[Elecraft] Down or Up conversions designs

[Alan Bloom]

As with any design decision, there are always trade-offs: each option
has its pros and cons.  I think an up-conversion architecture is easier
to design and potentially cheaper.  And as you say, there is no "hole"
in the coverage around the intermediate frequency.

However I believe it is harder to get top performance.  Because the
local oscillator is at a higher frequency it will tend to have more
phase noise, all other things being equal.  Also, with a VHF IF, it is
not possible to make a narrow-bandwidth, high-performance roofing
filter, which is important when working a big pileup or contest where
strong interfering signals are very close to the weak signal you are
trying to copy.

It is possible to mitigate the latter problem with a carefully-chosen
gain distribution using amplifiers and mixers before the second-IF
filter that have very high dynamic range.  However, that generally
requires high-power devices and/or complex feedback circuitry which
works against the advantages of simplicity and low cost.  I believe the
design goals of the K3 included small size and low power consumption,
which is more do-able in a high-performance receiver with a
down-conversion design.

> Perhaps, after the current trend to develop down conversion receivers 
> (Elecraft's rigs, Eagle, FT3K, TS-590, TS990) it will revert trend

Down conversion was universal before the late 1970s.  Then up conversion
became the fad.  Now we're back to down conversion.

Plus ca change...

Alan N1AL


On Fri, 2012-10-12 at 08:36 +0200, Georges Ringotte F6DFZ wrote:
> 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