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

wrote:
. . .
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. . .

Very nearly 30 years ago, I was looking into "feed forward" circuits, a
technique developed by someone at Tektronix for ultra-low distortion
amplification. It turns out that the topology of the MC1350 is similar
to what's needed, and a feed forward amplifier can be made from one plus
just a few external components. But even by then, I'd learned that it's
risky to use components for other than their intended purpose. So I
collected 8 or 10 samples from various vendors (the part was widely sold
then), and opened them up. Those in cans were easy, using a little can
opener that worked like a tubing cutter. Some of the plastic DIP ones
were more difficult, but one of the labs at Tek was able to dissolve the
plastic while leaving the chip intact. Then I examined them carefully
with an inspection microscope. Here's what I found:

1. There were at least three very different designs. The chip size of
the largest was several times that of the smallest.
2. Some designs were inherently better balanced than others. Some had
resistive "cross unders" where traces cross, which weren't the same on
both sides of the circuit.

Based on this, I decided it was too risky to make a design based on that
part number, since a vendor could change chip suppliers or designs
without notice.

Interestingly, about six months later, I got a call from the component
engineering group asking if I still had the chips. It seems that one or
more of the vendors supplying that part (which was used for other
applications at Tek) had changed their design, causing failure of some
products and the shutting down of their production lines. Tek was big
enough that vendors were often required to give advance notice before
such changes, but they hadn't given any notice in this case.

I'm bringing this up because I'm hearing the MC1350 being spoken of as
though all are the same. It wouldn't surprise me if, after all these
years, they're now all being made with one design from one foundry. But
those ones in your junk box might be way more different than you think.

This is almost certainly true of just about any IC.

Roy Lewallen, W7EL

On Sun, 27 Aug 2006 13:56:50 -0700, Roy Lewallen
wrote:

wrote:
. . .
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. . .

Very nearly 30 years ago, I was looking into "feed forward" circuits, a
technique developed by someone at Tektronix for ultra-low distortion
amplification. It turns out that the topology of the MC1350 is similar
to what's needed, and a feed forward amplifier can be made from one plus
just a few external components. But even by then, I'd learned that it's
risky to use components for other than their intended purpose. So I
collected 8 or 10 samples from various vendors (the part was widely sold
then), and opened them up. Those in cans were easy, using a little can
opener that worked like a tubing cutter. Some of the plastic DIP ones
were more difficult, but one of the labs at Tek was able to dissolve the
plastic while leaving the chip intact. Then I examined them carefully
with an inspection microscope. Here's what I found:

1. There were at least three very different designs. The chip size of
the largest was several times that of the smallest.
2. Some designs were inherently better balanced than others. Some had
resistive "cross unders" where traces cross, which weren't the same on
both sides of the circuit.

Based on this, I decided it was too risky to make a design based on that
part number, since a vendor could change chip suppliers or designs
without notice.

Interestingly, about six months later, I got a call from the component
engineering group asking if I still had the chips. It seems that one or
more of the vendors supplying that part (which was used for other
applications at Tek) had changed their design, causing failure of some
products and the shutting down of their production lines. Tek was big
enough that vendors were often required to give advance notice before
such changes, but they hadn't given any notice in this case.

I'm bringing this up because I'm hearing the MC1350 being spoken of as
though all are the same. It wouldn't surprise me if, after all these
years, they're now all being made with one design from one foundry. But
those ones in your junk box might be way more different than you think.

This is almost certainly true of just about any IC.

Roy,

That is my engineering experience as well. At the time I did my
testing I had Motorola, National and Hitachi parts Some fairly current
date codes and a few from early 80s and and while the general
behavour was similar I noted differences in gain, overall noise
and DC balance as well. The noise increase was enough to
be noteable in a particular case but on analysis understandable
and to be expected.

Then again I date back to when the Fairchild UA703 was a
breakthrough gain block for RF.


Allison
KB!GMX

wrote:

: 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.

Sorry for the short answer. The receiver is a Yaesu FR-101 with his internal
2m converter. Actually, I'm doing some recalibration but I didn't found at
now nothing really out. I just noted that, mostly noticeable in FM mode
(by noise increasing) but present in all modes looking s-meter, the response
of the IF was not a gaussian with a flat top. In the center there was a
little decreasing area. If you tune it perfectly, you had a sligtly less
signal than little up or down in freq.

So I started recalibrating. I had stopped because I had trouble with the
noise blanker. In the manual, there is a indication of a test point where
hook a voltmeter, but there is not any test point nor any indication on the
schematic where the exact point is. The transformer is T116, maybe someone
could help. Tomorrow I'll figure out what to do, I think I'll hook the
voltmeter as to measure the voltage rectified, at the gate of the FET.
If you have the schematic (it's freely downloadable in many places) maybe
you could tell if I'm right.

Anyway, thankyou very much you want to help troubleshooting my receiver,
but I was using what I noted (increase in noise with AGC on) mainly to
start a general discussion about AGC implementation and effects, and his
future in digital receivers as well as in homebrew very high-end ones. It
has started, and it's very interesting.

Ciao,
AB

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

On Mon, 21 Aug 2006 16:15:01 -0500, Andrea Baldoni
wrote:

wrote:

: 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.

Sorry for the short answer. The receiver is a Yaesu FR-101 with his internal
2m converter. Actually, I'm doing some recalibration but I didn't found at
now nothing really out. I just noted that, mostly noticeable in FM mode
(by noise increasing) but present in all modes looking s-meter, the response
of the IF was not a gaussian with a flat top. In the center there was a
little decreasing area. If you tune it perfectly, you had a sligtly less
signal than little up or down in freq.

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.

So I started recalibrating. I had stopped because I had trouble with the
noise blanker. In the manual, there is a indication of a test point where
hook a voltmeter, but there is not any test point nor any indication on the
schematic where the exact point is. The transformer is T116, maybe someone
could help. Tomorrow I'll figure out what to do, I think I'll hook the
voltmeter as to measure the voltage rectified, at the gate of the FET.
If you have the schematic (it's freely downloadable in many places) maybe
you could tell if I'm right.

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

Anyway, thankyou very much you want to help troubleshooting my receiver,
but I was using what I noted (increase in noise with AGC on) mainly to
start a general discussion about AGC implementation and effects, and his
future in digital receivers as well as in homebrew very high-end ones. It
has started, and it's very interesting.

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


Allison

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.

: 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.

: 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.

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.

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

Ciao,
AB

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

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

Andrea Baldoni wrote:



wrote:

: 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.

Sorry for the short answer. The receiver is a Yaesu FR-101 with his
internal 2m converter. Actually, I'm doing some recalibration but I didn't
found at now nothing really out. I just noted that, mostly noticeable in
FM mode (by noise increasing) but present in all modes looking s-meter,
the response of the IF was not a gaussian with a flat top. In the center
there was a little decreasing area. If you tune it perfectly, you had a
sligtly less signal than little up or down in freq.


................ snip ...........................


Ciao,
AB

Are you sure you are seeing an AGC problem? What you describe above, with
slightly less signal in the center, is typical of a *filter* with dip in
the middle of the passband. This is *not* a result of increased signal
strength causing more AGC and thus more noise when the receiver is tuned to
exact center on a signal. It is the result of a filter design choice (lower
cost) or the result of the filter tuning (tuned for better shape at the
edges which causes more dip in the middle).

Without knowing more about the receiver I can't make any guesses as to what
is in play here but I question if this is an AGC artifact.

tim ab0wr

tim gorman wrote:

: Are you sure you are seeing an AGC problem? What you describe above, with
: slightly less signal in the center, is typical of a *filter* with dip in

I have the filter dip; it was not cured with realigning, but realigning was
anyway helpful to gain a dB or two.
I also see that disabling AGC cause less noise in FM while listening to 2m
converted to HF by the internal converter of the Yaesu FR-101. I didn't check
if enabling or disabling AGC cause any change in the filter dip, anyway I'll
check and report soon.

: Without knowing more about the receiver I can't make any guesses as to what
: is in play here but I question if this is an AGC artifact.

It uses a MC1496G as mixer and two CA3053 as IF. Plus some DG FETs, in first
RF amplifier, after the mixer... What information do you need?

Ciao,
AB

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