wiki:Frequenzabgleich

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Frequency calibration


Calibration of the si570

In the FiFi SDR there is an Si570 VCO whose output frequency is derived from an internal reference oscillator. Each example of the Si570 therefore deviates slightly from the nominal reference of 114.285 MHz, which results in a different frequency error depending on the reception band. If the actual reference frequency is 114.3 MHz for example (an error of +131ppm), then a signal in the 40m band would appear 1 kHz too low, but at 15m this would be 3 kHz too low.

The calibration can be done either manually or automatically.

Manual Calibration

All Si570s leave the factory pre-adjusted to a set output frequency. If you know this output frequency, it is possible to work out the reference frequency measured in the factory with the adjustment values used by Silabs. The CFGSR program and rockprog use this principle to simply correct the referency frequency.

Note: before using CFGSR, the firmware of the FiFi SDR must be updated to the latet issue! See the instructions here. It is also recommended not to carry out this calibration when the onboard sound card of the FiFi SDR is active as this often gets out of step and can only be stabilised by turning off and on again.

  1. Getting the start frequency of the Si570

We're looking for the output frequency of the Si570 when it is first switched on. For this we need to know which type of Si570 is being used, and this is shown by an engraving of the type number on the module. The relevant part of the type number is shown in the following picture. The start frequency of this module can then be found in the table.

Si570 in the FiFi-SDR

Known Si570 versions

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Print

Start frequency [MHz]

000209

14.1

000266

15

000589

30

  1. Calibration with CFGSR

Start CFGSR (Version 2.3) and go to the Calibrate menu. The currently set reference frequency is shown at the bottom (3). If no calibration has been made so far (or after pressing the reset button on the left) then 114.285 MHz will appear. To carry out the calibration, enter the previously established start frequency in the Factory startup frequency field (1). In the example shown below this is 30 MHz (this is the value for the Si570 used by Box73). Then click on Calibrate (2) and, if no error was made when entering the start frequency, successful calibration will be shown by the corrected value for the referency frequency being displayed.

CFGSR-Screenshot. Bild von DF9DQ, bei Seitenerstellung neu hochgeladen.

Automatic calibration with rockprog

Automatic calibration is possible using our command-line tool rockprog. Rockprog determines which Si570 is in use by the register settings. The programm can be downloaded here.

To carry out the calibration it is sufficient to enter the command:

rockprog0.exe --autotune -w

Removing a frequency shift of 12 kHz

A constant 12 kHz shift can occur, depending upon the SDR software used. Some programs need the offset (e.g. RadioJet or Dream), while others are confused by it (e.g. Rocky). HDSDR automatically removes any shift.

The FiFi SDR can fundamentally be set to any chosen frequency shift. The safest way to change the offset is to use the Easy operation mode of the Easy operation mode of the Rockprog tool.

You can also run rockprog directly from the command line. The Offset can be removed using the command

rockprog0.exe -w --offset --subtract=0

removing 12 kHz offset with rockprog0

If it is necessary for any other programs, it can be reinstated with

rockprog0.exe -w --offset --subtract=0.012 

The programm RadioJet provides an additional option to quickly compemsate the unwanted offset. Just click on the "ExtIQ" button on the lower left:

removing 12 kHz offset with rockprog0
In the new window, enter the Xtal.Correcture value -600. Unfortunately there will still an offset of the center frequency remain in the optional "Panorama" window, but the scaling will be correct.


Automatic complete calibration

As explained above, all necessary settings for calibration of the si570 as well as for constant offsets can be made with Rockprog at command line level. It is therefore sensible to gather these commands in a batch file. A BAT-file with the necessary commands incl. programming of the switching points of our preselector can be found here.

Last modified 4 years ago Last modified on Mar 10, 2013 12:25:36 PM

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