[hpsdr] Frequency offset using Excalibur

[Rod Brink]

I'm seeing the same thing on Hermes....

73, Rod KQ6F

----- Original Message ----- 
From: "Joe Martin K5SO" <k5so at valornet.com>
To: "John Ackermann N8UR" <jra at febo.com>
Cc: "HPSDR List" <hpsdr at openhpsdr.org>
Sent: Saturday, December 08, 2012 11:58 AM
Subject: Re: [hpsdr] Frequency offset using Excalibur


> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi John,
>
> I have also noticed that and confirm your observations.  I want to check 
> to see how Hermes and Angelia responded for comparison.  Unfortunately the 
> pre-production versions of both of those boards that I have use incorrect 
> input transistors on the ext 10 MHz inputs so I will need to change those 
> transistors before doing more tests.  Maybe someone has a clue as to the 
> cause of the offset.  At the moment, I don't.
>
> 73,  Joe K5SO
>
> On Dec 8, 2012, at 12:45 PM, John Ackermann N8UR wrote:
>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> I just discovered something very odd with my Atlas-based HPSDR system 
>> (Hermes, Mercury, Penny, Excalibur, using W5WC PowerSDR 11/5/12 and 
>> latest firmware on all boards.)
>>
>> There seems to be about a 9 Hz frequency offset generated somewhere in 
>> the system.  I noticed this while monitoring WWV on 10, 15, and 20 MHz 
>> using the PowerSDR phase scope, tuning the carrier in CWU mode with a 25 
>> Hz filter to remove modulation.
>>
>> The minimum phase rotation occurs when I tune about 9 Hz below the 
>> nominal carrier frequency.  When I change to CWL, the offset reverses 
>> direction.
>>
>> I've tried using both GPSDO and lab-quality Rubidium frequency references 
>> plugged into Excalibur, and get the same result.  I've watched while 
>> interrupting the external reference, and see about a 12 Hz frequency 
>> shift between states, so Mercury seems to be locking to the external 
>> reference.
>>
>> To verify that it's not some propagation or interference issue, I routed 
>> the GPSDO output through a bunch of attenuation into the Mercury antenna 
>> input and see the same result on the very strong 10 MHz signal.  Since 
>> both the reference and the signal are coming from the same source, there 
>> should be exactly 0 offset.
>>
>> In the PowerSDR cal tab, the frequency correction is set to 0.  When I 
>> run the frequency calibration, the correction factor remains at zero --  
>> does this mean that PowerSDR think the frequency is accurate and the 
>> issue is just in the phase scope?
>>
>> Either I'm misunderstanding or misconfiguring something for this test, or 
>> there's some sort of offset being generated in the hardware or software. 
>> Anyone have any suggestions?
>>
>> Thanks,
>>
>> John
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