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Question: Why do you call "AGC" a simple automatic antenna "attenuator" in this forum? #ubitx #ubitxv6


IW4AJR Loris <lorisbollina@...>
 

From my tests and from the measurements made, it appears to me that the best known "AGC" modification module is a very bad "ATTENUATOR" placed in series with the receiving antenna!
Although I have tried, measured and studied the scheme and compared it with the reality installed on my µBITX-V6, the theory of the principle used confirms what the practice shows:
Although an automatic attenuator placed in series with the antenna is excellent (try it yourself with the best PIN attenuators on the market) the residual attenuation of the circuit at the minimum attenuation will never be less than -3dB, often, even in the most refined circuits. , yes it gets a minimum of -5 / -6 dB! ... not to mention what two poor quality MOSFETs can do (compared to a PIN diode attenuator) ... the best measurement I got was -10 / -15 dB, which is equivalent to losing at least 2 S points on the reception!
It is totally unacceptable! and technically from "geeks" and not from radio amateurs!
From my tests, only one module acts correctly on the "gain" of the MF amplification and, even if a bit obsolete and bulky (it is not built in SMD), it would seem the most efficient module, not changing the "sensitivity" of the receiver (very important thing in DX also in QRP).
Why do you insist on referring to these "ATTENUATORS" as "AGC" "Automatic GAIN Control"?


Bill Cromwell
 

Hi,

Attenuating the signal at the receive antenna input is not gain control at all. Attenuation can reduce overwhelming signals that distort the receiver front end and make those very weak signals copyable again. It may seem counterintuituve to improve weak signal reception by increasing attenuation at the antenna but that is *exactly* what happens.

Automatic Gain Control may be applied to an RF amplifier immediately following the antenna input and before the first mixer but that is not attenuation. Some os us still like to have manual gain control applied in the RF and IF sections. A manual attenuator that can be switched out completely is an excellent choice and one that I prefer. Not a PIN diode.

73,

Bill KU8H

bark less - wag more

On 11/18/20 5:11 AM, IW4AJR Loris wrote:
From my tests and from the measurements made, it appears to me that the best known "AGC" modification module is a very bad "ATTENUATOR" placed in series with the receiving antenna!
Although I have tried, measured and studied the scheme and compared it with the reality installed on my µBITX-V6, the theory of the principle used confirms what the practice shows:
Although an automatic attenuator placed in series with the antenna is excellent (try it yourself with the best PIN attenuators on the market) the residual attenuation of the circuit at the minimum attenuation will never be less than -3dB, often, even in the most refined circuits. , yes it gets a minimum of -5 / -6 dB! ... not to mention what two poor quality MOSFETs can do (compared to a PIN diode attenuator) ... the best measurement I got was -10 / -15 dB, which is equivalent to losing at least 2 S points on the reception!
It is totally unacceptable! and technically from "geeks" and not from radio amateurs!
From my tests, only one module acts correctly on the "gain" of the MF amplification and, even if a bit obsolete and bulky (it is not built in SMD), it would seem the most efficient module, not changing the "sensitivity" of the receiver (very important thing in DX also in QRP).
Why do you insist on referring to these "ATTENUATORS" as "AGC" "Automatic GAIN Control"?


IW4AJR Loris <lorisbollina@...>
 

Hi Bill ... I fully agree with you!
What I am wondering is "why" this forum continues to call AGC a very vulgar "ATTENUATOR" even if more or less automatic.
73 Loris


Bill Cromwell
 

Hi Loris,

There are a lot of newbies here and they just don't know. Don't be angry. Just help them alomng if they are actually open to help. I started learning about physics and electronics in the early 1950s and I still don't everything. I am more and moire aware of how much I don't know. I usually do know where to look:)

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:17 AM, IW4AJR Loris wrote:
Hi Bill ... I fully agree with you!
What I am wondering is "why" this forum continues to call AGC a very vulgar "ATTENUATOR" even if more or less automatic.
73 Loris


Curt
 

Loris

Interesting viewpoint, as I agree it is unusual to place AGC attenuation at the very front. I had not seen any loss measurements, and I imagine they vary with frequency. Perhaps they are not as high as you cite at HF.

The designer had a good motivation seeking to enhance dynamic range, but the ubitx already seems to have its gain conservatively distributed. My antennas are mere dipole, verticals and 2 element compact yagi. The ubitx is not swamped even with many big amateur stations here in Maryland.

On low HF atmospheric and man made noise dominate more than receiver noise figure, but up on 28 MHz this added loss, whatever it is, will have impact in not hearing as well.

As the designer, who left out agc to keep ubitx simple and affordable around the globe, suggests an IF agc such as w7zoi hybrid cascode would be ideal (I have one here not in use yet). In the ubitx design it can be integrated but with directional amplifiers that do TR switching it may not easily patch in.

Rigs do work without agc, it just means operator must regularly use the audio gain control knob. I find only that loud signals are uncomfortable when tuning across the band. The simple, novel circuit using an LED and photoresistor does this wonderfully.

Curt


Gary Rindfuss
 

Ok agree its not an agc. But it addresses the problem of having  your eardrums blown out with a sudden strong signal when you are wearing headphones. Plus it is easy to bypass. Just place a tiny reed relay across the board with nc contacts bridging the entire circuit. Relay off no "agc" relay on AGC
Does anyone have an easy way to implement an actual AGC ? 
One that can be managed by old eyes and old hands?

On Wed, Nov 18, 2020, 7:32 AM Bill Cromwell <wrcromwell@...> wrote:
Hi Loris,

There are a lot of newbies here and they just don't know. Don't be
angry. Just help them alomng if they are actually open to help. I
started learning about physics and electronics in the early 1950s and I
still don't everything. I am more and moire aware of how much I don't
know. I usually do know where to look:)

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:17 AM, IW4AJR Loris wrote:
> Hi Bill ... I fully agree with you!
> What I am wondering is "why" this forum continues to call AGC a very
> vulgar "ATTENUATOR" even if more or less automatic.
> 73 Loris
>






IW4AJR Loris <lorisbollina@...>
 

Hi Curt and hi Gary ...
to be honest I'm not even a little angry, on the contrary I'm stimulated to try to learn something more (as Bill rightly says, the more I study the more I realize that I still need to learn something else!).
I agree with Curt for the excellent judgment he gives to the µBITX, I own Version 6, but I believe that the judgment is applicable to all versions, especially I agree on the BF limiter that uses an LED and a photoresistor, it reminds me of a very old dynamic compressor circuit that I used 40/50 years ago on my HiFi to compress the dynamics of the Vinyl being recorded on magnetic tape.
I'm going crazy looking for the scheme among the old paperwork, there was a beautiful hardware solution that used an LED and a photostransistor (replaceable with a more convenient and less expensive photoresistor) stuck in a black plastic tube, if I find it again I scan it and I publish it, I will certainly remove the attenuator from the µBITX and insert this solution !!!
For Gary, I conducted my experiments in 40m (band on which I love to do portable QSO) and the data collected refer to old HP and TEK instruments and perhaps they are a bit too penalizing, I used an HP8454B generator with HP11710B downconverter , an HP436A Bolometer with HP8481D head, a TEK2535 oscilloscope and a TEK2465B oscilloscope ... obviously, since the bolometer head is characterized only starting from 10MHz, it may be that it gave a slightly worse result than the real one, but not by a lot (half a point S counts for little against 2 or 3 points S) ... and to conclude I would say that to not blow the eardrums Curt's solution is definitely better than any other.
Finally, I feel like finding a way to realize a real good AGC to be inserted in the µBITX, for example with an input amplifier MOSFET (perhaps with variable gain) driven by an AGC signal taken from the MF immediately after the quartz filter, I'm studying on ... if they are roses they will bloom.
a dear greeting ... IW4AJR Loris


Arv Evans
 

AAT (Automatic Attenuator Control)?
_._


On Wed, Nov 18, 2020 at 5:32 AM Bill Cromwell <wrcromwell@...> wrote:
Hi Loris,

There are a lot of newbies here and they just don't know. Don't be
angry. Just help them alomng if they are actually open to help. I
started learning about physics and electronics in the early 1950s and I
still don't everything. I am more and moire aware of how much I don't
know. I usually do know where to look:)

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:17 AM, IW4AJR Loris wrote:
> Hi Bill ... I fully agree with you!
> What I am wondering is "why" this forum continues to call AGC a very
> vulgar "ATTENUATOR" even if more or less automatic.
> 73 Loris
>






iz oos
 

Loris, would you think a vogad at the audio output could be a solution to save both sensitivity and hears to some extent?


Il mer 18 nov 2020 06:10 PM Arv Evans <arvid.evans@...> ha scritto:
AAT (Automatic Attenuator Control)?
_._


On Wed, Nov 18, 2020 at 5:32 AM Bill Cromwell <wrcromwell@...> wrote:
Hi Loris,

There are a lot of newbies here and they just don't know. Don't be
angry. Just help them alomng if they are actually open to help. I
started learning about physics and electronics in the early 1950s and I
still don't everything. I am more and moire aware of how much I don't
know. I usually do know where to look:)

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:17 AM, IW4AJR Loris wrote:
> Hi Bill ... I fully agree with you!
> What I am wondering is "why" this forum continues to call AGC a very
> vulgar "ATTENUATOR" even if more or less automatic.
> 73 Loris
>






Bob Lunsford
 

For headphones, I wonder if two diodes with polarities reversed from each other across the headphone line would provide some hearing protection...? Or a pair of reversed polarity in series if the volume were affected too much. Does this cause severe distortion? (I've read about doing this but never tried it.)

By the way, what is "vogad"???

Bob — KK5R

On Wednesday, November 18, 2020, 7:38:16 PM EST, iz oos <and2oosiz2@...> wrote:


Loris, would you think a vogad at the audio output could be a solution to save both sensitivity and hears to some extent?

Il mer 18 nov 2020 06:10 PM Arv Evans <arvid.evans@...> ha scritto:
AAT (Automatic Attenuator Control)?
_._


On Wed, Nov 18, 2020 at 5:32 AM Bill Cromwell <wrcromwell@...> wrote:
Hi Loris,

There are a lot of newbies here and they just don't know. Don't be
angry. Just help them alomng if they are actually open to help. I
started learning about physics and electronics in the early 1950s and I
still don't everything. I am more and moire aware of how much I don't
know. I usually do know where to look:)

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:17 AM, IW4AJR Loris wrote:
> Hi Bill ... I fully agree with you!
> What I am wondering is "why" this forum continues to call AGC a very
> vulgar "ATTENUATOR" even if more or less automatic.
> 73 Loris
>






Curt
 

Bob

I have seen that method used -- so give it a try if you have a pair of diodes handy.  I am trying to recall which rig I saw this used with.  silicon may work - but maybe some folk may use germanium or schottky - try what you have. 

At least with my v4, I am not hearing signals that would blast me out of the chair.  Its just a matter of 'enjoying tuning across the band.  If you have a photoresistor handy, then find an LED and a couple NPNs to try the circuit I recommend.  Otherwise, who knows see what the diodes will do. 

cool thing about the ubitx - 'have it your way' -- no reason not to be counter-culture.  forgive me for liking the dancing LED inside my ubitx case.  enjoy your listening and radiations. 

Curt


Bob Lunsford
 

Thanks Curt for your encouragement. I will try it soon and report to the group.

Bob — KK5R

On Wednesday, November 18, 2020, 8:30:24 PM EST, Curt via groups.io <wb8yyy@...> wrote:


Bob

I have seen that method used -- so give it a try if you have a pair of diodes handy.  I am trying to recall which rig I saw this used with.  silicon may work - but maybe some folk may use germanium or schottky - try what you have. 

At least with my v4, I am not hearing signals that would blast me out of the chair.  Its just a matter of 'enjoying tuning across the band.  If you have a photoresistor handy, then find an LED and a couple NPNs to try the circuit I recommend.  Otherwise, who knows see what the diodes will do. 

cool thing about the ubitx - 'have it your way' -- no reason not to be counter-culture.  forgive me for liking the dancing LED inside my ubitx case.  enjoy your listening and radiations. 

Curt


Bill Cromwell
 

Hi,

I have done and will be doing it again. When very loud signals suddenly appear in the cans, instead of making your ears bleed you will hear it suddenly very distorted and maybe a little louder. That is your cue to turn down the volume. It won't function like the AGC you are used to. It will protect your hearing.

73,

Bill KU8H

bark less - wag more

On 11/18/20 7:56 PM, Bob Lunsford via groups.io wrote:
For headphones, I wonder if two diodes with polarities reversed from each other across the headphone line would provide some hearing protection...? Or a pair of reversed polarity in series if the volume were affected too much. Does this cause severe distortion? (I've read about doing this but never tried it.)
By the way, what is "vogad"???
Bob — KK5R
On Wednesday, November 18, 2020, 7:38:16 PM EST, iz oos <and2oosiz2@gmail.com> wrote:
Loris, would you think a vogad at the audio output could be a solution to save both sensitivity and hears to some extent?
Il mer 18 nov 2020 06:10 PM Arv Evans <arvid.evans@gmail.com <mailto:arvid.evans@gmail.com>> ha scritto:
AAT (Automatic Attenuator Control)?
_._
On Wed, Nov 18, 2020 at 5:32 AM Bill Cromwell <wrcromwell@gmail.com
<mailto:wrcromwell@gmail.com>> wrote:
Hi Loris,
There are a lot of newbies here and they just don't know. Don't be
angry. Just help them alomng if they are actually open to help. I
started learning about physics and electronics in the early
1950s and I
still don't everything. I am more and moire aware of how much I
don't
know. I usually do know where to look:)
73,
Bill KU8H
bark less - wag more
On 11/18/20 7:17 AM, IW4AJR Loris wrote:
> Hi Bill ... I fully agree with you!
> What I am wondering is "why" this forum continues to call AGC
a very
> vulgar "ATTENUATOR" even if more or less automatic.
> 73 Loris
>


Bob Lunsford
 

Curt, I just tried two 1N914 type diodes connected anode-cathode and placed it across the earphone lines to ground. I tuned to a net where there were several hams with one really loud and the others weaker. The diode arrangement did help to the extent that while the loud station was still loud, it was not booming in my ears. If I had to guess, the signal in the headphones was about half what it was and it was livable. Therefore, it does help and I did not have to chase the volume control to keep the net comfortable as I did without the diodes.

The headphones were the common, 32 Ohm type.

I'd call it a clipper, a "leveler" or an equalizer but not an attenuator. Ha

It does not serve as a type of AGC but better if it is considered a crude safety device to help save the ears until something better comes along.

Bob — KK5R

On Wednesday, November 18, 2020, 8:30:24 PM EST, Curt via groups.io <wb8yyy@...> wrote:


Bob

I have seen that method used -- so give it a try if you have a pair of diodes handy.  I am trying to recall which rig I saw this used with.  silicon may work - but maybe some folk may use germanium or schottky - try what you have. 

At least with my v4, I am not hearing signals that would blast me out of the chair.  Its just a matter of 'enjoying tuning across the band.  If you have a photoresistor handy, then find an LED and a couple NPNs to try the circuit I recommend.  Otherwise, who knows see what the diodes will do. 

cool thing about the ubitx - 'have it your way' -- no reason not to be counter-culture.  forgive me for liking the dancing LED inside my ubitx case.  enjoy your listening and radiations. 

Curt


Bob Lunsford
 

I tried the scheme with two computer switching diodes from the earphone lines to ground and it does work although not as effectively as I'd hoped. I'll prepare a headset to do this for me and I expect it to still work as designed with other applications. May have to change the connector, though, to avoid cutting the headphone line.

Bob — KK5R

On Thursday, November 19, 2020, 2:12:45 AM EST, Bill Cromwell <wrcromwell@...> wrote:


Hi,

I have done and will be doing it again. When very loud signals suddenly
appear in the cans, instead of making your ears bleed you will hear it
suddenly very distorted and maybe a little louder. That is your cue to
turn down the volume. It won't function like the AGC you are used to. It
will protect your hearing.

73,

Bill  KU8H

bark less - wag more

On 11/18/20 7:56 PM, Bob Lunsford via groups.io wrote:
> For headphones, I wonder if two diodes with polarities reversed from
> each other across the headphone line would provide some hearing
> protection...? Or a pair of reversed polarity in series if the volume
> were affected too much. Does this cause severe distortion? (I've read
> about doing this but never tried it.)
>
> By the way, what is "vogad"???
>
> Bob — KK5R
>
> On Wednesday, November 18, 2020, 7:38:16 PM EST, iz oos
> <and2oosiz2@...> wrote:
>
>
> Loris, would you think a vogad at the audio output could be a solution
> to save both sensitivity and hears to some extent?
>
> Il mer 18 nov 2020 06:10 PM Arv Evans <arvid.evans@...
> <mailto:arvid.evans@...>> ha scritto:
>
>    AAT (Automatic Attenuator Control)?
>    _._
>
>
>    On Wed, Nov 18, 2020 at 5:32 AM Bill Cromwell <wrcromwell@...
>    <mailto:wrcromwell@...>> wrote:
>
>        Hi Loris,
>
>        There are a lot of newbies here and they just don't know. Don't be
>        angry. Just help them alomng if they are actually open to help. I
>        started learning about physics and electronics in the early
>        1950s and I
>        still don't everything. I am more and moire aware of how much I
>        don't
>        know. I usually do know where to look:)
>
>        73,
>
>        Bill KU8H
>
>        bark less - wag more
>
>        On 11/18/20 7:17 AM, IW4AJR Loris wrote:
>          > Hi Bill ... I fully agree with you!
>          > What I am wondering is "why" this forum continues to call AGC
>        a very
>          > vulgar "ATTENUATOR" even if more or less automatic.
>          > 73 Loris
>          >
>
>
>
>
>
>






Curt
 

Bob

That was kind of what I expected, call it 3 or maybe even 6 dB, but perhaps you think a bit less. Now if you had a photoresistor, I find these can suppress a signal around 20 dB. Amazing to me being resistive material, relative to light energy, it works without apparent distortion.  It cannot free us from needing to use a volume control, but its what i was seeking. Let there be light, dampening the signal chain. Or if one must have real agc, go to work in the IF signal chain. But cool this scheme merely wires to the volume control.

73 curt


IW4AJR Loris <lorisbollina@...>
 

Hello "iz oos",
normally a vogad, such as the SL6270 or similar, is used as a dynamics compressor for microphone input, obviously, with the appropriate checks of in / out levels and coupling impedances, nothing prevents it from being used even in a BF circuit to control the level output in the speaker or headphones, I just wonder if it's worth it, the cost will certainly be higher than a coupled LED / photoresistor that can give equally valid if not better results.
The advantage of the audio compression system that gives an LED / photoresistor circuit is the flattening of the dynamics practically independent of the input frequency (obviously within the limits of the LED's ability to react to high frequencies) on a much greater frequency spectrum than a chip vogad, this allows to keep the frequency response of the BF amplifier circuit intact.
Also consider the simplicity of the circuit (see the two attached diagrams), to add a LED / photoresistor compressor in the µBITX just insert the circuit between the output and the negative input of the LM386 and that's it.
Best wishes from IW4AJR Loris


IW4AJR Loris <lorisbollina@...>
 

Hi Bob,
 
why go crazy to try and try to invent hot water?
 
In HiFi amplifiers, the problem of ear protection in headphones has already been solved for almost a hundred years! just make a resistive divider on the output with two resistances between the signal (positive pole of the speaker) and ground (120Ω / 1W + 56Ω / 1 / 2W) and that's it, have you ever skipped eardrums listening to a vinyl ad high dynamics with headphones? if you love listening to Classical Music, sudden changes from a pianissimo to a fortissimo are the order of the day!
Look at the attached diagram, it is taken from a SUPERSCOPE A-260 from the 70s ... there are so many things to experience on the µBITX without getting lost in things that have already been solved for decades!
Good experiments and a dear greeting from IW4AJR Loris


IW4AJR Loris <lorisbollina@...>
 

Hi Bob,
 
If you want to try out an "ancient" headphone dynamics compressor, known to QST readers in the 60s, you can try the attached diagram.
 
The attached simulation is done with LTspice, the values are a little different because I have simulated different solutions, but the substance does not change ... "IT WORKS" !!! ...
P.S. the simulation is done by combining 3 frequencies (300Hz, 1.650Hz and 3.000Hz) to see the behavior in the normal OM reception range and check for any harmonic distortions introduced, the result is excellent, from the FFT the only harmonics present are over 5kHz , in an area where they should not give distortions.
 
Good experiments and a dear greeting from IW4AJR Loris


TD
 

What component is P1? Where in a transceiver would one connect this circuit?

Thanks,

---
Darrell Lee
Advanced Data Systems, Inc.
2801 Wade Hampton Blvd.
Suite 115-153
Taylors, SC 29687
864-230-9626 | dlee@adsi-sc.com

On 11/19/2020 10:27 AM, IW4AJR Loris wrote:
Hi Bob,
If you want to try out an "ancient" headphone dynamics compressor,
known to QST readers in the 60s, you can try the attached diagram.
The attached simulation is done with LTspice, the values are a little
different because I have simulated different solutions, but the
substance does not change ... "IT WORKS" !!! ...
P.S. the simulation is done by combining 3 frequencies (300Hz, 1.650Hz
and 3.000Hz) to see the behavior in the normal OM reception range and
check for any harmonic distortions introduced, the result is
excellent, from the FFT the only harmonics present are over 5kHz , in
an area where they should not give distortions.
Good experiments and a dear greeting from IW4AJR Loris
Links:
------
[1] https://groups.io/g/BITX20/message/83809
[2] https://groups.io/mt/78337317/4392615
[3] https://groups.io/g/BITX20/post
[4] https://groups.io/g/BITX20/mutehashtag/ubitx
[5] https://groups.io/g/BITX20/mutehashtag/ubitxv6
[6] https://groups.io/g/BITX20/editsub/4392615
[7] https://groups.io/g/BITX20/leave/defanged