Hi all,
With regard to SM5BSZ's SDR wideband-noise performance table
(http://www.sm5bsz.com/dynrange/dubus313.pdf):
The reason the K3 has excellent wideband transmit noise suppression is that it
is a down-conversion superhet with a very clean synthesizer and transmit chain.
This was intentional. We had several customers in our K3 focus group--VIP
contesters and DXers--who were within line-of-sight of each other's QTH's, up
on a ridge. They were tired of hearing their respective phase noise when they
all happened to be on the same band. They asked us to do something about this,
and we accepted the challenge.
To ensure as clean a signal from the transmit chain (DSP/DAC/TX mixer) as
possible, we routed it through the same crystal filters used by the receiver,
as well as through an additional 4-pole crystal filter that immediately follows
the transmit mixer. The two filters in series do a great job of band-limiting
transmit phase noise before it even gets to the mixer.
While this basic architecture is used by other superhet transceivers, they may
not have as clean a synthesizer as the K3, or they may lack the second filter
in series with the transmit path, or they may have such complex conversion
schemes that noise is allowed to leak in at various points. In some cases they
use a DDS without a following PLL/VCO, or their VCO may be operating a high
kHz/volt tuning ratio, leaving it susceptible to higher phase noise, especially
on the higher bands.
We went to extremes to keep the synthesizer's kHz/volt ratio as low as
possible, with a combination of switchable hi-Q inductance (1 uH/0.25 uH) and
lots of switchable fixed capacitors. The inductors and larger capacitors are
switched with relays to eliminate any loss of Q in the VCO's tank circuit. The
smaller capacitors are switched with PIN diodes, but because they're a small
part of the total C, this has a negligible effect.
The KX3 is no slouch, either, falling about in the middle of this particular
evaluation table. But the KX3 doesn't have the benefit of routing its TX signal
through crystal filters (there are no crystal filters in a "pure" SDR), so the
noise performance is determined almost entirely by the transmit DAC and its
following amplifiers. We made these as clean as possible within budgetary
constraints. We could have used a much more expensive DAC and added more
shielding and filtering, but that would have pushed the radio to a higher size,
cost, and current drain than we wanted.
It's nice to have further confirmation that, despite the relative simplicity of
the KX3's transmit architecture, it still comes in ahead of many radios touted
for their sophistication and high performance.
73,
Wayne
N6KR
On Dec 27, 2013, at 5:00 PM, "Bruce Beford" <bruce.bef...@myfairpoint.net>
wrote:
>
> You're not missing anything. It shows how good the receiver is. ('better'
> than the transmitter). It highlights the difference in performance between
> the receiver and the transmitter. But... does having a "way better" receiver
> than a transmitter necessarily make it the "best all around transceiver"? I
> don't believe so.
>
>
> I still believe (personally) that my K3 is a better all around transceiver
> than my KX3. But- they each have their strengths.
>
> 73,
>
> Bruce N1RX
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