From: on Sun, Aug 20 2006 9:06 am

On 19 Aug 2006 20:19:19 -0700, "
wrote:
wrote:
On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held"
wrote:
Michael Black wrote:

However be wary of ICs like the MC1350 as the gain reduction occurs
the internal noise is bad. I've built several recievers using this
part and at ~10db gain reduction the noise jumps way up. I've gone
to cascode JFETs as the noise is more predictable and generally
lower. The device used does make a difference.

Allison

I have to disagree on the MC1350 and way back 30 years to its
predecessor, MC1590. The prototype HF receiver presently on
my workbench has a NF of 5.5 and that hardly rises more than
that with AGC current applied to the AGC pin.

Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the
point where the gain cell has equal conduction on both legs the noise
rises significantly. I duplicated the test fixture and yes, it's
noisy, from around 6db to around 11db in my fixture when gain is
reduced by 10db and that was at 16mhz. In a reciever that used
it I went to two cascode stages using JFETs and the difference noise
was notable for weak signals just into the agc range. I restrict the
1590/1350/ca3028 for lower perfomance recievers now.

Apparently I hit some nerve on my disagreement.

My first experience with the MC1590 was in 1973 and a need to
operate over 55-64 MHz. Electronic gain control was essential
and it had to be fast. Motorola supplied some additional
information which was later incorporated into appnotes.

The MC1350 was marketed around '73 along with the MC1330
video detector as a TV IF system. It didn't sell that
well in quantities (presumably) and both were dropped
from active production (Lansdale acquired masks and now
makes the MC1350). The 1350 (8-pin DIP) should use the
same die in the metal can MC1590. While neither one was
ever touted as a super-champ low-noise device, it is what
I consider respectable as to NF. The fact that it has
differential input and differential output is convenient
from the standpoint of circuit design. Especially so when
input impedances (each side) has a dependable 5K R in
parallel with about 5 pF total capacitance. Gain of both
begins to fall above 75 MHz with output loads of 100 Ohms
resistive. I've found no noticeable difference between
differential input v. single-ended.

That IC is what I term a "double Gilbert cell" in that
AGC control current affects both differential inputs
equally (or very nearly so). Whether one connects to
both inputs or just one shouldn't make any difference
other than output gain.

I also verified that the 1590 does same and also the CA3028
wired as differential AGC. Even tried three 2n3904s and
same result. The agc range was good and at full gain the
noise was ok but the noise increase at partial agc was surprizing.

I've never encountered any "surprising" increase in noise
at any AGC input to a 1590 or 1350 causing partial gain
reduction. That is as true in 2005 as it was in 1973. If
there is a SNR of 10 db at an RF carrier input of 3 uV and
a gain reduction of 10 db for a 10 uV RF input results in
3 db more noise in the front end, the SNR with a 10 uV
input is still higher than the one at 3 uV. What has been
"lost" there?

Let's look at the original problem starting this thread:
There was a claim of "increased noise" with AGC on, but
no quantifiable data. The sudden segue to stating that a
certain IC is "bad" is a leap that defies good design
practices to me. I'm not impressed that the ARRL had some
test data in a publication; having been hands-on with this
Motorola design for a number of years, I have a number of
RCA lab notebook pages filled with my testing of it along
with a patent involving it granted 1974...besides my own
hobby notebooks.

Low-noise input amplifier design is an entirely separate
subject and there are a number of other active devices
which can do lower NFs than 5. What was orignally needed
was some way of getting some numbers and test configuration
of Andrea's problem...to pin down a possible reason for
alleged increased noise with AGC applied, presumably a
"partial AGC" application. [I can't quantify "partial"
as a numeric value...maybe others can?]

I do most of my RX experimentation at 6/ 2M and 70cm SSB so
noise and overload perfomance are important to me. Images
are also a big problem as I'm near a lot of VHF/hf broadcast.

[shrug I live about 6 miles from 50 KW KMPC on AM...]

If we can get back to the original claim of "increased
noise with AGC applied" we might be able to help Andrea
some. We don't know what Andrea has for a main receiver
and interjecting some "badness" remarks by the ARRL about
a certain IC isn't going to help clarify Andrea's problem.