|
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 |
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 |
Tom Holden wrote:
[snip] 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! Still looking for feedback on these targets. Had one private reply that raised an interesting point about noise blanker operation and 'delayed' RF AGC: "You might want to test out a system of this sort in the face of a significant amount of broadband impulse noise - e.g car-ignition noise or something like that. If the attack is made too fast, then impulse noise at a rate of, say, 60-100 Hz would tend to force the RF AGC into its low-gain state, and this could allow weaker RF signals to be buried in the noise of the first-stage mixer. Using a longer attack time constant will render the RF AGC a lot less vulnerable to the effects of impulse noise. You can then use a noise blanker, located prior to the first IF AGC, to trim out this noise. As long as the first-stage mixer and the RF amp stage aren't forced into excessive intermodulation by the content of the impulse noise, I'd expect that this approach would give you the best set of behavioral tradeoffs." 73, Tom |
Tom Holden wrote:
[snip] 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! Still looking for feedback on these targets. Had one private reply that raised an interesting point about noise blanker operation and 'delayed' RF AGC: "You might want to test out a system of this sort in the face of a significant amount of broadband impulse noise - e.g car-ignition noise or something like that. If the attack is made too fast, then impulse noise at a rate of, say, 60-100 Hz would tend to force the RF AGC into its low-gain state, and this could allow weaker RF signals to be buried in the noise of the first-stage mixer. Using a longer attack time constant will render the RF AGC a lot less vulnerable to the effects of impulse noise. You can then use a noise blanker, located prior to the first IF AGC, to trim out this noise. As long as the first-stage mixer and the RF amp stage aren't forced into excessive intermodulation by the content of the impulse noise, I'd expect that this approach would give you the best set of behavioral tradeoffs." 73, Tom |
In article , "Tom Holden"
writes: 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. According to terms used a half century ago in tube-type receivers, "delayed AVC" (they used 'AVC' back then instead of 'AGC') just meant that AGC action is delayed from taking effect until the signal level rises above wide-open gain. Wasn't a time delay, but closer to "hold off" voltage of some sort on the AGC control voltage action. "Attack" and "decay" meant the time of response of the AGC, in the same manner as audio-only control circuitry for broadcast audio modulation input. "Attack" being the onset of a signal and subsequent audio gain reduction time..."decay" being the release time of any audio gain reduction control signal, that 'decaying' back to original high-gain amplification. I suspect that the original "delayed AVC" came about from two reasons: First was that 'communications receivers' of olden times usually tuned a rather wide band, usually 1:3 in frequency range and the non-AGC gain varied by almost the same amount as a result of tuned circuit impedance changes (tubes would amplify almost entirely by only voltage changes when running Class A). A hold-off on the AGC action handled the difference in overall gain at the extremes of tuning. Second, and I suspect the stronger reason, was that customers could hear the front end noise in the speakers (with AGC held totally off and gain wide open) and get the impression of a "really hot receiver!" :-) It's easy to get any kind of "delayed AGC" or hold-off on AGC control with a DC bias on the detector. As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Tom Holden"
writes: 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. According to terms used a half century ago in tube-type receivers, "delayed AVC" (they used 'AVC' back then instead of 'AGC') just meant that AGC action is delayed from taking effect until the signal level rises above wide-open gain. Wasn't a time delay, but closer to "hold off" voltage of some sort on the AGC control voltage action. "Attack" and "decay" meant the time of response of the AGC, in the same manner as audio-only control circuitry for broadcast audio modulation input. "Attack" being the onset of a signal and subsequent audio gain reduction time..."decay" being the release time of any audio gain reduction control signal, that 'decaying' back to original high-gain amplification. I suspect that the original "delayed AVC" came about from two reasons: First was that 'communications receivers' of olden times usually tuned a rather wide band, usually 1:3 in frequency range and the non-AGC gain varied by almost the same amount as a result of tuned circuit impedance changes (tubes would amplify almost entirely by only voltage changes when running Class A). A hold-off on the AGC action handled the difference in overall gain at the extremes of tuning. Second, and I suspect the stronger reason, was that customers could hear the front end noise in the speakers (with AGC held totally off and gain wide open) and get the impression of a "really hot receiver!" :-) It's easy to get any kind of "delayed AGC" or hold-off on AGC control with a DC bias on the detector. As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person |
On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden"
wrote: 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." Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. Drake 2-B has 5Vpp from 6BA6, and R-4C somewhat less, but not too important. This is only a reference which may not apply to any other receiver because it is another AGC amplifier which also amplifies IF to the AM detector for 2-B, and product detector fed via a voltage divider. For R-4C everything is totally different, and the 4-5Vpp level seems more chosen for economical reasons. Another English LF communication receiver I checked had 80-100V pp IF output. The level to choose depends on good AGC characteristic, and usually the IF level should be certain level above the background noise, possibly 10-20dB for good operation. Some receivers have not particularly good AGC, Atlas 210X is one, and my Yaesu FT-902 is another, even worse is FT101B because carrier oscillator leaks into the IF and AGC threshold must be set 10dB above the level you would want it to to start, simply because it can't operate properly below it. It was a surprise to learn that the Lorenz 6P203 receiver operated nicely on SSB when BFO level was increased, in spite that it has only 6AV6 detectors, but IF is split, and it has two different 6BA6's with different diode detectors, one for AM/CW/SSB and one for AGC. http://home.online.no/~la8ak/b71.htm Siemens Rainbow receiver also had some improper connection between product detector and BFO, and could detect SSB well when this was corrected http://home.online.no/~la8ak/b72.htm Heathkit SB300/301 have bad AGC, but may be easily improved a lot by using 1N4148 AGC detector diodes (voltage doubler), simply because the original diodes have too much capacitance and do not rectify properly. I did some experiments using Drake 2-C type and R-4B type AGC detectors in my Kenwood TS-500, later in 2-B, and could set the detector output variation from AGC threshold to as little as 1dB, but it seem no real point since the subjective sound seem best with at least 6dB variation, but wasn't too important if it was kept as original 10dB for 2-B, it is only important when you have a lot of advanced measuring equipment in the shack, but not at all for the radio operator. 73, LA8AK ---- Jan-Martin, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/ |
On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden"
wrote: 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." Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. Drake 2-B has 5Vpp from 6BA6, and R-4C somewhat less, but not too important. This is only a reference which may not apply to any other receiver because it is another AGC amplifier which also amplifies IF to the AM detector for 2-B, and product detector fed via a voltage divider. For R-4C everything is totally different, and the 4-5Vpp level seems more chosen for economical reasons. Another English LF communication receiver I checked had 80-100V pp IF output. The level to choose depends on good AGC characteristic, and usually the IF level should be certain level above the background noise, possibly 10-20dB for good operation. Some receivers have not particularly good AGC, Atlas 210X is one, and my Yaesu FT-902 is another, even worse is FT101B because carrier oscillator leaks into the IF and AGC threshold must be set 10dB above the level you would want it to to start, simply because it can't operate properly below it. It was a surprise to learn that the Lorenz 6P203 receiver operated nicely on SSB when BFO level was increased, in spite that it has only 6AV6 detectors, but IF is split, and it has two different 6BA6's with different diode detectors, one for AM/CW/SSB and one for AGC. http://home.online.no/~la8ak/b71.htm Siemens Rainbow receiver also had some improper connection between product detector and BFO, and could detect SSB well when this was corrected http://home.online.no/~la8ak/b72.htm Heathkit SB300/301 have bad AGC, but may be easily improved a lot by using 1N4148 AGC detector diodes (voltage doubler), simply because the original diodes have too much capacitance and do not rectify properly. I did some experiments using Drake 2-C type and R-4B type AGC detectors in my Kenwood TS-500, later in 2-B, and could set the detector output variation from AGC threshold to as little as 1dB, but it seem no real point since the subjective sound seem best with at least 6dB variation, but wasn't too important if it was kept as original 10dB for 2-B, it is only important when you have a lot of advanced measuring equipment in the shack, but not at all for the radio operator. 73, LA8AK ---- Jan-Martin, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/ |
Jan-Martin Noeding, LA8AK wrote:
Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. [snip] Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. Tom |
Jan-Martin Noeding, LA8AK wrote:
Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. [snip] Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. Tom |
"Jan-Martin Noeding, LA8AK" wrote in message ... On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden" wrote: 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." Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. 73, LA8AK Jan-Martin, I think the issue is that it is NOT a time delay, but a signal level delay. The AGC does not begin to reduce receiver gain until the signal reaches some defined LEVEL. -- Steve N, K,9;d, c. i My email has no u's. |
"Jan-Martin Noeding, LA8AK" wrote in message ... On Mon, 23 Feb 2004 21:33:39 -0500, "Tom Holden" wrote: 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." Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. 73, LA8AK Jan-Martin, I think the issue is that it is NOT a time delay, but a signal level delay. The AGC does not begin to reduce receiver gain until the signal reaches some defined LEVEL. -- Steve N, K,9;d, c. i My email has no u's. |
"Tom Holden" wrote in message .. . Jan-Martin Noeding, LA8AK wrote: Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. [snip] Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. Tom Tom, I don't have the circuit, but have two diodes pointing down. Resistors on both anodes to +V. Common cathodes with resistor to GND. +V ___|______ | | R1 R2 a_| |_b | | V V - D1 - D2 | | | | __________ | R3 | | GND Changing the voltage at point "b" will cause "a" to also change. Another way to look at it is that you steal different amounts of current away from D1. Go too low at "b" and "a" won't change any more. It stops. However, go higher on "b" and "a" keeps going up. It is a limiter. -- Steve N, K,9;d, c. i My email has no u's. |
"Tom Holden" wrote in message .. . Jan-Martin Noeding, LA8AK wrote: Mentioning a certain voltage level doesn't really make so much sense when you don't know what the rest of detector stages looks like. [snip] Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. Tom Tom, I don't have the circuit, but have two diodes pointing down. Resistors on both anodes to +V. Common cathodes with resistor to GND. +V ___|______ | | R1 R2 a_| |_b | | V V - D1 - D2 | | | | __________ | R3 | | GND Changing the voltage at point "b" will cause "a" to also change. Another way to look at it is that you steal different amounts of current away from D1. Go too low at "b" and "a" won't change any more. It stops. However, go higher on "b" and "a" keeps going up. It is a limiter. -- Steve N, K,9;d, c. i My email has no u's. |
Avery Fineman wrote:
[snip] As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person Here is my survey on which I based my targets: Release-ms Receiver Attack-ms Slow Medium Fast AOR AR7030 2/8 SSB/AM 1800 400 - Collins 75S-3B/3C 1 600 190 - Drake R4C 1 1000 350 50 Drake R8B 1 2000 300 - Grundig Satellit 800 1 3000 300 - Rockwell/Collins HF-2050 30 3500-10000 250+/-50 - U.S. Federal 30 800-1200 SSB/ICW modes Telecommunication Recommendation 1050-1998 "HF Radio Automation Link" 13 25 Data Mode Design Target 1-13 1800-3000 300 25 I wonder if there is any merit in making the attack selectable within a range of 1-30 ms. Tom |
Avery Fineman wrote:
[snip] As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person Here is my survey on which I based my targets: Release-ms Receiver Attack-ms Slow Medium Fast AOR AR7030 2/8 SSB/AM 1800 400 - Collins 75S-3B/3C 1 600 190 - Drake R4C 1 1000 350 50 Drake R8B 1 2000 300 - Grundig Satellit 800 1 3000 300 - Rockwell/Collins HF-2050 30 3500-10000 250+/-50 - U.S. Federal 30 800-1200 SSB/ICW modes Telecommunication Recommendation 1050-1998 "HF Radio Automation Link" 13 25 Data Mode Design Target 1-13 1800-3000 300 25 I wonder if there is any merit in making the attack selectable within a range of 1-30 ms. Tom |
"Tom Holden" ) writes:
Avery Fineman wrote: [snip] As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person Here is my survey on which I based my targets: Release-ms Receiver Attack-ms Slow Medium Fast AOR AR7030 2/8 SSB/AM 1800 400 - Collins 75S-3B/3C 1 600 190 - Drake R4C 1 1000 350 50 Drake R8B 1 2000 300 - Grundig Satellit 800 1 3000 300 - Rockwell/Collins HF-2050 30 3500-10000 250+/-50 - U.S. Federal 30 800-1200 SSB/ICW modes Telecommunication Recommendation 1050-1998 "HF Radio Automation Link" 13 25 Data Mode Design Target 1-13 1800-3000 300 25 I wonder if there is any merit in making the attack selectable within a range of 1-30 ms. Tom Traditionally, the circuitry was too bulky to make it more complicated, so there was always tradeoff. With semiconductors the added circuitry to make attack and decay independent doesn't require much space, or added cost. Once one has that in place, one can fiddle with the actual times. Maybe making it completely variable is not useful, but it'll only cost a switch to add multiple times. Michael VE2BVW |
"Tom Holden" ) writes:
Avery Fineman wrote: [snip] As to the attack and decay times you specified, seems good to me. That can be set to suit the individual listener. Len Anderson retired (from regular hours) electronic engineer person Here is my survey on which I based my targets: Release-ms Receiver Attack-ms Slow Medium Fast AOR AR7030 2/8 SSB/AM 1800 400 - Collins 75S-3B/3C 1 600 190 - Drake R4C 1 1000 350 50 Drake R8B 1 2000 300 - Grundig Satellit 800 1 3000 300 - Rockwell/Collins HF-2050 30 3500-10000 250+/-50 - U.S. Federal 30 800-1200 SSB/ICW modes Telecommunication Recommendation 1050-1998 "HF Radio Automation Link" 13 25 Data Mode Design Target 1-13 1800-3000 300 25 I wonder if there is any merit in making the attack selectable within a range of 1-30 ms. Tom Traditionally, the circuitry was too bulky to make it more complicated, so there was always tradeoff. With semiconductors the added circuitry to make attack and decay independent doesn't require much space, or added cost. Once one has that in place, one can fiddle with the actual times. Maybe making it completely variable is not useful, but it'll only cost a switch to add multiple times. Michael VE2BVW |
Steve Nosko wrote:
Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. I don't have the circuit, but have two diodes pointing down. [snip] That's interesting, Steve but the circuit looks like this: RF Amp----MXR------IF Amp | | R R |------|---------| | | D | C R ---from AGC | | ----- | GND Just noticed an asterix by the 'delay' Diode that says it may be replaced by a test-selected resistor - that's the way the DX-394 is done. Tom |
Steve Nosko wrote:
Thanks for the interesting examples, Jan-Martin. The quote was from the 2004 ARRL HB page 17.23 in reference to a schematic/block diagram of a "typical superhet receiver with AGC applied to multiple stages of RF and IF". The RS DX-394 bears some resemblance to this with a resistor in place of the "Delay Diode". It beats me how the AGC in this HB example actually controls the RF gain - the diode appears to block control. A second one in parallel in the opposite direction seems to me to be needed. I don't have the circuit, but have two diodes pointing down. [snip] That's interesting, Steve but the circuit looks like this: RF Amp----MXR------IF Amp | | R R |------|---------| | | D | C R ---from AGC | | ----- | GND Just noticed an asterix by the 'delay' Diode that says it may be replaced by a test-selected resistor - that's the way the DX-394 is done. Tom |
| All times are GMT +1. The time now is 03:36 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com