[Elecraft] Down or Up conversions designs

[Georges Ringotte F6DFZ]

Out of an extremely interesting private mail, I don't  have got comments on this topic, which was much debated in the past. 
I post it once again  because I think the Elecraft team and Elecraft aficionados are more apt to discuss this subject than on other reflectors dealing with down conversion receivers. I added some comments. 
Sorry if I am boring you! 

George

Here it is :

Hi to the group 

Even though I own and appreciate very much my K3, in fact I believe it's the 
best sounding receiver I ever had, I have always been an advocate of up 
conversion design, namely because it's the only way to get general coverage and 
also because some characteristics, for a given cost, are better with an up 
conversion design (IF and image rejections for example). The probability is 
low, but if in the future a WARC give a new band very near the IF of a down 
conversion design, it will not be able to cover it. 

Due to the Sherwood chart, and the way receivers are classed, there is a trend 
for manufacturers to propose down conversion design, even though some 
characteristics may be poor ; I think of the Yaesu FT5000 for example, very 
high in the Rob Sherwood chart, but with poor image rejection, and extremely 
poor IMD2 figure for the second receiver. 
Even Kenwood comes with a high end transceiver with a strictly down conversion first receiver, and a mixed design second receiver. 
Only Icom seems to resist. The IC7700, even if costly and not perfect, employs some very interesting features (preselector, relays switching of RF filters, large coils, good gain distribution, etc...) 

I always thought the key to success was an up conversion design with very good 
gain distribution, like RS XK2100 transceiver for example. 
Ten Tec tried this with the Omni 7, but with limited success, even for remote signals IMD3. 

Now the 1st transceiver on Rob's chart is an up conversion design, with a VHF 
roofing filter 50 kHz wide, proof of the validity of optimized gain 
distribution ; it has 105 dB dynamic range with signals separated by only 1 
kHz. 
In the past, the Signal One CX7 and CX11 were up conversion design, with a first IF from 39 to 40 MHz (the real bandwidth was much wider than 1 MHz of course), and the CX11 managed to get very good IMD3 inside this bandwidth, proof of a very good gain distribution. 

Perhaps, after the current trend to develop down conversion receivers 
(Elecraft's rigs, Eagle, FT3K, TS-590, TS990) it will revert trend and we will see up 
conversion design with large dynamic range for closed spaced signals. 

Yes this German made transceiver, built like a measuring instrument, is very 
expensive, but surely it's concept can  be used to market simpler rigs (100W, 
13,8V, only one receiver, no VHF, not so luxurious, with true general coverage ...) for a more correct price. 

Any comments? 

Best regards. 




Georges F6DFZ