[Elecraft] Why no IF shift ?

[Geoffrey Mackenzie-Kennedy]

Ron AC7AC wrote:


Others pointed out the reason - single conversion design. That
single-conversion design is also what helps the K2's performance excel
compared to those with multiple-conversions that allow for an "IF Shift"
knob. Frequency conversions in receivers are like making copies in
photography. You can go to great extremes to make each copy very high
quality, but each generation degrades the image, just the same. In a
receiver, every conversion degrades the performance, no matter how hard the
engineers try to avoid it.

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Hang on <g>. One problem with single down conversion receivers is that their 
"stronger" spurious responses which, depending on the IF used, can be close 
to or even in one or more of the frequency bands covered by the receiver. If 
bandpass filters are used between the antenna connector and the mixer, their 
selectivity might offer some degree of protection against signals coming in 
at spurious response frequencies outside but close to the bands covered but 
obviously no protection against anything coming in at an "in-band" spurious 
response frequency. The choice of IF that reduces this problem in a single 
down conversion receiver is quite limited.

In a double conversion receiver, up and then down,  assuming that sensible 
design and construction practices are followed, the close or in-band 
spurious responses (if they exist) are considerably reduced.

Then there is the internal birdie problem, usually created by one or more of 
the receiver's oscillators and /or their harmonics getting together to 
produce a signal at some spurious response frequency of the receiver. If the 
Front End, LO and IF are not properly shielded in separate "boxes" with all 
associated DC and control lines filtered, then expect birdies in a single 
down conversion receiver. The same method of construction should be used in 
a double conversion receiver. I suspect that commercial double conversion 
amateur receivers have received a bad press because for reasons of cost this 
is usually not done.

It is true that every conversion degrades the performance, but for several 
years the technology has been available that allows a double conversion 
receiver to be built which exhibits an IIP3 of +40dbm
at an offset of 2 kHz while running at full gain, Noise Figure of 8db on 
10m. I have one here. The downside is that each one of the three VHF roofing 
filters / embedded amplifiers selected draws 240 mA.

With double conversion, in addition to true "IF Shift" a form of continuous 
bandwidth control can also be introduced.

73,
Geoff
GM4ESD