[Elecraft] Receivers?

[David Woolley]

I don't think these descriptions are accurate, particularly the K2 
versus K3 one.

The K2 is a fairly conventional, single conversion, analogue design.  As 
stated, it uses the crystal filter for primary selectivity.  It 
typically has up to two crystal filter options, one hard wired, and the 
other as an integral part of the SSB adapter.  The hard wired one is 
adjustable, and the SSB one is fixed.  The filters are constructed by 
the final assembler, from individual crystals.  Although there is a DSP 
option, it works purely on the audio.

The K3 and K3X are software defined radios (SDRs) of the non-direct 
sampling variety.  I use SDR in the technical sense, not in the amateur 
radio community sense; the latter requires the digital processing to be 
performed on a PC.  This means they have an analogue front end with at 
least one analogue mixer, but the final processing is done digitally.

The K3 has a double conversion superhet architecture, with an HF first 
IF and an extremely low second one.  There is a selectable crystal 
filter (using commercial sub-assemblies) in the first IF, which provides 
coarse selectivity.  The final IF processing is digital.  There is a 
quadrature path starting from the second mixer, analogue at that stage. 
  Combined with digital processing, this creates an analogue of a 
phasing design receiver to suppress the final IF image, rather than the 
audio image.  As the signal continues in quadrature, the digital 
processing may also act analogously to a phasing receiver to do the 
final conversion and audio image stripping, but it may be that the 
internal logic is more complex than that - the fine details are a trade 
secret, although they may or may not have release information about that 
part of it.

The K3 also does digital processing on the recovered audio, but this is 
done within the same digital processor as the final IF processing.

The K3X, for CW at least, implements a hybrid analogue/SDR direct 
conversion, phasing design.  For SSB it may do the same, but it is also 
possible that it actually implements a final passband centre at 0Hz, and 
then does a final frequency shift to move the centre of the passband to 
the correct audio frequency (i.e. they could have implemented it as a 
single conversion architecture).  Selectivity is provided entirely by 
digital processing.

For both the K2 and K3, first mixer image rejection is provided by a 
combination of band pass filters, optimised for each band, and a low 
pass filter, also optimised for the band.  For the KX3, the image is the 
one removed by the phasing, although there is also analogue band and low 
pass filtering - I'm not sure whether this is switched, or there is a 
single, compromise, filter.

Block diagrams for all three are fairly easy to find.  I have the K2, so 
did that from memory, but the K3 one is at 
<http://www.qsl.net/wb4kdi/Elecraft/K3/K3_Block.png> and the KX3 at 
<http://www.elecraft.com/manual/KX3%20Manual%20Block%20Diagram.pdf>. 
There are likely other places, including a better K3 image.

-- 
David Woolley
Owner K2 06123

On 11/12/15 21:40, Ron D'Eau Claire wrote:
> The K3 and K2 and conventional superhetrodyne formats with an Intermediate
> Frequency in the H.F. range and crystal filters to set the passband. The K2
> has an adjustable crystal filter and the K3 uses fixed crystal filter
> bandwidths. The basic K2 bandwidth is established by the crystal filter
> while the K3 adds an adjustable DSP filter after the crystal filter. (The K2
> has an optional audio DSP for enhanced filtering.)