RadioBanter - View Single Post

May 11th 05, 08:13 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. 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.

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).

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.

The CANbus has been used in the automobile industry for more than a
decade. The CANbus has a nondestructive collision system, so this
makes it possible to have a true peer-to-peer communication system,
without complex protocols (such as token passing).

IF and only if this SDR of the future NEEDS micro-
computer control...or even modular microcontroller
sub-systems.

Trying to use an EXISTING computer interface system
isn't always good because that system has worked for
a decade-plus. While automotive computer interface
system speeds are increasing with increasing control
demands, radios aren't quite vehicles. The control
needs aren't quite the same.

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.

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

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.

examples of previous control systems deleted

The basics have already been laid
down for the SDR system on what CAN work.

What is lacking is STANDARDIZATION.

This is definitely a big problem.

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. This can generate quite
different ideas (some useful, most less usable) than a "business as
usual" attitude. It is a good idea to have a lot of new ideas to chose
from than having no new ideas at all.

but requires much more organization...

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.

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. This makes the final standard hard
to implement properly or each manufacturer is implementing only a
subset of the complex standard and thus, there is no real guarantee
that two devices would actually communicate, even if both claim
compatibility with xxxx standard (remember the RS-232 "standard" :-).

Paul OH3LWR