Revision 1.0 3Copyright KE9H, 2009
Introduction
Excalibur is a small accessory card for the Atlas bus that enables the use of an external
10 MHz frequency reference for locking the frequency of an HPSDR radio to the same
accuracy of the standard, or GPS disciplined oscillator.
It also provides an on-card TCXO frequency reference for the HPSDR, that is better than
the on board 10 MHz oscillators, although not as good as an external reference standard
or GPS-DO.
Description
Excalibur can be configured in several ways, using the jumper clips, and also has an
auxiliary output connector.
First, it can be configured to take an external 10MHz signal, such as a sine wave or
square wave output from a reference standard or GPS disciplined oscillator, into a BNC
input and process it into a square wave and put it on Atlas bus 10 MHz clock line C16.
Second, it is a way to have an on-bus "instant on" TCXO with more accuracy than the
10 MHz oscillators on either Penelope or Mercury. When the on-card TCXO is selected
as the clock source, it will drive the Atlas bus via line C16.
No matter which source has been selected to drive the bus, the on board “beat” indicator
will always compare the frequency of the TCXO to the frequency of whatever signal is
coming in the “INPUT” BNC connector.
Excalibur also has an auxiliary output on two pin connector (J2), which can provide a
square wave output for direct connection to the AUXCLK-(J8) input on Mercury, for
special applications using that Mercury input.
The "Output" BNC provides a 10 MHz sine wave at + 8.5 dBm as a way to lock external
equipment to which ever 10 MHz source is driving the bus.
There is a multi-colored LED, hooked to the output of a frequency-phase detector
comparing the TCXO to whatever is coming in the "Input" connector. It gives both a
HIGH/LOW frequency color indication and a visible beat indicator. It is useful for setting
the TCXO to within a fraction of a Hertz.
The 10 MHz oscillators on the Mercury or Penelope cards have a rated stability of +/- 50
or 100 ppM over wide temperature, or +/- 500 Hz to 1 kHz at 10 MHz. Using the
Calibrate function built into PowerSDR, you can set them to WWV or other reference,
with an accuracy of about 10 to 30 Hz, but they could still move around +/- 50 to 100 Hz
over normal room temperature variation.
