On Fri, 08 Sep 2006 22:04:06 +0200, "i3hev, mario held"
wrote:

wrote:

This is not that hard and there are tricks to do this over
very wide ranges. The easiest is create a fixed 90 degree
reference and down convert it.

that's true but, imho, there's no meaning in removing the (moderate)
complexities of the typical superhet receiver just to put (trickier)
complexities in quadrature downconverting... apart from part count,
controlling the reciprocal phase in two "identical" converters is in
itself not an easy task, since just a single degree of dephasing is
enough to degrade unwanted sideband rejection.

I disagree with trickier vs moderate. For a given level of
performance the two are comparable. The only context is
one may be familiar vs different as in new.

The phase of two converters is pretty easy to maintain, having
done it. However, creative part was down converting the LO
rather than the more critical small signal paths. Since the LO
would only at most require low pass (VFO at 70-124mhz with
fixed IQ generation at 70mhz would give a simple dc-54mhz LO)
to prevent the 70mhz and higher components. If passive DBMs
are used phase shifts from gain and added LCs can be avoided.
Also it's possible to put inline measurement of the resulting LOs
and feed back any phase error (found unneeded) for correction.

To build a superhet (ignore selectivity and overload performance for
the moment) to cover dc-54mhz you need upconversion and then down
conversion to IF and lots of filters along the way with switching.
Then you have the problem of various LOs and carrier osc (SSB/CW)
getting back into previous stages causing undesired tones. The more
conversions the greater the problem and the better shielding must be
to avoid it.

Having built both each has challenges and to do it as a high
performing reciever (or TX) requires a very similar effort even if
the areas where the effort need be applied differ.

The one big difference is that the image reject system is less
hardware between antenna and the baseband and that hardware
is more frequency agile. The best available analog answer is the
KK7B MiniR2 (or R2-PRO), base design uses SBL-1 mixers good
from 1-500mhz making generation of an 90degree LO the only real
work. As you go up in frequency a quadrature splitter will have
both phase and level bandwidth that increases making a
tuneable VHF or UHF IF practical. (FYI: accepable (under 1degree
and under 1db level variation) bandwidth at 14mhz is 1mhz.)

Of course the image reject detector system could exist at any fixed
frequency such as 66 mhz with a fixed LO. Then the upconverter only
needs a conventional LO of 66 to 120mhz still yeilds an RX that tunes
from DC to 54mhz and has few if any adjustments. This would yeild a
Single conversion RX with a VHF IF that is remarkably simple
and needs few filters.

If you take that to the next step using DSP at baseband where it's
cheap (using PS and soundcard) you have a very straightforward
high performing multimode radio. This is the current SDR approach.
When you consider a mainboard with ram, sound and other needed bits
running at 1+ ghz is dirt cheap (found free sometimes) and small this
is achieveable. Especially considering the software is already out
there. Also unused CPU cycles could also do the "glass front pannel"
and manage things like tuning the DDS or PLL.

... Those two things
are not easy or cheap.. yet. But we are getting there.

I'm sure we will get there soon enough, and it may
well be that just a few years will suffice; but, as
for now... they are not easy or cheap!

Actually it's progressing faster than you'd notice. The number of Ham
radios with IF DSP as the detection is considerable. Most of the
upper models of Tentec, Yaesu, Kenwood and others are already
doing it. Also the vendors of the DSP processors do have boards
for proto work to venture into this realm. With more people doing it
and plenty of alreay published work it may be more assembly
of components (be they hardware or software) than
construction.

While this wanders far from the IF question posted it's an example fo
how you can have IFs from baseband through UHF or higher.

Allison