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" <[EMAIL PROTECTED]> To: "PE1E" <[EMAIL PROTECTED]> Cc: "'List Elecraft'" <[email protected]> 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 > > _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
