I've been working on making a decent chart out of MS-Excel and still
trying to figure out the best "AVERAGE" syntax to use for the Y axis.
I've been browsing around the web, stopped by Tu-Cows.com to see if
they had any templates for excel. Any ideas for making a chart to
plot AGC curves? I have several (background) links on my voting
system and wanted to make some bench marks. At work we do it manually
in pencil, for our microwave RSLs.

Any input from you would be appreciated. It's for Amateur Radio.

Regards,

I didn't know FM rigs had an AGC curve. FM does not need any AGC.

Once the input signal is strong enough to fully drive the Limiter, typically 10
uV, the magnitude of the input signal is a don't care.

Karl wrote:

I've been working on making a decent chart out of MS-Excel and still
trying to figure out the best "AVERAGE" syntax to use for the Y axis.
I've been browsing around the web, stopped by Tu-Cows.com to see if
they had any templates for excel. Any ideas for making a chart to
plot AGC curves? I have several (background) links on my voting
system and wanted to make some bench marks. At work we do it manually
in pencil, for our microwave RSLs.

Any input from you would be appreciated. It's for Amateur Radio.

Regards,

Some FM rigs use FM Rx chips that have a signal strength output, Lke the
Signetics NE-605. These can have a dynamic range of more than 80 dB with
excellent liniarity and temperature stability. In fact some DIY spectrum
analyzers use these in the 'video' IF for the level detector.

On Sat, 26 May 2007 14:01:53 -0400, Dave wrote:

I didn't know FM rigs had an AGC curve. FM does not need any AGC.

Once the input signal is strong enough to fully drive the Limiter, typically 10
uV, the magnitude of the input signal is a don't care.

Karl wrote:

I've been working on making a decent chart out of MS-Excel and still
trying to figure out the best "AVERAGE" syntax to use for the Y axis.
I've been browsing around the web, stopped by Tu-Cows.com to see if
they had any templates for excel. Any ideas for making a chart to
plot AGC curves? I have several (background) links on my voting
system and wanted to make some bench marks. At work we do it manually
in pencil, for our microwave RSLs.

Any input from you would be appreciated. It's for Amateur Radio.

Regards,

Dave,

I guess I didn't explain myslef. Acually all FM receivers can and may
use the the IF to generat a DC
voltage depending on the strength of the signal coming down the IF.
This voltage can be used to drive
an "S" meter or other measuring device. If you calibrate this voltage
by plotting on a graph for the Y axis
with a sig gen in the into for the X axis you can plot an AGC curve.
Real handy for determing RSLs.
We do it in microwave work all the time.

Regards,
Karl

Ok, we have a terminology problem then. AGC (Automatic Gain Control) is found in
AM receivers to control the gain of the RF amp stage and one or two IF amplifier
stages.

FM receivers almost never use any gain control (some designs that have to work
under very high signal strength conditions may use a simple AGC to keep the
impedance constant at the IF filter), depending instead on the IF limiter(s) to
produce a constant output. They may have an RSS (Relative Signal Strength)
output to drive S-meters. It is usually derived from current changes in the
limiter/last IF stages as the signal input rises. Any signal that falls within
the IF filter passband can make this rise, be it spark/arcing noise from car
ignition, broadband noise from computers, or spill over from nearby tranmitters.

In FM voting systems, this RSS may not be the best signal to use since any of
these sorts of interference may cause this level to jump up without opening the
squelch. The squelch circuit senses the audio noise level at the detector ouput
(though some old designs may use the lack of audio noise on the last limiter
RSS) and is a much better indicator of signal quality. If you can dig out of the
receiver schematic where this noise detector is and use it instead you'll have
far fewer false votes to a jammed receiver.

Now, to your original question, Excel and graphing. I'm not sure exactly what
you're looking for. Simple instructions on how to produce a graph for print-out?
Or something a bit more involved like curve fitting to get a smooth curve
instead of straight lines through the data points? Excel ought to have examples
of the former in the 'help' tab. For curve smoothing, look for 'statistical
analysis' in the help also. My search on google.com for 'Excel "polynomial
regression" ' yielded several good sources of more information.

73's

On 27 May 2007 22:10:47 -0700, Karl wrote:

Dave,

I guess I didn't explain myslef. Acually all FM receivers can and may
use the the IF to generat a DC
voltage depending on the strength of the signal coming down the IF.
This voltage can be used to drive
an "S" meter or other measuring device. If you calibrate this voltage
by plotting on a graph for the Y axis
with a sig gen in the into for the X axis you can plot an AGC curve.
Real handy for determing RSLs.
We do it in microwave work all the time.

Regards,
Karl

Karl, I think you misunderstand me.

My understanding of AGC [my college days were almost 50 years ago] is briefly
described as follows.

An AGC [Automatic Gain Control] curve is a plot of the negative feedback to
control [reduce] the gain of the rf amplifier and/or first i.f. stage necessary
to maintain a constant audio level. Typical AGC dynamic ranges may easily exceed
90 dB of attenuation or more. In a modern receiver the MDS is typically -134
dBm. An average audio output may be +16 dBm [earphone]. With an input variation
from -134 dBm [S0] to S9+40 [typically -34 dBm] the AGC dynamic range has to be
100 dB for 'constant audio output'.

In FM radios the constant audio level is typically determined by the saturation
level of the discriminator/limiter. Any further increase in the input signal
cannot produce more audio/output because the discriminator/limiter have
saturated [assuming a TX/RX matched deviation system]. In amateur radio
equipment a relatively low level rf signal will fully saturate the limiter
circuit [10 to 15 uV]. This will indicate S9++ on a typical amateur receiver.
[Note: 10 to 15 uV will produce an average S meter reading of S7 +/- in amateur
SSB/AM/CW modes.]

I understand from your description that the measure of i.f. voltage amplitude
is simply a measure of the open circuit gain and associated normal non
linearities of the rf/i.f string before limiter saturation. Is this correct? Is
there a negative feedback system to reduce the gain of the rf stages in your
description?

Are you using the term in a manner that I am not familiar with? I am trying to
understand exactly what you are saying.

/s/ DD, W1MCE

Karl wrote:

Dave,

I guess I didn't explain myslef. Acually all FM receivers can and may
use the the IF to generat a DC
voltage depending on the strength of the signal coming down the IF.
This voltage can be used to drive
an "S" meter or other measuring device. If you calibrate this voltage
by plotting on a graph for the Y axis
with a sig gen in the into for the X axis you can plot an AGC curve.
Real handy for determing RSLs.
We do it in microwave work all the time.

Regards,
Karl

Lynn Richardson wrote:

Ok, we have a terminology problem then. AGC (Automatic Gain Control) is found in
AM receivers to control the gain of the RF amp stage and one or two IF amplifier
stages.

FM receivers almost never use any gain control (some designs that have to work
under very high signal strength conditions may use a simple AGC to keep the
impedance constant at the IF filter), depending instead on the IF limiter(s) to
produce a constant output. They may have an RSS (Relative Signal Strength)
output to drive S-meters. It is usually derived from current changes in the
limiter/last IF stages as the signal input rises. Any signal that falls within
the IF filter passband can make this rise, be it spark/arcing noise from car
ignition, broadband noise from computers, or spill over from nearby tranmitters.

SNIPPED

Lynn, sorry for my direct response to this thread. I posted it before I read
your response.

We are both making the same point. AGC is a negative feedback subsystem in a
receiver to maintain a constant output. I was wondering how the term was/is
being used.

/s/ DD