[hpsdr] Precise receive frequency measurement

[Brian Lloyd]

On Fri, Mar 8, 2013 at 1:08 AM, John Sager <john at sager.me.uk> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Or measure the actual error in the onboard 10MHz reference. I did that
> using
> a standard frequency broadcast (RWL), and the 'fmt' program in Joe Taylor's
> WSPR software. Mine is about 1.5Hz high after it warms up.
>

The built-in 10MHz reference is (if I recall properly) rated at 1ppm
accuracy. So your 1.5 Hz error at 10MHz is well within spec.

But the thing I noticed about the built-in reference, at least on the
Hermes board sold by Apache Labs, is that the internal temperature
correction is not smooth but corrects in jumps. Phil figured that one out
after I reported frequency instability. I had hoped to be able to quickly
calibrate the Hermes board to WWV and then be able to do non-critical
frequency measurements, i.e. under 1Hz but not quite down to 1mHz, but that
appears not to be possible using the built-in reference. Of course, for
people using the radio for "normal" operations, they wouldn't notice a
thing. You just have to be anal-retentive like me to catch it. :-)

(And I really appreciated Phil's and the folks at Apache Labs' efforts to
figure out what was going on.)


> 73 John G8ONH
>
> On 08/03/13 08:06, Phil Harman wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > Yes,  but it will be if you use an external 10MHz reference that has your
> > required accuracy.,
> >
> > 73 Phil...VK6APH
> >
> >
> >
> > -----Original Message----- From: Bruce Beford
> > Sent: Friday, March 08, 2013 10:05 AM
> > To: hpsdr at lists.openhpsdr.org
> > Subject: Re: [hpsdr] Precise receive frequency measurement
> >
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > Thanks, Phil.
> > Doesn't the overall accuracy of this formula assume that the 122.88 MHz
> > clock -is- in fact, exactly correct?
> >
> > 73,
> > Bruce, N1RX
> >
> >> The frequency is sent to Hermes/Mercury as a 32 bit value in Hz.  This
> is
> >> converted to a 32 bit phase word as follows:
> >
> >> phase =  frequency * 2^32 / 122880000
> >
> >> You will be able to calculate the actual frequency from the above.
> >
> >> 73 Phil...VK6APH
> >
> >
> >
> >
> >
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> --
> John Sager
>
> Computer Security Consultant
>
> Tel:    01473 736620
> Mobile: 07920 867261
> E-mail: john at sager.me.uk
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-- 
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
brian at lloyd.com
+1.767.617.1365 (Dominica)
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