http://elecraft.85.s1.nabble.com/K3-frequency-accuracy-versus-displayed-precision-tp4016138p4024560.html
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!
> From: Wayne Burdick <
[hidden email]>
> Subject: Re: [Elecraft] K3 frequency accuracy versus displayed precision
> To: "ab2tc" <
[hidden email]>
> Cc:
[hidden email]
> 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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