Re: K3 frequency accuracy versus displayed precision

Posted by wayne burdick on Nov 17, 2009; 10:04pm
URL: http://elecraft.85.s1.nabble.com/K3-frequency-accuracy-versus-displayed-precision-tp4016138p4021632.html

Here are the technical details regarding the K3's VFO tuning accuracy:

The K3's PLL synthesizer is phase locked to its 49.38 MHz reference  
via a DDS chip used as a precision divider. The DDS output is in the  
3.6 MHz range, with a step size of 0.184 Hz (49.38 MHz DDS clock  
divided by 2^28 DDS word size).

Taking the 8.215 MHz 1st IF into account (added to the VCO on 160-10  
m, subtracted from the VCO on 6 m), this translates to a VCO step size  
of about 0.5 Hz on 160 m, 1 Hz on 20 m, and 2 Hz on 6 m. Other bands  
are between these values.

Example:  At RF=14 MHz, the VCO is running at around 22 MHz. The ratio  
between 22 and 3.6 is about 6, and 6 * 0.184 = about 1 Hz. This is the  
worst-case DDS-related tuning error on 20 m.

If REF CAL (in the CONFIG menu) is not set to the exact frequency of  
the reference oscillator, there will be a small additional offset on  
all bands that is proportional to the operating frequency.

In diversity mode (main/sub VFOs and filter bandwidths set  
identically), the DDS chips on the two synthesizers are set to exactly  
the same tuning word, and the VCO outputs are at exactly the same  
frequency. The two synths are phase-locked to the same signal, so  
there is never any beat note between the two. This is critical for  
diversity use, and is lacking in some other transceivers with a sub  
receiver.

73,
Wayne
N6KR


On Nov 17, 2009, at 10:05 AM, ab2tc wrote:

>
> Hi,
>
> First time I have heard about this. How much frequency shift are we  
> talking
> about here? A fraction of a Hz, 10Hz? I have certainly never noticed  
> this,
> but of course on speech anything less than 10Hz is practically  
> inaudible
> unless you do a USB/LSB comparison on an AM station.
>
> AB2TC - Knut
>
>
>
> Wes Stewart wrote:
>>
>> Guys,
>>
>> I've mentioned this caviet before:
>>
>> http://n2.nabble.com/Ref-Osc-Cal-Method-4-td2595451.html#a2595451
>>
>> The way the K3 implements the passband shift, the same signal, with  
>> the
>> same VFO setting, will have a different audio beat note if the SHIFT
>> control is changed. *
>>
>> <snip>
>>
>
> --
> View this message in context: http://n2.nabble.com/K3-frequency-accuracy-versus-displayed-precision-tp4016138p4020213.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html