DSDPlus PPM correction

[DaveNF2G]

I am having trouble making the connection. One of my dongles has a large tuning error, which FMP24 indicates in Hz. To correct it, a PPM adjustment has to be entered. How do I calculate PPM from Hz?

 

[Unitrunker2]

Warp = Error(hertz) * 1,000,000 / frequency

Frequency​	Error (Hz)​	Warp​
150000000​	75​	0.5​
850000000​	425​	0.5​
450000000​	225​	0.5​
150000000​	150​	1​
850000000​	850​	1​
450000000​	450​	1​

 

[prcguy]

One PPM (parts per million) is one Hz at one MHz, 10Hz at 10MHz, 100Hz at 100MHz and so on.

 

[mtindor]

I am having trouble making the connection. One of my dongles has a large tuning error, which FMP24 indicates in Hz. To correct it, a PPM adjustment has to be entered. How do I calculate PPM from Hz?

I'm not sure what you are asking, but if you are seeing a large tuning error on your dongle, while focused on FMP24 hit 'p/P' to go up and down to correct. It will show at the top of the FMP24 window how much it was corrected. Use p/P to get yourself dead nuts on frequency and then you can use the value reported at the top of the FMP24 window to add a -P0.0 (no correction) or -P-1.3 or -P1.3 or something like that to your FMP24-CC.bat file.

 

[prcguy]

I'm not sure what you are asking, but if you are seeing a large tuning error on your dongle, while focused on FMP24 hit 'p/P' to go up and down to correct. It will show at the top of the FMP24 window how much it was corrected. Use p/P to get yourself dead nuts on frequency and then you can use the value reported at the top of the FMP24 window to add a -P0.0 (no correction) or -P-1.3 or -P1.3 or something like that to your FMP24-CC.bat file.

As others have said, PPM error would be different on every band. So I'm not sure hard coding is best option. Just use p/P when focused on FMP24 to get it dead nuts on when you need to. And it's likely good for as long as you have that FMP24 session running.

The PPM error would be the same on all bands, the amount of frequency offset will be different and higher on higher bands. 5PPM would be 500Hz off at 100MHz but 5KHz off at 1,000MHz.

 

[mtindor]

The PPM error would be the same on all bands, the amount of frequency offset will be different and higher on higher bands.

Ok cool. I'll modify my last post.

tnx

 

[prcguy]

Ok cool. I'll modify my last post.

tnx

But then I would have to remove mine.

 

[mtindor]

But then I would have to remove mine.

Not at all. You explained what I couldn't regarding the correlation between PPM error and actually frequency offset.

 

[DaveNF2G]

I'm not sure what you are asking, but if you are seeing a large tuning error on your dongle, while focused on FMP24 hit 'p/P' to go up and down to correct. It will show at the top of the FMP24 window how much it was corrected. Use p/P to get yourself dead nuts on frequency and then you can use the value reported at the top of the FMP24 window to add a -P0.0 (no correction) or -P-1.3 or -P1.3 or something like that to your FMP24-CC.bat file.

Yes, that is the immediate solution. But when I start the same dongle again, I need to wait for traffic and then repeat the process. This can take forever on low traffic systems.

However, your third sentence is one way to accomplish my goal.

 

[DaveNF2G]

The PPM error would be the same on all bands, the amount of frequency offset will be different and higher on higher bands. 5PPM would be 500Hz off at 100MHz but 5KHz off at 1,000MHz.

This is the way I understand it also.

 

[Ubbe]

You can use something like SDR# that has a spectrum view and use a digital transmitters frequency, most of them are frequency controlled by a GPS, and zoom in on the channel and adjust the ppm value in SDR# and the graphical bandwidth can be clicked and drag to just cover the signal at both the upper and lower end of the modulated signal. Then write that ppm value on the dongle.

My two RTL-SDR v1 dongles had both a +1,5ppm offset and two new v3 had both a -4ppm offset.

/Ubbe

 

[DaveNF2G]

I have 24 RTL-SDR v3 dongles. I'd like to hardcode the offsets for the ones that need it just once per dongle.

What made me aware of the problem is that one dongle received a particular DMR channel perfectly when tuned .0125 MHz high. I'll have to check the PPM error again and see if it is any multiple of 12.5.