RadioBanter - AGC Questions

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Could some kind soul please explain the whole AGC thing to me ?
And keep it simple please.
What does it do?
When should I use AGC Fast?
When should I use AGC Normal?
If I turn off AGC, and manually adjust the RF Gain, what am I doing, and why
am I doing it?
Anything else I should know?
FYI, I have an Icom R75 with all the Kiwa mods, and an Antenna Supermarket
Eavesdropper dipole.
Thanks,
Lenny

In article ,
"Lenny" wrote:

Could some kind soul please explain the whole AGC thing to me ? And
keep it simple please. What does it do? When should I use AGC Fast?
When should I use AGC Normal? If I turn off AGC, and manually adjust
the RF Gain, what am I doing, and why am I doing it? Anything else I
should know? FYI, I have an Icom R75 with all the Kiwa mods, and an
Antenna Supermarket Eavesdropper dipole.

Basically the radio has to adapt to the signal level you are tuned to by
changing its sensitivity. A weak signal will cause the AGC voltage,
which stands for Automatic Gain Control (this is a feedback loop in the
radio) to change to a level for maximum sensitivity. If the radio is
tuned to a strong signal the AGC voltage will change to cause the radio
to be less sensitive. This same AGC voltage also drives the "S" meter on
most radios so when the "S" meter swings up to indicate a strong signal
the radio is also being driven to be less sensitive.

Generally I set the AGC to the fastest settings that does not cause an
annoying pumping of the signal for AM. If the AM broadcast has a rapidly
changing amplitude I would then switch the AGC to a slower rate of
change.

For SSB usually you want a slow AGC so you don't get the noise between
the audio peaks for SSB broadcast. Generally you will want slow AGC for
ham SSB for the same reason.

For any kind of digital mode you want fast AGC and if it is an option on
your radio you would also want to use fast response IF filters.

Band scanning for any type of signals I use the fastest AGC.

The ICOM R75 AGC is a more complicated thing than on most radios and
some of the "Mods" may well have changed the way the radio AGC behaves
so I'll have to leave the commentary to someone else that owns one.

--
Telamon
Ventura, California

"Telamon" wrote in message
Basically the radio has to adapt to the signal level you are tuned to by
changing its sensitivity.

.... in order to avoid overload distortion at any stage in the receiver by
excessive signal and to even out the loudness between weak and strong
signals so that the listener does not have to adjust the volume control.

For any kind of digital mode you want fast AGC ....

Telamon, I've read that elsewhere and it seems to be embedded in specs but I
don't understand why after having experimented with modifying a DX-394A. My
impression from the literature I have read is that the 'ideal' receiver is
supposed to have fast attack (under 10 milliseconds) and variable release
(slow 1 second, medium ~ 100's of milliseconds, fast ~ 10's of
milliseconds).

1. A problem is if the attack is under 10 ms and the release is slow, then a
very short noise burst reduces the receiver gain for a long time. Seems to
me that the attack should be proportional to the release to avoid that
problem but that might not be ideal for SSB or Morse. I can see that, for
most data, a fast attack/release combo would likely minimise dropouts. Maybe
the ideal has independently variable attack and release to accommodate all
conditions.

2. Many DRM experimenters have found that the maximum DRM SNR is achieved by
turning AGC off but that may result in a higher long term error rate due to
signal fades and surges after the gain has been set. The reasons why the Max
SNR is reduced by AGC are not fully understood - one, specific to the
DX-394A and original DX-394 but possibly a factor in other radios, is that a
local oscillator was pulled as the mixer gain varied, whether by manual RF
Gain control or by AGC. This results in a phase noise and, in extreme swings
in gain, in the loss of lock. There are other more subtle factors degrading
the Max DRM SNR that remain that I don't know about. (Note that the DRM SNR
has nothing to do with the perceived audio SNR but is a simple
representation of the quality of the transmission channel calculated from
the scattering of the transmission coding constellation.)

Tom

On Sat, 7 Jan 2006 21:42:48 -0500, "Lenny"
wrote:

Could some kind soul please explain the whole AGC thing to me ?
And keep it simple please.
What does it do?
When should I use AGC Fast?
When should I use AGC Normal?
If I turn off AGC, and manually adjust the RF Gain, what am I doing, and why
am I doing it?
Anything else I should know?
FYI, I have an Icom R75 with all the Kiwa mods, and an Antenna Supermarket
Eavesdropper dipole.
Thanks,
Lenny

Basically, if there is a lot of fading and variability in the signal,
the Automatic Gain Control can smooth it out. It might help you on a
fading and choppy foreign broadcast. It can smooth out a ham SSB
signal. I like to set the AGC to off on CW because then I can hear a
"pop" at the beginning of each dit or dah; otherwise the reception is
smoother and I hear no pop. For RF gain, I like to set it on the
threshold between increasing and decreasing the signal. Any higher a
setting, and atmospheric noise seems excessive, to me. I'd let your
ears be your guide.

The choppier the signal, the more you might use AGC, and the faster
the settings.

The smoother and stronger the signal, the less it might be needed.

RF gain, I just keep at the threshold, and don't diddle with much.

Other ears may vary...

bob
k5qwg

Ok, Lenny, here is a (somewhat) simple explanation. The Automatic Gain
Control attempts to keep the volume level the same on all stations so
that if you turn the volume up on a weak station and then tune to a
stronger one, you won't get "blasted" out of the room. The reverse also
holds true - if you've set the volume based on s atrong station and
then tune to a weak one, the volume will be about the same and you'll
be able to hear it.

"Fast" AGC and "Slow" AGC are just what they say they are. One setting
makes it work quickly and one makes it work slowly.

Of course your question is "why" and what does it mean to me?

When tuning the radio, you want a fast AGC to keep all the volume
levels approximately the same so that, if you tune to a weak signal,
you can hear it, and if you tune to a strong signal, the volume level
will not be too great. AGC has been featured on radios since the 1930's
(maybe even earlier) but, on most consumer AM/FM radios, it is so
common that no one mentions it any more (just like the "number" of
transistors - no one "cares" any more). But on shortwave, not only is
it very useful, it is necessary to be able to adjust it. You cannot
take advantage of a radio's DXing abilities unless you have an
adjustable AGC (among other things!).

Once you have tuned in a station and have decide to listen to it, you
switch to slow AGC. Why? Because signal strength fluctuates greatly on
SW signals and a slow AGC, not responding as fast as fast AGC, will
tend to keep the volume level the same and will not fluctuate up and
down as it would if you used a fast AGC. (You'll hear background "hiss"
rising and falling as the signal-strength increases and decreases -
look at your S-meter, but the volume level of the broadcast will stay
about the same.)

So, to make a long story short, when tuning your radio, use fast AGC.
Once you find something to which you want to listen, switch to slow
AGC.

Now what about AGC "off?" You use that in conjunction with your RF gain
control when chasing the most exotic and weak signals (it is also
sometimes useful when listening to Hams on SSB). The AGC can attenuate
(lessen) the apparent strength of some signals so, by turning it off
and adjusting the RF gain to maximum, you may be able to hear the
station. Note that you do NOT want to casually tune with the AGC off.
You'll get "blasted!" (Just try that and see!) You CAN tune slowly in,
say the tropical band, where you might expect to find some unusual or
difficult-to-hear country. But always you must do this carefully. You
can also (rarely) use this to minimize "overload" from a strong station
located close to your location; in this case, you would turn the RF
control all the way down. With an ICOM IC-R75, though, you should
rarely, if ever, need to do this. Remember, most AGC sections,
certainly the one on your ICOM, work extremely well and rarely, if
ever, cause problems that would make you want to turn the AGC off.

So, in conclusion, you will use the AGC fast setting for tuning and the
AGC slow setting for listening. AGC off will be used rarely, if at all.

I hope that's of some help to you.

Best,

Joe

"Lenny" wrote in message
...
Could some kind soul please explain the whole AGC thing to me ?
And keep it simple please.
What does it do?
When should I use AGC Fast?
When should I use AGC Normal?
If I turn off AGC, and manually adjust the RF Gain, what am I doing, and
why
am I doing it?
Anything else I should know?
FYI, I have an Icom R75 with all the Kiwa mods, and an Antenna Supermarket
Eavesdropper dipole.
Thanks,
Lenny

One other thing comes to mind: when listening to AM DX, turn off your AGC.
You may want to set AF gain (Volume) up quite high, after first turning down
RF Gain. Control the "volume" by the RG gain (instead of the usual AF gain.)

Mark.
Auckland
New Zealand.

Great explanation.
Now I get it.
Thanks Joe and everyone else who answered my question.
Lenny

"Joe Analssandrini" wrote in message
ups.com...
Ok, Lenny, here is a (somewhat) simple explanation. The Automatic Gain
Control attempts to keep the volume level the same on all stations so
that if you turn the volume up on a weak station and then tune to a
stronger one, you won't get "blasted" out of the room. The reverse also
holds true - if you've set the volume based on s atrong station and
then tune to a weak one, the volume will be about the same and you'll
be able to hear it.

"Fast" AGC and "Slow" AGC are just what they say they are. One setting
makes it work quickly and one makes it work slowly.

Of course your question is "why" and what does it mean to me?

When tuning the radio, you want a fast AGC to keep all the volume
levels approximately the same so that, if you tune to a weak signal,
you can hear it, and if you tune to a strong signal, the volume level
will not be too great. AGC has been featured on radios since the 1930's
(maybe even earlier) but, on most consumer AM/FM radios, it is so
common that no one mentions it any more (just like the "number" of
transistors - no one "cares" any more). But on shortwave, not only is
it very useful, it is necessary to be able to adjust it. You cannot
take advantage of a radio's DXing abilities unless you have an
adjustable AGC (among other things!).

Once you have tuned in a station and have decide to listen to it, you
switch to slow AGC. Why? Because signal strength fluctuates greatly on
SW signals and a slow AGC, not responding as fast as fast AGC, will
tend to keep the volume level the same and will not fluctuate up and
down as it would if you used a fast AGC. (You'll hear background "hiss"
rising and falling as the signal-strength increases and decreases -
look at your S-meter, but the volume level of the broadcast will stay
about the same.)

So, to make a long story short, when tuning your radio, use fast AGC.
Once you find something to which you want to listen, switch to slow
AGC.

Now what about AGC "off?" You use that in conjunction with your RF gain
control when chasing the most exotic and weak signals (it is also
sometimes useful when listening to Hams on SSB). The AGC can attenuate
(lessen) the apparent strength of some signals so, by turning it off
and adjusting the RF gain to maximum, you may be able to hear the
station. Note that you do NOT want to casually tune with the AGC off.
You'll get "blasted!" (Just try that and see!) You CAN tune slowly in,
say the tropical band, where you might expect to find some unusual or
difficult-to-hear country. But always you must do this carefully. You
can also (rarely) use this to minimize "overload" from a strong station
located close to your location; in this case, you would turn the RF
control all the way down. With an ICOM IC-R75, though, you should
rarely, if ever, need to do this. Remember, most AGC sections,
certainly the one on your ICOM, work extremely well and rarely, if
ever, cause problems that would make you want to turn the AGC off.

So, in conclusion, you will use the AGC fast setting for tuning and the
AGC slow setting for listening. AGC off will be used rarely, if at all.

I hope that's of some help to you.

Best,

Joe

In article ,
"Tom Holden" wrote:

"Telamon" wrote in
message
Basically the radio has to adapt to the signal level you are tuned
to by changing its sensitivity.

... in order to avoid overload distortion at any stage in the
receiver by excessive signal and to even out the loudness between
weak and strong signals so that the listener does not have to adjust
the volume control.

For any kind of digital mode you want fast AGC ....

Telamon, I've read that elsewhere and it seems to be embedded in
specs but I don't understand why after having experimented with
modifying a DX-394A. My impression from the literature I have read is
that the 'ideal' receiver is supposed to have fast attack (under 10
milliseconds) and variable release (slow 1 second, medium ~ 100's
of milliseconds, fast ~ 10's of milliseconds).

1. A problem is if the attack is under 10 ms and the release is slow,
then a very short noise burst reduces the receiver gain for a long
time. Seems to me that the attack should be proportional to the
release to avoid that problem but that might not be ideal for SSB or
Morse. I can see that, for most data, a fast attack/release combo
would likely minimise dropouts. Maybe the ideal has independently
variable attack and release to accommodate all conditions.

2. Many DRM experimenters have found that the maximum DRM SNR is
achieved by turning AGC off but that may result in a higher long term
error rate due to signal fades and surges after the gain has been
set. The reasons why the Max SNR is reduced by AGC are not fully
understood - one, specific to the DX-394A and original DX-394 but
possibly a factor in other radios, is that a local oscillator was
pulled as the mixer gain varied, whether by manual RF Gain control or
by AGC. This results in a phase noise and, in extreme swings in gain,
in the loss of lock. There are other more subtle factors degrading
the Max DRM SNR that remain that I don't know about. (Note that the
DRM SNR has nothing to do with the perceived audio SNR but is a
simple representation of the quality of the transmission channel
calculated from the scattering of the transmission coding
constellation.)

Maybe I should not have commented on the digital mode since it has been
years that I have used a decoder on short wave but what I recommended is
what I recall working best for me most of the time. I was using an R8
with a decoder and looking at the R8B manual the AGC spec is:
Mode Attack release
Slow 1 ms 2 sec
Fast 1 ms 300 ms

Assuming the R8 and R8B AGC specs are the same the Fast AGC was the
right choice for most situations.

Although the R8 and R8B have a filter for RTTY I also used the CW filter
for the small shift 450Hz signals. Then there were some modes where I
had to use the LSB filter so 500Hz, 1.8 KHz and 2.3 KHz. Generally you
used the smallest filter for the shift you were trying to get as the
smaller filter lowers the noise floor.

I have no experience with DRM other than the noise it generates on the
bands but since it is a digital mode I would think Fast AGC would the
best choice. If your radio is programmable then I would set the fastest
settings for attack, hang and release. Here you don't actually care what
the received digital signal sounds like and you want the AGC on to
prevent an kind of overloading together with as rapid response as
possible to signal increases or decreases. This would be the best case
for getting the most bits in the stream on a continuous basis. The sound
you do care about is after the digital processing and then conversion to
audio. One problem DRM has on the receiver end is the high noise floor
due to the very wide bandwidth needed. This is yet another maddening
tradeoff when one looks at the entire system of transmitter, ionosphere
and receiver. Increased intermodulation products, high noise floor and
probably other things I have not thought about.

I generally stay away from turning the AGC off. The only reason to do
that is if the signal you are trying to get is very weak and noise is
desensitizing the radio though the AGC action.

--
Telamon
Ventura, California

In article ,
Telamon wrote:

I have no experience with DRM other than the noise it generates on the
bands but since it is a digital mode I would think Fast AGC would the
best choice.

Wouldn't the AGC action stick some quick phase shifts in the signal? That
would be readly bad news for a DRM signal. I'd think that no or a very
slow AGC would be best and let the decoder algorithms deal with the short
term variations.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)