Topics

SDR and Ubitx freq not the same.

Werner de Vos
 

I have a uBITX V5 with the CEC v1.2 fw and installed a RTL SDR dongle as mentioned on de CEC blog.
Everything works only the RX frequenty on SDR is not the same as on the radio itself.

So RX on SDR console is for example 14.138MHz, when I switch back to the uBITX I have to tune to 14.1395 (which is the actual freq)

Is it possible to adjust that somewhere?

73 Werner pa3gyv

Arvo W0VRA
 

To compare the frequencies of the µBITX and the RSP1A, which I use together, I did an FT8 transmission with the µBITX set to 20M, 14.074MHz and the FT8 signal at 2000Hz.  Without an RX antenna connected to the RSP1A, there was still a strong signal showing up in the WSJT-X FT8 screen.  So the PC with the µBITX was doing the 14.074MHz + 2000 Hz, and the laptop with the RSP1A running FT8 was showing that signal as 14.074MHz + 1986 Hz.

I then applied the CEC principles of calibration to the µBITX and got them within 1 Hz, sometimes 0 Hz.

They move around a little bit, probably because their temperatures are changing and affecting them differently.

Also, if someone is using a transceiver with a common oscillator for RX and TX (like another µBITX) and responds to you on FT8 with their frequency the same as yours, you'll see the same difference between the SDR and the µBITX.

W2CTX
 

Can you verify if your last sentence is true no matter what you have selected
in the Memory Manager for frequency display( Send or Receive )?

Thanks, rOn

On September 16, 2019 at 1:01 PM "Arvo W0VRA via Groups.Io" <arvopl=yahoo.com@groups.io> wrote:


To compare the frequencies of the µBITX and the RSP1A, which I use together, I did an FT8 transmission with the µBITX set to 20M, 14.074MHz and the FT8 signal at 2000Hz.  Without an RX antenna connected to the RSP1A, there was still a strong signal showing up in the WSJT-X FT8 screen.  So the PC with the µBITX was doing the 14.074MHz + 2000 Hz, and the laptop with the RSP1A running FT8 was showing that signal as 14.074MHz + 1986 Hz.

I then applied the CEC principles of calibration to the µBITX and got them within 1 Hz, sometimes 0 Hz.

They move around a little bit, probably because their temperatures are changing and affecting them differently.

Also, if someone is using a transceiver with a common oscillator for RX and TX (like another µBITX) and responds to you on FT8 with their frequency the same as yours, you'll see the same difference between the SDR and the µBITX.


Arvo W0VRA
 

Very good question, and I might not be answering it.
 
I don't (at least I don't mean to) have a different frequency in my µBITX for TX and RX; I'm not running split.  So for me they should always be the same.  And if I understand the theory of operation, no matter how screwed up the calibration is, the µBITX is running the same frequency whether in or out in that state.
 
If that's the case then:
 
Suppose I send out what the µBITX says is 14.074MHz + 2000Hz FT8.  Let’s call this 14.076000 MHz.  It could be that the transceiver actual transmitted frequency is 14.073992MHz, and the audio tone is 1996 Hz or all together 14.075988, but my system says we’re sending out 14.076000.
 
Suppose the contact that replies does not have “Hold TX Freq” checked and responds to the CQ.  His FT8 screen might show me at 1500 Hz or 2500 Hz, but it doesn’t matter, because if his rig is using the same oscillator for TX and RX, the math will wash out all the errors and I’ll get back exactly what I sent him, a signal at 14.075988.
 
Since I’m getting back exactly what I sent him, if there is a difference between what I sent at 2000 Hz and what I get back, it’s the difference between my absolute TX and RX frequencies.  So if the FT8 screen shows his reply at 2017 Hz, I know I that my µBITX and my SDR disagree by 17 Hz at 20M.
 
At least that’s the way I understand the math, and it's a very elegant way that FT8 naturally keeps all of us neat and clean and polite with the small slice of band.
 
Someone did a youtube showing how to use split for FT8 that takes advantage of that.
 
Suppose you want to use a 3000 Hz tone for FT8, but your system works better at sending out a 1000 Hz tone than a 3000 Hz tone.  You’re working 20M, set your TX freq to 14.076 and use the 1000 Hz tone.  To the person listening on 14.074, it sounds like a 3000 Hz tone.  Of course, you listen at 14.074 so that the system hears things normally.
 
Explaining this stuff would be a lot easier with a whiteboard and a pot of coffee.
 

Dexter N Muir
 

Make that pot a "proper copper coffee pot"! ... and say it fast when you order it ... :)  Brownie points for Captain Janeway, too.
73
Dex, ZL2DEX

Werner de Vos
 

I think I have to explain a little more. My ubitx is calibrated and works on the right frequency. I can oparate FT8 or WSPR without any problem.

The thing is, when I connect a RTL Dongle (hooked up the CEC way, with cat control via SDR console) all works fine exept the RTL-SDR RX frequency is off. So for example. receiving with SDR 14.076MHz is 14.0775. 

It can possibly be the SDR dongle that is somehow off frequency.

Bill Lamm
 

I have had the coffee today.. maybe need to keep a white board at my side when I read these posts?!

Arvo W0VRA
 

On Tue, Sep 17, 2019 at 12:55 AM, Werner de Vos wrote:
The thing is, when I connect a RTL Dongle (hooked up the CEC way, with cat control via SDR console) all works fine exept the RTL-SDR RX frequency is off. So for example. receiving with SDR 14.076MHz is 14.0775. 
From this example, are you saying that your software shows the SDR at 14.076, but it's actually receiving at 14.0775?

Then the RTL-SDR and your software need to be calibrated.

Which SDR software are you using?