On Sat, 26 Aug 2006 14:50:32 -0500, Andrea Baldoni
wrote:

wrote:

: Dual conversion for HF and triple conversion of 2m. There would
: not be RF agc on the 2m converter. In FM modes you should see
: limiting so agc is behavour is different.

By looking closer at the AGC circuits, there is RF AGC also in the 2m
and 6m converters. However, the AGC involves not all amplifiers in this
receiver, just the first and the last but one. I often see different
configurations in receivers, where every amplifier is controlled.
Maybe they thought it would suffice.

Often not every amplifier needs to be controlled. It's a matter of
van the overall gain change be achieved with fewest control points.


: Likely a test point that is not marked on the board. I don't have
: a print handy.

I had calibrate it for the minimum voltmeter reading, because it appears that
the signal is negative over a positive DC bias, supplied by the JFET.
Probably there is another point where the signal is positive, anyway it
should be the same.

Generally will be. What I have seen in some cases is where the no
signal resting point for gain control bias voltage is not correct and
the gain can go up a bit before going down. Often seen on oder
recievers where the large part of the radio is discrete devices
and the various setpoints have drifited from age or componenet
changes.

: Having experimented with recievers since before EE school and still
: many decades later I find it challenging. Always looking for and at
: new ideas.

I have read very interesting articles on QEX (by downloading them in PDF
format from the site) following back the chain of cross references starting
from "A software defined radio for the masses" to the R1 and R2.

I've build R1, R2, MiniR2. I happen to like the miniR2 and use on
with the matching T2 in 6m as my highest performing RX. Highest
performing in this case is best dynamic range and lowest noise.

SDR is an extension of that work. However since most of the digital
work is done at low IF (under 50khz) or at baseband all of the
frontend, filter and IF issues remain though the tradeoffs may be
different.

Every designer has his own ideas about AGC, dynamic range, et all. Often
very different. While I never (as now) tried to design a receiver, I want
to carefully understand the reasons behind every implementation. So I could
build my own opinion and in future choose what I think better for a receiver
project.

Very true. What was considered best in class for 1960, 1970, 1980
and so on has changed considerably. However it's possible to
use older topologies with newer devices and obtain perfomance
unattainable back then. What is easiest to build and make perform
is usually a low gain approach using amplifers that do not overload
easily to get a balanced dynamic range and noise figure. I've seen
too many chase for a high gain for sensitivity at low HF only to
be overloaded with man made and atmospheric noise.

A good example of a strong staple topology is the Elecraft K2. A
very solid single conversion with medium high IF transceiver. The
basic design topology is 25 years old (look up Progressive RX,
QST) but, it's well executed using current tech parts. The manual
is available on line and worth reading and reviewing.

Another interesting argument is LO: DDS, PLL or DDS+PLL?

I use Analog with premix for lowest close in noise. Though I
also have a DDS with PLL tracking filter and a straight PLL
system that has proven satisfactory at my favorite band (6M).
In each case considerable care was taken to well shield and
filter the signals used while using the best techniques. Executed
with care and with an eye for how it fits into the system all work
well. Of those (opinion follows) I find DDS has agility and stability
with the very noticeable tradeoff for spurious outputs. Due to that
I prefer to restrict raw DDS to narrow band systems or clean
them up with a tracking PLL. The cost is power, and great
care in shielding as the various oscillators and the concurrent
need for a microprocessor to do the translation of knobs, buttons
and tuning displays into the digital control for DDS can contribute
to a lot of undesired signals (birdies).

So the whole subject of the LO system can be as complex as the
rest of the reciever and be a significant factor in it's total
performance.


Allison