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AGC - why delayed? good properties?
In researching desirable AGC characteristics that might be applied to the RS
DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". On the DX-394, I found and removed an electrolytic capacitor in the AGC line of the RF stages that has the effect of slowing the attack relative to that of the IF. Can't say I hear any difference with it out. I would have thought that we would want the AGC attack to be fast on all stages in order to avoid momentary overload. I concluded the following to be good targets for AGC behaviour after surveying a number of radios - corroboration or otherwise appreciated. Attack: 1-13ms Release: - fast: 25ms - medium: ~300ms - slow: 1.8-3 seconds I thought the fast release to be too fatiguing for human listening to SSB speech and ICW code but desirable for machine decoded data formats to minimise loss of data. Also, with audio derived AGC, the distortion on heavy bass modulation of all AM modes would be excessive. In applying mods to the DX-394 by others and some designed by myself, stretching the release time towards the 'slow' target has the side effect of lengthening the attack time to potentially a few hundred milliseconds. My version is the fastest so far with an attack of about 100 ms on a release of 2 seconds. I'm wondering if there is much to be gained by struggling to bring that down to the target of 1-13 ms. Comments on my assumptions, logic, conclusions and questions most welcome! TIA, Tom |
Tom Holden wrote:
In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Tom Holden wrote:
In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Tom,
The term "Delayed AGC" originally referred to the fact that the AGC action (reduction of gain) does not occur until the received signal reaches some signal level and NOT to a time delay. I believe this is to allow the signal to get up to a desirable level at the detector before any gain reduction occurs, otherwise it won't get to "maximum" level at the detector. AGC time constant is quite a personal preference and also depends on different conditions. I prefer slow AGC to prevent pumping and the annoying bursts of noise between SSB syllables, but if QSB is fast, this can loose syllables. -- Steve N, K,9;d, c. i My email has no u's. "Tom Holden" wrote in message .. . In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". On the DX-394, I found and removed an electrolytic capacitor in the AGC line of the RF stages that has the effect of slowing the attack relative to that of the IF. Can't say I hear any difference with it out. I would have thought that we would want the AGC attack to be fast on all stages in order to avoid momentary overload. I concluded the following to be good targets for AGC behaviour after surveying a number of radios - corroboration or otherwise appreciated. Attack: 1-13ms Release: - fast: 25ms - medium: ~300ms - slow: 1.8-3 seconds I thought the fast release to be too fatiguing for human listening to SSB speech and ICW code but desirable for machine decoded data formats to minimise loss of data. Also, with audio derived AGC, the distortion on heavy bass modulation of all AM modes would be excessive. In applying mods to the DX-394 by others and some designed by myself, stretching the release time towards the 'slow' target has the side effect of lengthening the attack time to potentially a few hundred milliseconds. My version is the fastest so far with an attack of about 100 ms on a release of 2 seconds. I'm wondering if there is much to be gained by struggling to bring that down to the target of 1-13 ms. Comments on my assumptions, logic, conclusions and questions most welcome! TIA, Tom |
Tom,
The term "Delayed AGC" originally referred to the fact that the AGC action (reduction of gain) does not occur until the received signal reaches some signal level and NOT to a time delay. I believe this is to allow the signal to get up to a desirable level at the detector before any gain reduction occurs, otherwise it won't get to "maximum" level at the detector. AGC time constant is quite a personal preference and also depends on different conditions. I prefer slow AGC to prevent pumping and the annoying bursts of noise between SSB syllables, but if QSB is fast, this can loose syllables. -- Steve N, K,9;d, c. i My email has no u's. "Tom Holden" wrote in message .. . In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". On the DX-394, I found and removed an electrolytic capacitor in the AGC line of the RF stages that has the effect of slowing the attack relative to that of the IF. Can't say I hear any difference with it out. I would have thought that we would want the AGC attack to be fast on all stages in order to avoid momentary overload. I concluded the following to be good targets for AGC behaviour after surveying a number of radios - corroboration or otherwise appreciated. Attack: 1-13ms Release: - fast: 25ms - medium: ~300ms - slow: 1.8-3 seconds I thought the fast release to be too fatiguing for human listening to SSB speech and ICW code but desirable for machine decoded data formats to minimise loss of data. Also, with audio derived AGC, the distortion on heavy bass modulation of all AM modes would be excessive. In applying mods to the DX-394 by others and some designed by myself, stretching the release time towards the 'slow' target has the side effect of lengthening the attack time to potentially a few hundred milliseconds. My version is the fastest so far with an attack of about 100 ms on a release of 2 seconds. I'm wondering if there is much to be gained by struggling to bring that down to the target of 1-13 ms. Comments on my assumptions, logic, conclusions and questions most welcome! TIA, Tom |
That's the meaning I know -- and it's probably more appropriate for the
purposes of keeping the receiver noise figure up. "Doug Smith W9WI" wrote in message ... Tom Holden wrote: In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
That's the meaning I know -- and it's probably more appropriate for the
purposes of keeping the receiver noise figure up. "Doug Smith W9WI" wrote in message ... Tom Holden wrote: In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
On Mon, 23 Feb 2004 10:28:19 GMT, Doug Smith W9WI
wrote: Tom Holden wrote: In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. Yes, this is correct for the proffesionals, a certain amplitude level must be reached before the AGC threshold is hit, while amateurs started to talk about time delay, rise and fall times when AGC was optimized for SSB reception. But even receivers designed before WWII had some degree of mode-dependent time delay optimization, fast for AM and somewhat slower for CW I experienced the importance of proper time constant when I tried to improve the Collins 51-S, see http://home.online.no/~la8ak/b35.htm 73 JM ---- Jan-Martin, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/ |
On Mon, 23 Feb 2004 10:28:19 GMT, Doug Smith W9WI
wrote: Tom Holden wrote: In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. Yes, this is correct for the proffesionals, a certain amplitude level must be reached before the AGC threshold is hit, while amateurs started to talk about time delay, rise and fall times when AGC was optimized for SSB reception. But even receivers designed before WWII had some degree of mode-dependent time delay optimization, fast for AM and somewhat slower for CW I experienced the importance of proper time constant when I tried to improve the Collins 51-S, see http://home.online.no/~la8ak/b35.htm 73 JM ---- Jan-Martin, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/ |
Jan-Martin Noeding, LA8AK wrote:
On Mon, 23 Feb 2004 10:28:19 GMT, Doug Smith W9WI wrote: Tom Holden wrote: In researching desirable AGC characteristics that might be applied to the RS DX-394 over a year ago, I came across the terms 'delayed' and 'hang'. Thought they were interchangeable but on reading the ARRL 2004 Handbook, it seems that 'delayed' means that the attack speed on the RF stages is slower than on the IF stages or is relatively delayed. According to the HB, "This prevents a premature increase in the receiver noise figure". I always thought "delayed AGC" means that there's no gain reduction unless the strength of the incoming signal reaches a certain threshold. It's not a delay in time, but in amplitude. Yes, this is correct for the proffesionals, a certain amplitude level must be reached before the AGC threshold is hit, while amateurs started to talk about time delay, rise and fall times when AGC was optimized for SSB reception. But even receivers designed before WWII had some degree of mode-dependent time delay optimization, fast for AM and somewhat slower for CW I experienced the importance of proper time constant when I tried to improve the Collins 51-S, see http://home.online.no/~la8ak/b35.htm I seized on the word "delayed" and linked it to the inclusion of a longer time constant in the AGC to the RF stages of the DX-394 than the time constant in the AGC to the IF stages. The Handbook example actually says "As an option, the AGC to the RF amplifier is held off, or 'delayed', by the 0.6V forward drop of the diode so that the RF gain does not start to decrease until larger signals appear. This prevents a premature increase in the receiver noise figure. Also, a time constant of one or two seconds after this diode helps keep the RF gain steady for the short term." I think 'delay' is a misnomer if what we have in effect is a higher threshold of signal strength for activation of RF AGC than for IF AGC. 'Two-step' AGC might be a better description. 'Delay' seems a more appropriate term for the way AGC is implemented in the DX-394. RF stage attack speed is slower (extra RC time constant probably on order of 100ms) than that for the IF stage and is clearly 'delayed' in reaching steady state, no matter what the signal level change is, as long as the AGC is activated by the higher, later level. Release speed of the RF AGC is similarly slowed or 'delayed' relative to IF AGC, certainly not by the 1 or 2 seconds in the HB example. Having removed the 'delay' capacitor, I have not noticed any deleterious effect. What should I be looking for? Would there be some advantage in revising the RF AGC to achieve a 2-step AGC? 73, Tom |
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