Chris Johnson <elecr...@ozy.us> wrote: > 1) Is phase noise the measurement of the instability of an DDS?
That's one contributing factor. But the DDS signal drives a mixer, and the other input to the mixer may be the larger source of phase noise in transmit mode. > 2) How does this impact TX only, and why does it create such an issue to > nearby listeners? In the case of the KX3 and Flex radios (e.g. Flex 1500/3000/5000), the limiting factor is probably the transmit D-to-A converter. Its performance is limited by quantization and sampling noise, setting an upper bound on phase noise. The KX3 has an advantage in that its synthesizer uses a DDS-driven PLL, while the Flex radios are DDS only, without a following PLL. But the TX DAC is still a factor in both cases. In a well-designed superhet like the K3, the TX DAC's noise floor affects only the in-band portion of the transmitted signal, i.e. the portion within the I.F. crystal filter passband. The K3 has two crystal filters in series in transmit mode, which results in very high rejection of noise outside the SSB passband (typically 100-3000 Hz). This means that the limiting factor on wideband transmit noise is not the IF injection into the mixer -- it really *is* the synthesizer. The K3's synthesizer is extremely clean at wide offsets (another DDS-driven PLL), and that is why its transmit phase noise is so low. > 3) Does phase noise go down if you use a faster master clock? The Flex 6700 > uses a 983.04mhz vs a 122.99Mhz clock in the 6300. That is a completely different design (direct digital up/down conversion), which requires very high clock speeds, very expensive ADC and DAC components, and high current drain in receive mode. It can be made clean in transmit and receive modes, although in receive mode this architecture will still have typically 15-20 dB lower blocking dynamic range than a well-designed superhet. None of this is applicable to the KX3, which is obviously intended to be an ultraportable radio with low current drain and low cost. > 4) Do low phase noise radios allow in-band use, such as someone on CW on 20M > and someone up on voice on 20M? Yes. In general this is only a problem if you have stations in very close proximity. In this situation, the K3 is better than any other radio on the market. This is why the K3 is highly favored for Field Day and DXpeditions. 73, Wayne N6KR ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
