From: Paul Keinanen on Wed,May 11 2005 12:13 am

On 10 May 2005 13:59:13 -0700, wrote:

From: Paul Keinanen on 10 May 2005 09:11:19 -0700

In a radio receivers, the signal levels vary
from less than a microvolt to several volts, so the crosstalk issues
are much more demanding.

I will disagree on radio receivers on such wide dynamic
ranges. "Several volts" INTO a receiver front end?
No. Such levels aren't encountered in practical
locations and would, definitely, cause enough IM
that would create much distortion and spur products.

Look at a multitransmitter contest site with one transmitter on each
band, the voltage induced to the receiving antennas for other bands
can be quite large.

In a production model receiver? Mais non. That's
not a design prerequisite, never was, not even with
the Rhode & Schwarz designs featuring very high
3rd IP specifications.

I've been IN such situations on aircraft installations
where the potential RFI was much stronger than in ham
DXpedition or Field Day setups. The work-arounds to
make the receivers operate is NOT a design criteria,
not in avionics-oriented design plans.

Of course, in a competent receiver design only the
frequency band of interest is filtered out before processing. However,
if the antenna is connected directly to the backplane and the modules
do their own filtering, the large composite signal on the backplane
will radiate all around the system.

Possibly, IF and only IF the antenna IS connected
to the "backplane" (or motherboard). Why must it be
so? Look at the PC. Sound cards have their audio
input (at microphone levels) on a separate connection).
No interference doing that.

In non-contest sites large wire or log-periodic antennas can collect a
quite large signal voltage (in the order of 0 dBm, 220 mV or more).

Perhaps, but that still isn't a design criterion for
present-day ham receivers.

Also if the final IF is within or below the receiver tuning range and
a diode ring mixer is used as the SSB demodulator with +7 or +17 dBm,
you must keep this BFO signal and harmonics from entering the front
end.

Yes...but that was a problem a half century ago, too! :-)


Even the SDR is going to need some switchable front end band pass
filters in order to survive in the hostile RF environment these days
with a lot of strong signals even in ordinary sites.

Diode switching. My two-decade old Icom R-70 has that
to select approximate octave-bandwidth bandpass filters
to cover 50 KHz to 30 MHz. Has its own little PCB,
probably because every single L, C, diode, and resistor
is included on that board...no shielding except from
the side wall of the cabinet and part of the cast frame.

I've had that little receiver within a city block from
AM BC station KMPC running 50 KW into its towers. Worked
fine with a temporary long-wire antenna despite the RF
around that station.

In transceivers, there would be several points that would need
switching.

Of course. That's what was done two decades ago.

I used the CANbus as an example, since the cable can be tens or
hundreds of meters long depending on speed and thus, it could be used
to control some internal points in a transceiver as well as wire all
devices in the ham shack as well as in the tower. For instance, the
same controller could control the antenna rotator, command the antenna
preamplifier to bypass mode, turn the transvertter into transmit mode,
select the VFO frequency for transmit (in split operation) and finally
turn the transmitter on.

Right, no problem...except for the individual ham
installer who then has to set up the "program" to
do all those things. Can they? :-)

I think a better approach is something like SGC does
in their automatic antenna tuners. They add a
frequency meter function to their tuner micro-
controller, a small section of Flash memory to
hold data, measure an RF input, adjust the coupler
switches to compensate for VSWR, then record that
data in memory. Any future frequency close to the
recorded memory can use the same settings.

Near-ultimate in modularity is thus achieved. Needs
only DC power to operate and doesn't care what kind
of transmitter is connected to it...as long as its in
specification for power and frequency. Absolutely
"plug-and-play!" :-)


What is lacking is STANDARDIZATION.

This is definitely a big problem.

Yes and no. :-) It's like a recipe for "tiger
soup:" "First, you have to catch a tiger..."

In a similar way, there must be SOME idea of
what kind of control range, modulation, etc.,
etc. would be expected...and for what radio
service. The FCC in the USA can't yet come to
grips on that, nor has industry made much
progress outside of their own product lines.

Right now, it is more like Pandora's Box.


That can't be worked out in newsgroups,

A newsgroup is a good place for open ended discussions between people
with experience in quite different fields.

I agree. But, like the infamous "John Smith,"
it can be infiltrated with someone who doesn't
have either the experience or the courage to
use his/her real name. Raises the noise
level enough to make some go QRT for a while.


Writing a formal specification may require some formal organisation,
but on the other hand quite a few successful RFCs in the IT sector are
written by a single person or a small group.

Ahem...that INDUSTRY specification is going to
range considerably farther than some small group
within one company. As to IT (Information
Technology), I've not seen ANY industry-wide
softwares which extend beyond corporate levels
and that's been for the last three decades.
LANGUAGES not counted there.

and willingness to compromise

That is the problem in formal committees, in which most delegates from
various vendors have large commercial interests in the subject and in
order to be able to produce even some kind of standard, all features
from various vendors are included.

I'll just cite the ARINC standards which are
generally used internationally for all civil
avionics, from radio to radar, radionavigation
systems. ALL the interfaces to every avionics
box and the physical shape and mountings. NOT
a big commercial venture in terms of profit.
If you've been able to read the verbatim minutes
of ARINC meetings (I have), then you would see
that it can be done.

ARINC = Aeronautical Radio INCorporated, once a
radio communications provider for airlines,
later evolving into a combined industry-government
central standards organization for civil avionics.
[they have a website, BTW, but the documents are
horribly expensive now...]