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
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