[hpsdr] Excalibur

[wully]

Hi all

A topic which I have not seen recently in the reflector:
Today, I have completed the assembly of the Excalibur (call it Ec) kit. 
When putting in the Ec into Atlas, both mercuries (one from TAPR, the 
other Mercury-EU from Gerd Loch) show with DEBUGLED 3, that they are 
locked with the 10MHz PLL.

But when I have made a QSO today at 7.146000 MHz with a german station, 
I had to tune my frequency to 7.146100 MHz to be on a "reasonable" 
sounding SSB signal. The other station has claimed, that its frequency 
is very good defined at 7.146000 MHz.

So, I tried to check Ec with my DCF77-based 10MHz frequency normal (from 
the german magazin "Funkamateur"), this normal is specified to be at the 
range of 10e-9 (0.01 Hz at 10MHz) precision, when the Oscillator is 
locked to the frequency of the DCF-77.
(This is a very elaborate PLL circuit which compares the phase of the 
local oscillator divided to 625Hz with the  77.5 kHz also divided to 
625Hz. The phase comparison is done with a time constant of about 1 
minute, as soon as the local oscillator is locked, which occurs very 
fast (some seconds of reception). So, I expect, that this normal is 
really quite precise.

Now feeding this signal into "IN" of Ec, I can not adjust the CXOH20 
such, that there is a slow variation between red and green Led, as 
described in the Ec-Manual. This sounds reasonable, if the Ec is off by 
approx. 100Hz as suggested by the QSO above. But on the other hand, the 
CXOH20 is specified at +-10e-6 with a variation of +-3*10e-6 with the 
adjusting capacitor, which would vary 40 Hz maximum.

So, either the Ec is not on the frequency or the my DCF-normal is not at 
the frequency....

Another experiment shows strange behavior: switching from the TCXO to 
the external Source (J2 on Ec), the TAPR-mercury does show the lock on 
debugled 3 but the reception frequency is frequency-modulated with a 
shift around 100Hz (not measured, just heard...) the FM-frequency is 
something around 0.25 to 0.5 second. It looks as if the PLL would loose 
the lock and then lock again within this time.

Are there any ideas what could be wrong here?

73, HB9EPU, Alfred