FMPA ppm

[dave3825]

Is there is any way to force an Airspy to run @ +1200 Hz. I have a mini I would like to use with DSDPlus but its off. In Unitrunker about 0.9 ppm seems to be dead on in 700mhz and decodes perfect. In Fmpa its showing +1200Hz when using autotune @ 700MHz and +509hz @ 477 MHz.

 

[cg]

There is an Airspy calibration tool but not sure if it works for the Mini.

 

[wgbecks]

Is there is any way to force an Airspy to run @ +1200 Hz.

FMPA to my knowledge does not support PPM (frequency compensation) adjustment parameters. You'll need to run the calibration tool that works with the Mini's as well as with the R2's to dial in your device and is a process that I've successfully performed numerous times.

 

[dave3825]

I do not have anything to inject a signal. What did you use?

 

[wgbecks]

I do not have anything to inject a signal. What did you use?

I have obtained good results calibrating against a P25 control channel frequency from a site that uses GPS referenced frequency supplies such as would be typical of simulcast networks that require high frequency precision.

I will tell you that calibrating an Airspy with the calibration tool and regardless of your frequency source often times requires several attempts to get the device properly calibrated. When using an off-air signal as a calibration source the signal must be fairly strong and stable meaning that there is virtually no fading due to multipath or extraneous RF noise or interference.

The Airspy devices have a bit of a warmup cycle in that the internal TCXO will drift from a cold start but will gradually settle down after the device runs for several minutes and as the internal components build heat that ultimately effects the internal oscillator (reference) frequency. It's a bit of a bit of a challenge but I clear the existing calibration using the tool the reconnect the device to an SDR application, let it run for a few minutes then note the degree of frequency error.

From here I do the math to relate that frequency error into terms of parts per million (PPM) either positive or negative and then proceed back
to use of the calibration tool. Here I keep pressing the calibrate function repeatedly while observing the correction (PPM) as indicated until
the tool outputs a value that is very close to the calculation.

Retest the device using your SDR application to verify the calibration and repeat the above as necessary until you obtain the necessary
correction (PPM) value that results in the least frequency error one the Airspy has been operating for several minutes and has had a change
to come up to operating temperature.

The screen capture below is of an Airspy Mini running with FMPA following calibration against an off-air P25 control channel. You can see
that the overall error is only -19 Hz at an operating frequency of 853.925 MHz. or 0.0225 PPM.

 

[wgbecks]

If you're looking for the Airspy calibration tool, it can be downloaded from their website but it's not at all obvious in that the executable
file is contained within the sdrsharp-x68-dotnet4.zip archive.

This file downloads when clicking on the hyperlink "For the latest dotnet 4.x build (1784) click here" (See the screen capture below)
or you can directly enter URL:

https://airspy.com/download/sdrsharp-x86-dotnet4.zip

 

[slicerwizard]

you can directly enter URL:

https://airspy.com/download/sdrsharp-x86-dotnet4.zip

No, it's https://airspy.com/downloads/sdrsharp-x86-dotnet4.zip

 

[thewraith2008]

I wouldn't be so quick to run the calibration of the Airspy.

You said the Airspy is OK in Unitrunker.
Do frequencies in other programs look OK?

If frequencies look OK in other programs, then FMPA or it's options need correcting, not the Airspy.
If you calibrate for FMPA and it's at fault, then the Airspy and every other program will now be wrong.

 

[wgbecks]

No, it's https://airspy.com/downloads/sdrsharp-x86-dotnet4.zip

Sure enough. I dropped the "s" at the end of ----/download/---- Thanks!

 

[dave3825]

I wouldn't be so quick to run the calibration of the Airspy.

You said the Airspy is OK in Unitrunker.
Do frequencies in other programs look OK?

If frequencies look OK in other programs, then FMPA or it's options need correcting, not the Airspy.
If you calibrate for FMPA and it's at fault, then the Airspy and every other program will now be wrong.

In UT 2.0 it takes a -1.3 ppm setting to get good decodes @ 171.xxx, 500.xxx @ 7/800.

In Sdrtrunk, the reported error showing is +1.2 @ 171.xxx, 477.xxx and@7/800 So setting -1.2/3 yields good results. Problem is when using with Sdr# and Fmpa there is now way for me to apply that -1.2. If ppm on an airspy can be adjusted within UT and Sdrtrunk, then it should be the same with Fmpa and Sdr#

 

[TDR-94]

The screen capture below is of an Airspy Mini running with FMPA following calibration against an off-air P25 control channel. You can see
that the overall error is only -19 Hz at an operating frequency of 853.925 MHz. or 0.0225 PPM.
View attachment 120365

When I look at the same FMPA spectrum display, on my computer, the center frequency is never correct. It's always way off from what I set it to. My Airspy R2 is typically operating at around 0.3-0.4 PPM, so it's not the Airspy R2. Is there a way to get the FMPA spectrum display to display the correct frequencies?

 

[slicerwizard]

When I look at the same FMPA spectrum display, on my computer, the center frequency is never correct. It's always way off from what I set it to. My Airspy R2 is typically operating at around 0.3-0.4 PPM, so it's not the Airspy R2. Is there a way to get the FMPA spectrum display to display the correct frequencies?

With FMPA, you don't tune the center frequency. The center frequency is never of much interest.

 

[wgbecks]

I think the OP might be confused by the Spectrum Zoom Overlay that normally appears at the center of the FMPx spectrum (FFT) screen.
Here FMPA is tuned to 853.925 MHz as seen in the screen capture below, and illustrated by the mouse pointer aligned with the narrow spike
at the left side of the screen. The larger display at the center being he Zoom Overlay that can be toggled on/off with the "Z" key.



Here again is the same signal with the mouse pointer positioned at the center of the Zoom Overlay that happens to represent the offset frequency of the SDR that doesn't represent the actual RF frequency the SDR is tuned to, but rather the standard offset of 750000 Hz
associated with the Airspy Mini's. Doing the math, subtract the RF tuned frequency of 853.935 from the Zoom Overlay frequency of 854.675 for a difference of 0.750 MHz (750000 Hz).