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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html