Posted by
Georges Ringotte F6DFZ on
Nov 18, 2010; 7:05am
URL: http://elecraft.85.s1.nabble.com/Down-conversion-tp5750644.html
Hi to the group,
Here is a post I made on the TS-590 Yahoo Group about down or up conversion, close spaced signals dynamic range, and the
way receivers are classed in a few charts (Sherwood or W8JI for exemple).
I ordered very recently a K3, and it is on its way to France.
I must add that Elecraft took some measures to limit the problems associated
with down-conversion ; for exemple, transmit low pass filters are used to improve image rejection. There is also a complex
netwoork in the receiver path with notches on image frequencies of the upper ham bands. For remote signals IMD2 and IMD3,
relays are used to limit them.
Down-conversion is not a fatality, only a technical choise aimed to get the maximun performance on the ham bands.
If it's well implemented (as on the K3, I do hope !), all is fine. If not, you will have problems much more important than
limited 2 kHz dynamic range.
Professional receiver must cover the whole HF spectrum, and up-cpnversion is the only choise to do it.
It's possible to have a wide (aka 15 kHz) VHF roofing filter, and still very good 2 kHz dynamic range, but it comes
at a cost.
All receiver are compromises, between technical choise, cost, and even marketting. You must understand this to
make the better choise with the money you want to spend.
I have a lot of respect for the work done by Rob Sherwood. But
its chart, and the way receivers are classed, describes only a
very small part of receivers performances, even if it has the
merit to provoque the interest of prospective buyers and by way
of consequence the interest of marketing departments of
ham equipment manufacturers.
2kHz spacing IMD3 describes the behavior of a receiver in presence
of close spaced signals, mainly CW and data. A good figure
can be had by a tight selectivity near the frontend obtained
by a narrow bandwidth roofing filter (as narrow as 200 Hz for
the K3), and/or by a wideband roofing filter and judicious
gain distribution (as the RS XK2100 does).
As Rob pointed it, 2 kHz IMD3 has little to mean for SSB because
IMD generated by nearby transmitters is more prevalent than
IMD generated by the receiver itself ; there, selectivity obtained
by the DSP is the main factor.
The chart of Rob Sherwood is very incomplete ; for exemple,
he could add image rejection, and IMD2 and IMD3 generated by
remote signals.
Image rejection is not a problem with a good up-conversion
design, figures well over 100dB are mesured (FT-2000). But it's
not the same case with downconversion designs (K2, K3, high in
the Rob chart, Eagle, FT-5000, TS-590). With a 9MHz high IF,
it's difficult and costly to reject images over 70dB on the
higher ham bands, even more difficult with a lower IF (K2).
One must understand that these low figures are a real threat
when cycle 24 will peak. Can you imagine your "chart topping"
$5k+ receiver with the upper ham bands full of BC and utilities
image signals ?
Remote signals IMD2 and IMD3 are even more prevalent.
These measurements describe the behavior of a receiver in
presence of signals for which the sum and/or difference
(F1+F2 or F1-F2 for IMD2, 2xF1-F2 or 2xF2-F1 for IMD3) falls
were you want to receive.
Take real life exemples, described in CQDL magazine in the 80's
by the late DL1BU. Imagine for IMD2 large BC signals from
the 31m band combining with large BC signals from the 25m band.
If your receiver has poor IMD2 (see ARRL review of some
"chart topping" receivers), your 15m ham band will be full
of strong combs of BC signals each 5kHz.
The same problem exists for 31m and 19m BC bands falling inside
the 12m ham band, and for 21m and 19m BC bands falling inside
the 10m ham band, but there are a lot of over combinaisons doing
the same.
For IMD3, the 41, 31 and 21 m BC bands can generate
tremendous spurious signals on the 40, 30 and 20 m ham bands.
Speaking of real life, I bought in the mid 80's a brand new
TS-830s, a very good reveiver, with down-conversion and
a preselector. This receiver is quite high into Rob chart, and
highly regarded (see eHam.net). When, for the first time
I connected it to my Sommer XP507 antenna (a kind of
log periodic wide band beam), the receiver was useless during
certain parts of the day when the propagation was good on the
BC bands. I could solve the problem from IMD2 with 2 switchable
high-pass filters (18 MHz high-pass for 17 and 15 m ham bands, and
24 MHz for 12 and 10 m ham bands). For 40, 30 and 20 m ham bands,
the only solution was to use the 20 dB attenuator to reduce IMD3.
I could also have used an outboard preselector.
I can attest that with the peak of cycle 24, if you have a
large and/or broadband antenna, these same problems will occur
if your receiver has poor IMD2 and remote signals IMD3. I can
also certify that 2kHz IMD3 will have very little interest
because statisticaly you will have a few ham signals inside
your roofing filter, and thousands large BC and utilities
signals outside your roofing filters. RF selectivity
(VRF or Digisel), relays switching of RF band-pass filters
will be most important, not 2 kHz IMD3, even if it has its
own relative importance.
Think about your trafic, about your antenna, make your
own jugement and enjoy your rig, no one is perfect. Think,
it's just a hobby.
Best regards to the group.
Georges F6DFZ
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