[Elecraft] Re: Unique synthesizer LO ?

[PE1E]

Wayne,

Thanks for your dedicated response to my phase noise question.
I understand now why and how  K2's LO design differs from the usual.

Indeed, I noticed that at less than 5 KHz/dbc your VCXO+DAC>varicaps
approach show less oscillator noise than the famous AOR7030 design (
the7030Plus should be even bit better ).
However, after 15 KHz/dBc the AOR 7030 graph claims a 10 dB better phase
noise.
Though, I see on the 7030's graphs the DDS spurs where you are talking about
( and as you suggest, more spurs may remain unseen ).
Besides, I presume the more the sweep goes to the right the less reciprocal
mixing will occur / harm.

I may frankly confess that I am gladly surprised by your extensive an
dedicated effort you make for me to explain things.
If this an omen of Elecraft's customer service, I may have taken the right
decision to buy a K2.

I was so fortunate to buy today a high S/N completely built/aligned/tested
K2  ( ssb+nb+160Rx ) from a nice US person.
He built many K2's before and sold them.

Since I want more options I just ordered with you the DSP, the internal ATU,
the RS232 i'face and the internal battery options.
This nice OM will built all of these options for me.
So, shortly I will own a K2+ which will enrichten my life :-)

Thanks again,
Peter PE1E
Amsterdam.


----- Original Message -----
From: "wayne burdick" <n6kr at elecraft.com>
To: "PE1E" <PE1E at chello.nl>
Cc: "'List Elecraft'" <Elecraft at mailman.qth.net>
Sent: Tuesday, August 01, 2006 6:11 AM
Subject: Re: Unique synthesizer LO ?


> PE1E wrote:
> >
>
> > It is said that the synthesized LO of the K2 is of a unique design.
> > Though, when I compare the LO phase noise specs of the K2 and the
> > AOR7030,
> > the latter seems to show much better specs.
>
> Hi Peter,
>
> Eric (WA6HHQ) and I designed the K2's synthesizer, so I'll try to
> answer your questions.
>
> When we designed the K2 synth we were faced with some difficult
> criteria. Since the rig is a kit, we wanted to keep cost and complexity
> low and avoid using surface-mount parts. And since it's targeted at
> portable operation, we also needed to minimize current drain. But we
> also needed a reasonably high-performance synthesizer to match the
> possibilities of our down-conversion superhet receive architecture.
>
> For all of the above reasons, we decided against using a DDS (direct
> digital synthesizer). "High-performance" DDS chips generally are
> expensive, have high current drain, and require a lot of support
> circuitry. And lately just about all of them are fine-pitch SMDs. In
> 1998 when we first started prototyping synthesizers, the choices
> considerably bleaker.
>
> DDS chips have a bigger problem, which for a rig like the K2 was not
> acceptable: close-in spurs that could compromise performance in high
> QRM conditions. These spurs can be managed by adding a lot of
> additional filtering and various techniques that are
> component-intensive. For example, putting a PLL after the DDS will
> help, but without great care, this can actually make things worse (the
> PLL can multiply or fold back in spurs that occur way outside the DDS's
> nominal output frequency). As is clearly shown in QST reviews of radios
> that use DDS without a PLL (e.g., the SDR-1000), a simple low-pass
> filter won't do the trick either. Numerous large spurs can be seen near
> the carrier in phase noise plots of these radios. But a bare DDS is a
> good choice for an "SDR," since it permits the very fast, very accurate
> tuning needed for sophisticated digital modes.
>
> I'm not familiar with the AOR7030's synth design. But if it uses a DDS,
> it must either be managing the spurs as I mentioned above, or it has a
> *lot* of spurs. You won't necessarily see then in the phase-noise plot;
> doing the phase-noise sweeps with a typical bandwidth of 100 Hz won't
> show any but the largest spurs (such as the loop spur shown in the
> AOR7030's plot). But you'll hear them in the form of reciprocal mixing.
>
> Back to the K2. Since we were avoiding DDS, we used a clean VCXO as the
> PLL reference oscillator, tuning over a small range. To tune the
> oscillator, we used a 12-bit DAC driving varactor diodes, and (as you
> may recall) there's a calibration pass where we measure the VCXO and
> store constants to tune it over its full range.
>
> Again, I'm not familiar with the AOR design, but over the range we
> typically measure (carrier + 2 kHz to 20 kHz), their "smoothed" phase
> noise appears to be similar to the K2's. I don't know what bandwidth
> they used for their phase-noise plots, so it's hard to say whether
> they're mitigating DDS spurs or not. As I mentioned, they can be hidden
> by the analyzer's bandwidth. (Note that the upper curve of the two
> shown for the AOR7030 is the one I'm referring to. They provided an
> additional "far out" curve below it, and we don't have a plot of this
> type available for the K2.)
>
> The other piece of the puzzle was the VCO. Somehow, using a minimum of
> parts, we had to make the VCO cover 9 or 10 bands. The resulting
> circuit, with three latching relays to switch fixed capacitors and
> varactors, and only one VCO inductor, did the trick.
>
> 73,
> Wayne
> N6KR
>
> ---
>
> http://www.elecraft.com
>
>