i3hev, mario held wrote:
Reg Edwards wrote:

If an amplifier incorporates a noise generator...
... then it is no longer just an amplifier. ....

... so, we must conclude that amplifiers do not exist!

of course, there is no real amplifying device which does not generate
noise; or, more precisely, there is no non-noisy real device at all

If you choose not to call "amplifier" a device which amplifies signals
if it has a non-unity noise figure, please feel free to do so - we live
in a (at least partially) free world... but you will be alone! )

Regards!

Don't be too hasty. Reg has verified he's got some coax that's much
lower loss than anyone else can buy (it actually meets the predictions
of his coax loss program), and has verified it by measuring it many
times. Maybe he also has a secret source of noise-free amplifiers. You
never know.

Roy Lewallen, W7EL

wrote:

: I'm researching about the matter and I just read that, in a BJT for
: instance, emitter current is inversely proportional to the noise. So,
: if AGC reduces the gain (so current), SNR degrade?

: Not necessarily true. Noise, true natural noise, in a bipolar
: ...
: of how many factors go into noise generation within the
: transistor. :-)

So, if you have to engineer a (let's start with HF) receiver, do you think
it may better to:

1)
find a way to insert automatically a stepped attenuation (maybe
using a diode switched resistor network) and leaving amplifiers without AGC,
thus optimizing them for a particular gain

2)
build circuits with so high dynamic range that's completely impossible to
have input signals overload them (what's the dynamic range one should
normally expect at the antenna input, excluding obvious limit-case situations
where the transmitting output is fed into the receiver input...?)

3)
use the usual AGC

....I'm thinking the 1 could be a good solution if the demodulator had to be
a digital one. That way, a calibrated attenuator simply add bits to the ADC.
Hovewer, the 2 is very attractive, providing that all is analog, or the
ADC dynamic range is better than the one that could come from the antenna...

I've read most use the 3, digitizing the AGC signal maybe with a second
ADC channel, to have anyway a sort of more bits of resolution.
So probably I'm wrong and the right solution is the 3... but only if adding
an AGC never ruin amplifiers performance.

: There isn't much FM on HF. What there is would be in
: narrow-band Data mode signals. Some of that Data is a
: combination of AM and PM similar to a wireline modem's
: modulation.

In fact I was receiving 144MHz using a converter.

: What is needed in an investigation of this is a reasonably-
: well-calibrated signal generator with a calibrated attenuator.

Unfortunately I have only a Instek function generator, and I'm not
very satisfied with any intrument I bought from this firm...

Anyway, sooner or later I'll build a dds one...

Ah, another question. I have a very precise digital voltmeter. Very
precise, 6.5digits (this time from Agilent)... Unfortunately, it's
absolutely unable to handle RF.
I would like to build a "RF" frontend for it... Any ideas?
I'm thinking to a precise rectifier built with an OP AMP followed by a
OP AMP integrator...

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.

Andrea Baldoni wrote:
. . .
2)
build circuits with so high dynamic range that's completely impossible to
have input signals overload them (what's the dynamic range one should
normally expect at the antenna input, excluding obvious limit-case situations
where the transmitting output is fed into the receiver input...?)
. . .

One night I heard audio in the background when listening to my direct
conversion 40 meter receiver. It was designed specifically to be as
immune as possible to AM demodulation, and since I had finished its
optimization several years before, I hadn't heard any audio from
demodulated AM. (It was common when I was using mixers with poorer
balance and dynamic range.) It didn't take long to find the station with
my home receiver. It was at about 7335 kHz, a religious HF broadcast
station in San Francisco (about 600 miles from here). The broadcast was
in Russian, so they were evidently beaming to Russia and I wouldn't be
far off the main beam.

Some careful measurements showed a signal strength of 250 mV RMS at my
receiver terminals. (That's 74 dB over the typical S9 value of 50 uV.) I
was using a vertical 4-square array, which isn't at all optimum for that
path. I hooked the antenna directly to my oscilloscope and could see the
carrier and modulation.

I took the receiver on a visit to England, and heard the audio from a
large number of AM stations in the background, so I believe the signal
levels there from HF broadcasters commonly exceeded the 250 mV I saw
only once at home.

The problem can of course be reduced by use of very narrow filters, but
they're often so close to the 40 meter band edges that even that
wouldn't be enough in most cases.

I've also encountered some staggeringly strong signals when operating
Field Day, when a group with a high or even moderate power transmitter
is on the next ridge or otherwise very close.

The bottom line is that I'd be hesitant to trust just about any number
for a "worst case" maximum signal strength. Be sure to test any proposed
design on 40 meters for a while from your location in Europe.

Roy Lewallen, W7EL

On Fri, 18 Aug 2006 16:33:25 +0200, "i3hev, mario held"
wrote:

tim gorman wrote:

You might also ask yourself whether it really matters or not. If your signal
is strong enough to begin driving the AGC to limit the system gain do you
really care what the noise level actually is? ....

But you may very well care for the SNR on an interesting weak signal
which you are trying to listen to, while a strong nearby signal
activates your AGC...

In theory the system bandwidth should not allow that strong signal to
hit the AGC. Of course practical systems this may not be true.

However, manual gain control helps if the stronger signal is not
overloading the front end causing gain compression and
intermodulation.

Allison

On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held"
wrote:

Michael Black wrote:

In that case though, you should be worried about that nearby strong signal
dropping the gain of the receiver so you can't hear the weak signal...

may be you are not thinking of cw...

Provided the SNR is good enough, you can filter and post-amplify your
weak signal, e.g. with a good AF filter, or a dsp.

Of course, if the IF stages are (reasonably) linear in response, you can
disable the AGC, but this would be no answer to the original question

Since the SNR is established by the frontend the IF system can have a
more relaxed SNR.

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

From: Andrea Baldoni on Sat, Aug 19 2006 4:05 pm


wrote:

: I'm researching about the matter and I just read that, in a BJT for
: instance, emitter current is inversely proportional to the noise. So,
: if AGC reduces the gain (so current), SNR degrade?

: Not necessarily true. Noise, true natural noise, in a bipolar
: ...
: of how many factors go into noise generation within the
: transistor. :-)

So, if you have to engineer a (let's start with HF) receiver, do you think
it may better to:

There's no "engineering" involved, just a crunching of numbers
AFTER you find the input levels versus AGC and how much noise
is actually generated...and approximately WHERE this excess
noise is coming from.

1)
find a way to insert automatically a stepped attenuation (maybe
using a diode switched resistor network) and leaving amplifiers without AGC,
thus optimizing them for a particular gain

I see no need of that at this point. "Getting fancy" with
extra circuitry is rather useless without knowing what the
problem all this fancy circuitry is supposed to cure.

2)
build circuits with so high dynamic range that's completely impossible to
have input signals overload them (what's the dynamic range one should
normally expect at the antenna input, excluding obvious limit-case situations
where the transmitting output is fed into the receiver input...?)

That's NOT the issue here.

Noise and signal-to-noise ratios are only important at LOWEST
signal levels, not the highest.

3)
use the usual AGC

Why not? Decades of designs in many countries have successfully
operated with "usual AGC." [voltage-controlled, sometimes
current-controlled gain stages driven by a DC control line]

...I'm thinking the 1 could be a good solution if the demodulator had to be
a digital one. That way, a calibrated attenuator simply add bits to the ADC.
Hovewer, the 2 is very attractive, providing that all is analog, or the
ADC dynamic range is better than the one that could come from the antenna...

Experiment any way you want but I can't see that as your cure.

I've read most use the 3, digitizing the AGC signal maybe with a second
ADC channel, to have anyway a sort of more bits of resolution.
So probably I'm wrong and the right solution is the 3... but only if adding
an AGC never ruin amplifiers performance.

A rather common (for decades of designs and production) AGC action
is no more than 6 db change in output for 60 to 100 db of input
signal (carrier) change.

AGC should be approached from the standpoint of a servo loop.
The "error signal" is the change in carrier level at the
detector. The controlled items are the RF and IF amplifiers.
The time-constant of the error feedback loop (what is commonly
called "the AGC line") is quite slow but fast enough to try to
keep detector level constant through flutter (rapid reflections
at VHF and up) and ionospheric path variations.

If "the AGC line" somehow has some noise in it, that noise is
probably going to change RF-IF amplifier gain. However, the
frequency of that noise is going to be low; it is band-
limited by the usual AGC line decoupling.

Let's look at SNR with low to higher antenna input levels:
1. Assume you have (for example) 1 uV of noise at no-signal.
2. If the RF signal is 3.16 uV then the signal-plus-noise
to noise ratio is 10 db.
3. If the RF signal is 10 uV then the signal-plus-noise to
noise ratio is 20 db.
4. If the RF signal is 31.6 uV then the signal-plus-noise
to noise ratio is 30 db.

The common (for about 40+ years, internationally) level of
receiver sensitivity for AM mode signals is a 10 db signal-
plus-noise to noise ratio. That's an easy test, done by
connecting an AC voltmeter (that can measure RMS voltage)
to the detector output. With no signal input, all you get
is front-end noise; note that. Apply a known-level RF
source to the antenna input, adjust that level to be 10 db
higher than the noise level measured with no signal input.
Note the RF source level; that is the "minimum
sensitivity" level for the common "10 db S+N:N" criterion.

For FM or PM it is a bit more complicated. FM and PM
rely on quieting through the Limiter stages ahead of the
FM detector. For most tests of FM/PM sensitivity you NEED
a known-signal-source-level to determine the quieting.

: There isn't much FM on HF. What there is would be in
: narrow-band Data mode signals. Some of that Data is a
: combination of AM and PM similar to a wireline modem's
: modulation.

In fact I was receiving 144MHz using a converter.

That data was omitted. Have you checked out the converter
insofar as adding noise? You can get a rough comparison
by using another HF receiver.

Have you checked your internal (to HF receiver) FM
demodulator characteristics? Do you have the manufacturer's
specifications on that? Since nearly all FM/PM demods
use Limiters, they normally operate with AGC off.

: What is needed in an investigation of this is a reasonably-
: well-calibrated signal generator with a calibrated attenuator.

Unfortunately I have only a Instek function generator, and I'm not
very satisfied with any intrument I bought from this firm...

Anyway, sooner or later I'll build a dds one...

You can't work in the dark (without instruments) when
trying to troubleshoot electronics.

A DDS (Direct Digital Synthesis) signal generator gives
you very precise FREQUENCY. For years there have been
L-C oscillator based signal generators which have been
stable enough in frequency to determine AGC action.

What you really need to investigate the AGC is PRECISE
RF ATTENUATION -and- a way to calibrate the maximum RF
output. [an ordinary diode detector could do that if it
was itself calibrated against a known RF source LEVEL]

Ah, another question. I have a very precise digital voltmeter. Very
precise, 6.5digits (this time from Agilent)... Unfortunately, it's
absolutely unable to handle RF.
I would like to build a "RF" frontend for it... Any ideas?
I'm thinking to a precise rectifier built with an OP AMP followed by a
OP AMP integrator...

The usual method of making a "precise" RF voltmeter is to
begin with a wideband video amplifier with gain controls
setting the gain in the full-scale ranges desired.

However, the BACK END needs attention, particularly if you
want TRUE RMS measurement. The "less precise" HP3400A
AC voltmeter could do that True RMS within 1% using an
analog meter readout (mirrored scale on meter). The
3400A used a pair of matched heaters and thermocouples.
Amplified AC heated one heater. A high-gain DC op-amp
had inputs (opposing) from both thermocouples. Op-amp
output heated the second heater. This was self-balancing.
The AC Voltage indicated actually came from the DC op-amp
output.

If you are going to measure AC-RF volts of both sinewaves
and noise, you need True RMS indication. Without that the
noise (random stuff) read by simple averaging rectifiers
will be DOWN by as much as 50% compared to a sinewave input.
There are three basic types of AC voltmeters made: Rectify-
average (common to handheld multimeters); Logarithmic (now
a standard of high-end bench multimeters) using special ICs
for True RMS conversion to DC; Thermal (now out of favor
in new designs but using the first-principles of measuring
the effective heating of a resistive load). Thermocouple
sensors are reliable, can handle overloads, but a diode
string biased for forward conduction can produce DC voltage
changes of -2 mV / degree C heating.

For some references, you can search the Internet for
"RMS to DC" conversion, or begin at www.ednmag.com, go to
their Archives button, select issue for May 11, 2000, and
look at the "How It Works" article by Jim Williams of
Linear Technology Corporation. LTC made an IC that was a
dual heater-sensor, the LT1088, but that IC is now
discontinued. The article shows a "front end" as well as
the whole AC voltmeter circuit.

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.

BTW, that receiver, single-conversion with one IF at 21.4 MHz,
uses only MC1350s up to the detector, including the one mixer
stage. [ LO is a separate PLL board ]

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.

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.

BTW, that receiver, single-conversion with one IF at 21.4 MHz,
uses only MC1350s up to the detector, including the one mixer
stage. [ LO is a separate PLL board ]

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.

Allison

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.

On 20 Aug 2006 12:46:25 -0700, "
wrote:

shortend

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.

Wide open and at 20db reduction the noise figure is not bad at
all. Only that the apparent increase is noteable. I've seen it
occur with other topologies including simple bipolar or FET
stages (even tubes).

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.

Those are the feature of the part that makes them desireable.

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 call that circuit an analog four quadrant multiplier for what it
does not how it's made.

The feature of the Gilbert cell that applies for the AGC use is the
lack of DC shift at the output points keeping downstream
DC coupled stages at their undistrubed bias points.

However the active device that is in the agc control
positions is still a noise generator (as are all active devices)
and as agc increases it's contribution is additive to the
RF path devices. Makes little difference if the node where
outputs are combined see no DC shift the various diff amp
transistor as individual pairs do see a significant shift (100%
collector current to near 0).

It's easier to see using the older MC1550 or CA3028 diffamps
or even discretes in a diffamp with current source.

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?

The surprize is that I'd not considered the possibility that
the SN+N/N could degrade unevenly due to applied agc.
So I'd never paid attention until I was trying to improve an
earlier reciever design (ca1978) of my own and at the same
time aquired a copy of EMRFD and did some testing to verify
their resuts. Since the design was optimized for low RF
gain and high overload thresholds I was revisiting anything
that could better the design without loosing those features.
Note it's a single conversion system with high IF. The
problem was a MDS of -136dbm but the 10db Signal+N/N
point was around -110dbm and at ~121dbm it was worse
than at -130! The front end was common gate RF amp
(2n4416s) driving a pair of 4416s in a single balanced
mixer. Low noise but limited gain for better overload
performance. No agc before the IF. If needed there
are switchable resistive attenuators (3, 6,12db or 19db total).
Measured gain from antenna to IF is only 16db (after all losses).
Disable the agc or increase the threshold and it wasn't as
measurable or appeared to disappear.. The hunt was on.
The results were a surprize as there is no data for noise output
with no input or signal to noise with gain reduction. My Moto
databooks go way back, as do my National, RCA and
Signetics library. More current datasheets do not reflect any
improved information.

The revised RX used two stages of mpf102 Jfet in cascode
plus a diferential pair of 2n3904s to resolve the 5-10V agc to be
compatable with the new fet amp to replace the two MC1350s
and the problem of decreasing signal to noise as signal increased
with agc active disappeared. Not to say the fet amps did not do the
same thing only that the rate of noise increase was a smoother curve
from max gain to min gain. In retrospect a delaying AGC to the first
of the two 1350s could potentially have the same effect but was not
investigated. I may revisit it at some time as I still have the
original if module in the junkbox.

Let's look at the original problem starting this thread:
There was a claim of "increased noise" with AGC on, but
no quantifiable data.

Thats a problem, the lack of data or information on the circuit.
Also I've repaired a few commercial radios that due to component
failure or "golden screwdriver" had the various operating conditions
sufficiently altered as to cause a similar problem.

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.

That's nice but are the test results in error from two different
sources? No.

However, it's was a noteable weak point. But calling it bad is
your words. It's a point that needs to be understood and
allowed for. In a design with more RF gain and/or less mixer
noise it many not have been a factor or less of one. Also in the
case that brought it to a point for me even altering how agc is
applied might have achieved a better result.

Since the 1350 is at IF for most designs the noise is likely from
front end causes should be investigated first. One would hope the
design had secured the system noise performance before
the IF. However in low gain systems or system with no gain
before the mixer and first filters this may be problematic.

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...]

10 miles from the Needham towers in MA. Not less than 8 VHF
broadcasters, then the usual crowd of UHF and now the
HDTV-UHF broadcasters and no small party of FM
broaccasters.

Oh and WKOX 1200 AM three miles away.

Then I have 9 hams within a 1 mile circle and two within 1500ft
running KW level at VHF. RFI are us. I understand overload
as +15dbm on coax is common here.

It's an interesting design challenge to do low noise figure RX
and at the same time be overload resistant in a harsh environment.

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.

Having tested and understood the problem I would say the
authors of EMRFD did a fair job of pointing out the points where a
device needs better understanding. A blanket "it's great" is lore,
testing it and understanding it is engineering. Having done the
work to understand it better I can appreciate the perfomance of
the part and it's limitations.

I still use it and have a tube of them because it's a useful part.
Just like the often reviled SA602 mixer.

It's relevence is I've seen this before and understood it's origin
and also elsehere. The other aspect is that if a commonly
accepted part is not fully understood and can lead to undesired
effects then, why not others. AGC is not a trivial thing to be
tacked on and considered a problem solver. Protects the ears
but it's place in the reciever is not always understood. It does
not always solve things like gross overload at the front end or
possibly further down or outside the agc detectors bandwidth.

But a lack of information about his radio doesn't help us either.
We do not know for instance what topology is used for RF
and if any agc is even applied to it.


Allison