[Elecraft] K3 frequency accuracy versus displayed precision

[juergen piezo]

Hi Wayne

If 2 X K3's can be phased locked ON TX and then combined with a Hybrid combiner, it should be possible to substantially reduce the TX IMD. This is a feature that the R&S Series 4100 DSP transmitters use to improve TX IMD  performance. They do the same with their 1KW PA's combining  and phase locking the amplifiers  with powers upto 4kw of output.

I am no expert in area, so I am not entirely sure if it is as simple as they indicate. It would however  prove very interesting if this could be achieved with two K3's?  A  good reason to buy more K3's!  

Is phase locking 2   K3's transmitters possible?

John

--- On Tue, 11/17/09, Wayne Burdick <n6kr at elecraft.com> wrote:

> From: Wayne Burdick <n6kr at elecraft.com>
> Subject: Re: [Elecraft] K3 frequency accuracy versus displayed precision
> To: "ab2tc" <ab2tc at arrl.net>
> Cc: elecraft at mailman.qth.net
> Date: Tuesday, November 17, 2009, 2:04 PM
> 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.
> >
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