR is non-trunked systems at this point, I had been using the frequency scanner to hit wide swaths of spectrum at once. I have not tried any trunked systems. I have used HDSDR from time to time,but will check out the other software listed and see what I can get into.

 

[dave3825]

I have not. Is this something that can be done via software, such as the "SV PPM" setting under configuration, or do I need external devices? While I have test equipment, including a signal generator, its max output of 1 MHz is well below the lower end of the mini's reception.

You need to first, be able to inject a known and accurate signal (preferred GPSDO) into the airspy. Then you use the software designed to calibrate it.

Calibration​

Starting from June 2016, Airspy R2 units are calibrated in the factory up to ~0.05 PPM. This calibration is written in a different memory location than the one used for the firmware, so upgrading the firmware won’t delete it unless you do it by yourself.
The procedure to recalibrate a unit is as follows:

• Open the calibration tool (AirspyCalibrate.exe),
• Clear the correction,
• Reset the device (unplug/plug again),
• Inject a known and accurate signal,
• Set its exact frequency in the tool,
• Click on “Calibrate”,
• Reset the device.

Note that this procedure requires firmware version 1.0-RC9 or better.

Quick start guide - airspy.com

High Performance Low Cost Software Defined Radio Receivers

airspy.com

 

[thewraith2008]

The small spectrum in the channel analyzer panel will show the same as the SDR# frequency spectrum but you generally need to alter the FFT display options (FFT resolution and Zoom) to see that.

I've seen center frequencies all over the place in what can be described a intentional de-tuning as no-one could tune a radio that badly.
In the image above, it is 510 Hz off the expected center frequency.