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6.6
27MHz reference clock value, register 02
The 27MHz crystal used in the Si5351A Synth Module crystal does not oscillate at exactly 27.000000MHz.
It varies from one crystal to the next, but in general the frequency will be 3-5 kHz too high, e.g.
27,004,000Hz.
This is why the default reference frequency in register 02 is 27,004,000.
If you are using the OCXO/Si5351A Synth module kit, the frequency of this oscillator tends to be about
2kHz too low, e.g. 26,998,000Hz.
If you are using GPS discipline by applying a 1pps signal from a GPS receiver, the kit will self-calibrate in
less than half a minute. This will update register 02 in EEPROM. Next time ProgRock is powered up the
calibrated value will be used. Thereafter, the GPS makes corrections to the reference oscillator value in
register 02 to compensate for any measured temperature-induced drift in the crystal frequency.
The reference oscillator value is used in the calculation of the Si5351A registers. So it is important that it is
accurate, if you need the output frequencies of ProgRock to be accurate.
If you are not using a GPS, you can manually calibrate ProgRock by entering the correct value into register
02.
It is possible that you have an accurate means to measure the 27MHz reference oscillator frequency itself
by probing the correct point on the Si5351A module. However most kit builders will not have the necessary
equipment, and such a measurement is not easy to make without itself risking disturbing the frequency.
The easiest method is to set the output frequency to something convenient such as 13.500000 MHz, and
measure it. Measurement can either be by using an accurately calibrated frequency counter, or by setting
up an accurately calibrated receiver with Argo and monitoring the output signal frequency that way.
Once you have measured the actual output frequency, you can calculate the required correction to the
27MHz reference frequency and enter it in the “Ref. Frq.” configuration setting. For example, suppose you
set the output frequency to 10.000000 MHz but you actually measure 10.000075. Your output frequency is
75 Hz too high. Assuming the default reference frequency (27,004,000), you should change this to
27,004,000 * 10.000075 / 10.000000. So enter the new value 27,004,203
in the “Ref. Frq.” setting. Be sure
to write down somewhere the value you enter. If you need to re-calibrate in future, you will need to know
what value you entered! Otherwise it will be necessary to do a “factory reset” (see section 6.5).
6.7
GPS correction threshold, register 03
This register has a default value of 5Hz. But you can set it to anything you like. It controls how and when
the microcontroller makes updates to the Si5351A output frequencies. For example, at the default setting
of 5Hz, the microcontroller will only correct the Si5351A output frequencies when the 27MHz reference
value has changed by more than the 5Hz threshold. The threshold is always referred to the 27MHz
reference frequency, not the output frequencies.
If you set the register to have a value of 0, then the microcontroller will adjust the Si5351A output
frequencies once per second, every second, depending on whether it thinks the 27MHz frequency has
drifted upwards or downwards. This continuous change in output frequency may not be desirable. It is
more practical to have small jumps in output frequencies, less often
– i.e. only when the reference
oscillator has changed by more than a threshold.
The QRP Labs website has some notes on how to obtain excellent frequency stability with the Si5351A
Synth module
– please refer to the resources section below.
