SDR-4+ frequency calibration using off-air signals

Chris Moulding

One of the problems with the Yahoo group is that we lost the facility to search through the posts. I'm going through my email to find some of the replies I sent to questions about the SDR-4+ receiver and other products.

When we test the SDR-4 the frequency calibration is checked against a GPS disciplined oscillator or on-air standard which has an accuracy measured in parts per billion. The calibration correction for the Si570 oscillator is stored in a microcontroller in the SDR-4.

If the correction data gets changed or corrupted then the Si570 reverts to a factory default figure which may be several kHz from the true value.

I've devised a method for SDR-4 users to check the calibration using off air frequency standard signals using HDSDR.

First connect the SDR-4 to the PC and an antenna. Open HDSDR and tune the LO to 10.010 MHz. In the UK a time signal beacon should appear on 9.996 MHz. This is the frequency of the frequency standard station RWM.

If the receiver is in calibration the time signal or morse transmission will be seen exactly on the 9996 kHz line on the waterfall.

If the frequency is offset by any significant amount then in the HDSDR program, left click on the ExtIO button just to the right of the Tune frequency display. Click on Open to start the program.

This will bring up the SoftRock Configuration Tool. Click on the Calibrate tab.

In this window enter the frequency that RWM appears on the waterfall in the Tuned box.

In the Real box enter the actual transmitted frequency of RWM (in this case 9.996 MHz.

Click Calibrate.

If the frequency was out by several kHz you may have to retune the receiver to 9.996 MHz to find RWM again. It should now be on frequency.

As a final check set the receiver to USB and put the red line in the centre display on 9.995 MHz so that RWM appears as a 1 kHz audio tone in the speakers. In the bottom waterfall RWM should be on the 1 kHz line. If not repeat the calibration process again to correct any small errors seen.

When the calibration process is completed click on Close on the SoftRock Calibration Tool and close it completely by clicking on the X at the top right hand corner.

I've suggested the use of RWM as it's usually a strong signal in the UK. Here's a link to a list of other frequency standard stations that may be used:

Regards and 73,

Chris, G4HYG 

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