[Elecraft] Elecraft technology

[N2EY@aol.com]

Just my $.02...

In a message dated 12/26/03 11:48:34 AM Eastern Standard Time, aa4lr@arrl.net 
writes:

> why did it take until 1998 for this architecture to make it
> >into ham receivers, why didn't it happen say ten years earlier?

The basic concept goes back to at least 1957 (see the QST article "What's 
Wrong With Our Present Receivers?" by W1DX in that year).

The problem was always how to generate the local oscillator signal.


Back in the 70s or so, there were many tranceivers (or receivers) that 
> 
> used a dual-conversion architecture. Typically this would involve 
> conversion to a broad-band IF from a fixed heterodyne oscillator, then 
> conversion through a 5-5.5 MHz LMO to a crystal filter for the IF. 
> Sometimes there was a third IF frequency, but this dual conversion seemed 
> typical.

This technique goes back to the 1950s and Collins' 75A series receivers and 
the S-line. 


> 
> The dynamic range on this architecture wasn't fabulous, but it was stable 
> and reliable. Since the LMO only covered 0.5 MHz, a fixed heterodyne 
> crystal was needed for each band (or 4 for the full 10m band). On some 
> rigs, the LMO was even more narrow, so more expensive crystals were 
> required.
> 

The reason this concept caught on so solidly for ham gear was that it results 
in a minimum of receiver spurs for the complexity involved. And it can be 
used to transceive.

The competing methods were:

1) Use a VFO with multiple ranges. Swan championed this idea, but dial 
calibration and stability were real challenges given the available technology. 

2) Use a premixer chain (one VFO range, with the VFO signal mixed with the 
signal from an xtal osc.) to get the desired injection. Works well but requires 
careful premixer design, and the premixer output is never perfectly "clean". 
The premixer idea was used by Drake in the TR3 and TR4 transceivers as well as 
the R4/T4 family. Hallicrafters used it in the SX-146/HT-46 pair, and Ten Tec 
used it in almost all their ham-band-only rigs (Omni A thru 6, Argosy, 
Corsair)

> Then in the 80s, a rash of solid-state rigs hit the market with 
> general-coverage receivers. 

Ah, you missed a step, I think.

in 1979, we hams got the so-called "WARC bands"  - 30, 17 and 12 meters. The 
methods described above worked fine for rigs covering just the ham bands 
80/40/20/15/10, but many of them could not be easily modified to handle the new 
bands without lots of spurs. So the up-conversion tactic was used, and general 
coverage receive was a side benefit.

This required a different approach. Instead 
> 
> of an LMO, PLLs were used to generate the conversion frequencies. This 
> got rid of a lot of expensive heterodyne crystals. To make the rig 
> general coverage, the first broad-band IF was moved up to the 45-70 MHz 
> range, and was broader than the 0.5 MHz of before. This worsened the 
> dynamic range situation a bit, but the worst part of this change had to 
> do with the PLLs. 
> 
> Those early PLLs generated a lot of low-level broadband noise, called 
> Phase Noise. They didn't generate pure frequencies like those old crystal 
> heterodyne oscillators. This meant these rigs received (and transmitted!) 
> a low-level broadband response. For weak signals, this didn't matter 
> much, but it was terrible for adjacent, strong signals to leach through. 

Actually, I think a noisy LO is trouble at all signal levels. But it's most 
noticeable when there are lots of strong signals on the band. In many cases 
it's not exactly clear what is going on because the effect of LO noice may be to 
raise the apparent background noise level.

All of this is one reason why some folks swear their old receivers sound 
"better" than the more modern Ikensu stuff. 


> 
> Many of these rigs also supported FM as a mode, so the first roofing 
> filter was typically 15 kHz or more. 
> 

Exactly. Besides, it's not practical to make a really narrow filter for 70 
MHz.


> Designers have since cleaned up their PLLs quite a bit. But the middle 
> 90s saw the introduction of the Ten-Tec Omni V. It solved the phase noise 
> problem by going back to crystal-controlled heterodyne oscillators. Like 
> its predecessors, it isn't general coverage. But the Omni V showed it was 
> possible to get better performance by going back to earlier designs.
> 

Ten Tec's ham-bands-only rigs had always used that approach. There's only one 
conversion between antenna and sharp filter in almost any of their rigs, 
going back into the '70s. They got the premixer method to work even with the WARC 
bands. Rigs like the Corsair have the second conversion *after* the first 
selective filter, and they only have the second conversion for things like 
passband tuning.

Strictly speaking, a K2 is dual conversion, but the second conversion is just 
for the AGC.

> In one way, the K2 goes back to even earlier, single-band superhet 
> design. That's why its dynamic range figures are so good. Most rig 
> manufacturers aren't likely to move back to this simpler design, largely 
> because they don't want to give up general coverage receive.

Agreed. They took certain tradeoffs. Elecraft took different ones. Guess 
which tradeoffs I prefer ;-)

And the K2's PLL design is elegantly simple, yet very clean because it does 
not try to do everything at once, so to speak. As I understand it, the 'basic' 
K2 PLL tunes in 5 kHz steps, and the spaces between steps are handled by 
"pulling" the reference frequency, not by multiple dividers. This is why you can't 
get 1 Hz accuracy on a K2 at every frequency it covers - but I'll take that 
tradeoff in order to have the K2's clean PLL output!

> 
> Ten-Tec has come up with a new rig, the Orion, which combines techniques 
> of single conversion to a narrow IF and multiple conversions. They use a 
> combination of roofing filters which are significantly narrower than 15 
> kHz in many modes.
> 
> The Orion design works, but it is expensive.

And complex. Note that the Orion's main receiver is ham-bands-only, while its 
sub receiver is general coverage and does not have quite the performance of 
the main rx.


> 
> The K2 receiver performance is right up there, still. 

It must be remembered that the Orion is the latest in a long line of TT rigs. 
It was designed to outdo everything on the market, with considerations like 
price, size, weight and the ability of the average ham to work on it being 
secondary, or simply not considered. 

 > My friend Gary, K9AY told me a couple of years ago, "The K2 is a simple 
> radio superbly executed." There's something to be said for simplicity.
> 
> 

Agreed! And what's really great is that we have choices undreamed-of not too 
many years ago.

73 de Jim, N2EY


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