[Elecraft] Down or Up conversions designs

[Geoffrey Mackenzie-Kennedy]

Georges,

Although the Hilberling PT-8000 series of transceivers are very expensive, 
this could be because they have been designed for Military and 'commercial' 
use and not only for amateurs.  I believe that it should be possible to 
design and market a high performance transceiver using an up-conversion 
architecture, whose selling price is reasonable for most amateurs.

While I agree with Alan's N1AL comment about the higher density of phase 
noise generated by the receiver's first LO in an up-conversion scheme, it 
was possible back in 1994 to devise a LO system whose phase noise density 
allowed a SFDR3 > 100db at 2 kHz spacing to be obtained from an 
up-conversion HF receiver, whose IF was at low VHF.  However the task was 
not easy at the time.

With regard to roofing filters there is a problem which is often overlooked, 
and is present in both HF and VHF crystal filters.  This is the level of 
filter generated IMD vs. filter bandwidth.  The results I have of tests 
performed by other people and myself, show that the level of odd order IMD 
generated by a crystal ladder filter using a given quality of quartz 
increases as the filter's bandwidth is decreased.  This effect can be seen 
when the two parent signals used during the tests are either inside or 
outside the filters passband, which places a question mark over the use of 
very narrow bandwidth roofing filters in a receiver designed for high 
performance.

Again I agree with Alan that the use of amplifiers and mixers before the 
second IF filter, which have very high dynamic range, should be used to 
offset the effects of signals passing through a wider bandwidth roofing 
filter.  IMHO these are not very complex circuits, but the amplifiers 
usually operate at high values of standing current.  Comprehensive Gain 
Distribution calculations are essential, of course, when designing a 
receiver.  I would also suggest that the frontend gain between the antenna 
input and roofing filter's input should be negative in value to avoid 
overdriving the roofing filter, provided that a useful receiver Noise Figure 
is obtained.

73,

Geoff
LX2AO



On Friday, October 12, 2012 at 8:36 AM, 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.

<snip>

> 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.