[hpsdr] 1pps correction method

[K4KV]

 From an email I received from QRPLabs:

Hi Glen,

The Si5351A requires a reference of 25-27MHz. It does not operate 
directly with a 10kHz, 10MHz, or 1pps output from a GPS.

In the various QRP Labs kits, I use a microcontroller to configure the 
Si5351A. There is also a continuous GPS discipline function, or an 
occasional calibration function (depending on the kit). In both cases, I 
use the 1pps output of the GPS as a frequency counter gate to the 
microcontroller. The microcontroller acts as a frequency counter, to 
measure an output frequency produced by the Si5351A. The microcontroller 
knows what output frequency it configured the Si5351A to produce. And it 
measures what it actually got. It can then compute an error, and infer 
the actual oscillation frequency of the 27MHz reference crystal. It can 
then use this actual oscillation frequency and recalculate the Si5351A 
configuration necessary to produce accurate output frequencies. In all 
cases the inferred 27MHz reference frequency is also stored in EEPROM so 
that it is available at next power up.

This method is therefore a software error compensation; it does not 
require any phase locked loop of a reference frequency to a 10kHz 
output; or generating a 25MHz reference from a GPS 10MHz output; it does 
not require any actual adjustment at all of the 27MHz reference crystal 
(no trimmer capacitors etc). It is all done in software, using the 1pps 
signal as a frequency counter gate.

It produces a high accuracy output with very minimal component count. It 
is used in all the QRP Labs kits very nicely!

73 Hans G0UPL
......

73
Glen K4KV