A single line of code....

A while back I was trying to receive AIS (ship traffic, bit like radar) messages with a rtl_sdr stick. Those are around 160 MHz or so, but need an extremely precise frequency setting. Using rtl_fm and aisdecoder to receive those did not work until I used the parts per million correction in rtl_fm, experimentally found out (used a loop that incremented it, let it run all night, and then found the best reception value possible from that, and programmed it in the scripts).

Now yesterday I was trying to get the Raspberry Pi PLL (500 MHz source) to make a precise

9.something MHz for the 2.4 GHz /256 prescaler phase comparator in the DVB-S transmitter. So one can set output frequency on the command line. Any error is multiplied by 256, there is also the fractional divider 'wobble', got to about 6 kHz steps, but needed to be absolutely sure about the exact frequency. As I use the freq256 program: the Linux interface to my PIC frequency counter: that idea of adding a ppm correction to that little frequency counter interface seemed very attractive.

So I added the basically one line of code, and measured my Rubidium standard with it:

The -y command line flag lets you specify the ppm error of the crystal.

The amazing thing is that after 30 minutes with warming and cooling by the sun on it and than no sun etc (clouds passing) it still shows 10,000,000. OK I did some fine adjustment of the capacitors in the crystal 2 years ago, but very impressive indeed.

Yesterday added the ppm correction to the Raspberry DVB-S program:

This Raspi needs -440 ppm correction for the correct frequency..

As things are moving maybe later I will release the modified sources.

6 kHz accuracy is enough for the DVB-S transmitter at 2.4 GHz, it is not very good if I want to do SSB on the sats narrow band transponder... That would need an FPGA ..

Adding one line of code versus a trimmer...

Its later now: freq256 is now updated on the website:

Also released a new version of freq_pi:

The ts2iq_pi program and hardware is here:

This is not a real release, just experimental. You can either use the frequency output for symbolrate generation (-s command line flag), or use it to drive a phase comparator that compares transmitter frequency, and set the transmitter frequency that way, for this use the -x command line option. Both hardware and software in a changing mode, when parts come in and tests are run, then more things will change.

But this program also has the -y parts per million correction option now.