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AGC Design?
I'm looking for some advice/guidance on the design of AGC detection and
timing circuits, prompted by some level of frustration with a modification I have been doing to a DX-394 SW radio. My questions, though, probably apply to receiver design generally. I have a problem with stability - the receiver gain oscillates at medium and fast release speeds. Previously I had done a mod that pretty successfully provided 3 release speeds for the DX-394 but fell short of what I thought was the ideal: an attack time of ~1 millisecond, independent of the release time. That was based on a survey of receivers from which I concluded that the attack should be less than 13 ms and that 1 ms seemed to be the goal. Release speeds should probably be on the order of 30ms, 300ms and 3 seconds, for fast, medium and slow, respectively, although there seems to be lots of scope for subjective preference. My mod required a rather large capacitor for Slow release so my Slow was more like 1.2 seconds and the attack was slowed to maybe 50-70 ms for the slow release.. The objectives of the enhanced mod are to: a) improve the attack speed to better less than 13ms for all release speeds b) extend the Slow release using smaller cap c) reduce the loading of the AGC detector on the output of the 2nd IF amp and also possible distortion due to the AGC and AM/Product detectors fed in parallel I used a JFET to buffer between the IF amp and the diode detector and an emitter follower between the attack R-C circuit and the release R-C circuit, dc coupled to the stock AGC amplifier. On the release side, about 1/10 the capacitance vs the earlier mod is required for slow release and the attack does seem to be similarly less affected by the release network. However, at the fast and medium release settings, the receiver gain literally oscillates at a rate that seems to be a function of attack and release time constants, manual RF/IF gain setting, AGC gain setting and signal strength. The depth of this gain modulation is affected by AGC and RF gain. In order to get stability, it seems that I have to slow down the attack (and/or release) time constant and carefully tweak the AGC gain between the onset of oscillation and receiver peak distortion caused by not enough gain reduction. Have I completely misunderstood the meaning of attack/release speeds? My 'ideal' attack circuit has a R-C time constant of 1 ms, which means it will even respond substantially to 1kHz modulation. That seems high. The R-C time constant for my target fast release of 30 ms means that it will substantially follow a 30Hz signal. I have had to pad these out to ~20ms attack, 50ms release for stability or tolerably low gain oscillation depth at medium and lower signal strengths. With this slower attack, stability is much improved with the 500ms medium release speed. The target attack/release of 1ms/30ms is not good for AM reception anyway as it causes considerable distortion on heavy bass modulation - it is for data services on steady carriers, e.g., PSK, FSK, DRM. But if the AGC causes oscillation, then that's interference of another kind that would adversely affect error rates. Several, including myself, have noted that DRM SNR is improved by defeating AGC, on a wide variety of receivers. Is this a typical problem for receiver design? Would 'hang' AGC stabilise the AGC loop? Are my design objectives reasonable? Comments from experienced radio designers/builders/experimenters much appreciated. Tom |
This sounds like a classic negative feedback oscillation. You sense the
signal is too large, so you send a signal to kill the gain, and then you sense the signal is too small, so you send a signal to increase the gain. Having different attack and release time means you have two different time constants My guess is the quick attack leads to the instability, since it is the lesser damped system. If this is true, then you should concentrate on the attack time, i.e find how slow it has to be for the sytem to be stable. Of course this is really had to do without seeing the circuitry in action. |
I agree with you and slowing the attack is the only way that I have
been able to approach stable operation with a fast release. But 20ms or longer attack runs counter to what I understand to be the objective - an attack speed of less than 13 ms and ideally about 1 ms. So, unless I have this wrong, how do other receivers accomplish similar speeds without self-oscillation? The way my circuit operates (I think) is as follows (I'd be happy to send a schematic to anyone who is interested) : a) assume an impulse of signal of duration very much longer than the attack time b) the rectified signal is filtered of RF by a series-parallel R-C attack network whose adjustable output feeds an emitter follower c) the emitter follower pumps current as a low resistance source into the release R-C network so the attack is not greatly slowed - its output feeds the AGC driver amp d) at some point, equilibrium should be reached - the current flow through the release resistor and AGC driver base should equal the flow though the emitter follower - but maybe the emitter follower pinches off and that could be a cause of instability? e) the signal drops, the attack network discharges at attack speed and shuts off the emitter follower, so the release capacitor discharges through its parallel R at release speed, the voltage to the AGC driver falls so the AGC bias rises at roughly release speed to increase RF/IF gain. Having written that out, I have an idea or two I will try. Tom |
In article ,
"Tom Holden" wrote: I'm looking for some advice/guidance on the design of AGC detection and timing circuits, prompted by some level of frustration with a modification I have been doing to a DX-394 SW radio. My questions, though, probably apply to receiver design generally. I have a problem with stability - the receiver gain oscillates at medium and fast release speeds. Snip I don't know receiver design but I have a RX340 that uses the following settings. Attack is in dB/mS, Hang in seconds, and Decay are in dB/S. Attack Hang Decay Fast .8 0 1600 Medium .8 0 100 Slow .8 0 25 Programable attack .01 to 1 dB/mS Programable hang 0 to 99.9 seconds Programable decay .01 to 99.9 dB/S -- Telamon Ventura, California |
Telamon, those are interesting numbers, expressing the action of a more
sophisticated, programmable, digital AGC. Classic analog AGC speeds are expressed as the length of time it takes to reach a certain percentage or within a few dB of the desired gain setting, i.e., similar to and based on RC time constants as that was the foundation of the classic AGC control system. With an RC derived control, whether the gain change is 10 dB or 100 dB, it takes the same time. With your digital control in 'Fast' mode, attack would be 8ms for 10 dB and 80 ms for 100 dB; release would be hang time plus 6ms or 60 ms respectively. It's interesting how these compare with my target of 1-13 ms attack, 25-50 ms release. I'm wondering how your RX340 behaves when you program to 0.01 dB/ms attack, 0 hang, and 1600 dB/s decay (but I see that the programmable decay is limited to 99.9? probably for good reason!). That would correspond to my Fast target when you tune from no signal to S9+50. Regards, Tom |
From: "Tom" on Wed,May 25 2005 10:30 am
I agree with you and slowing the attack is the only way that I have been able to approach stable operation with a fast release. But 20ms or longer attack runs counter to what I understand to be the objective - an attack speed of less than 13 ms and ideally about 1 ms. So, unless I have this wrong, how do other receivers accomplish similar speeds without self-oscillation? The way my circuit operates (I think) is as follows (I'd be happy to send a schematic to anyone who is interested) : a) assume an impulse of signal of duration very much longer than the attack time b) the rectified signal is filtered of RF by a series-parallel R-C attack network whose adjustable output feeds an emitter follower c) the emitter follower pumps current as a low resistance source into the release R-C network so the attack is not greatly slowed - its output feeds the AGC driver amp d) at some point, equilibrium should be reached - the current flow through the release resistor and AGC driver base should equal the flow though the emitter follower - but maybe the emitter follower pinches off and that could be a cause of instability? e) the signal drops, the attack network discharges at attack speed and shuts off the emitter follower, so the release capacitor discharges through its parallel R at release speed, the voltage to the AGC driver falls so the AGC bias rises at roughly release speed to increase RF/IF gain. Having written that out, I have an idea or two I will try. Having encountered a similar problem many years ago, I'll offer this as a suggestion: Analyze the behavior of the total signal amplification chain at LOW frequencies, not at the RF or IF carrier. Know the control characteristics of the AGC voltage input to the amplifier versus the total amount of gain of the receiver chain. Approach the whole receiver AGC action as a low-frequency servo loop (which is what the AGC actually does). Think servo control systems theory. Control systems theory is a rather abstract thing and there probably will be no sudden bright light of understanding switched on, but here's a bit of that: The AGC loop action works by BOTH magnitude and phase at low frequencies. "Nyquist" and "Bode" plots are helpful there, even though both of those subjects are also rather abstract. In general, if the AGC control action results in instability or even motor- boating, the overall receiver gain - related to the control voltage range - is too high. Adding a voltage divider at the low-pass R-C filter of the AGC voltage input will demonstrate that. Also, the low-frequency phase shifting in the AGC voltage "decoupling" can upset the phase versus magnitude of the control voltage. Note: Vacuum tube or FET RF/IF controlled amplifiers probably use such R-C decoupling, working only on AGC voltage; other amplifier types might have some other form of R-C filtering at low frequencies. That low- frequency magnitude AND phase relationship is important for total loop stability. What has to be considered in the AGC loop is the response through all the decoupling newtorks between the ACG control source and the controlled device(s). For a "non-linear" loop (separate attack and decay times) that analysis will be difficult. It is much easier to analyze with a Spice simulation that has the capability to model a controlled-gain amplifier. The whole loop at low frequencies can be modelled that way. In starting that, forget the RF and IF components and consider only the amplifications at low frequencies; the source of the AGC control (detector output) may have to be modelled slightly differently in that the detector is, in effect, similar to a power supply rectifier. If that model is tweaked to be stable with sudden transitions on its input, then it will be stable at RF and IF. |
Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while the AGC circuit is trying to operate with millivolts. Unless you're very careful with layout, shielding, and balance, a lot of BFO signal can get into the AGC circuit and cause disturbances and malfunctions of various kinds. The last AGC circuit I did was very conventional, and it's the sweetest operating one I've ever used. But I went to great pains to keep the BFO out of it, and feel that was one of the essential ingredients in getting it to operate so well. Roy Lewallen, W7EL |
From: Roy Lewallen on May 26, 5:39 pm
Let me add one more general note about AGC design. The BFO frequency is very close to the IF, and it typically puts out volts of signal while the AGC circuit is trying to operate with millivolts. Unless you're very careful with layout, shielding, and balance, a lot of BFO signal can get into the AGC circuit and cause disturbances and malfunctions of various kinds. I agree on the need for isolation of various circuits but fail to see the relevance. A "BFO" is on for OOK CW reception and normally a manual RF/IF amplification control is used to set a comfortable listening level. Yes, AGC could be used on OOK CW but it would be a mistake to derive the AGC control from an AM detector getting "BFO" input...that would be the same as introducing a DC bias into the AGC control loop...which would change the AGC servo-action control...perhaps severely so. Note: A "BFO" source is steady-state. The detector mixes the incoming signal (usually at the IF) with the "BFO" to derive the audio. If the AGC control line is picked off this same detector, the DC component is akin to having a nearly fixed DC bias inserted. To use AGC on an OOK CW signal, the audio tone would have to be used...and that necessaitates a different sort of AGC control source derivation. A peak-riding, perhaps selective audio circuit could do that, but the complexity of that part of the receiving chain is growing. It might be easier all-around to just pick off the IF to a separate AM detector as the AGC control line source. The "original" detector could remain as the OOK CW output with isolated BFO feeding it. For SSB voice reception, a "BFO" is still present but a single diode detector all-purpose sort of detector is far from optimum as a combined audio source and AGC control line source. It WILL work, but it is non-linear for both audio and AGC purposes and that alone could be the source of AGC instability. It depends on the IF signal level at the detector diode (or "product detector" which is really a mixer stage). A single diode with large time-constant on its voltage output is a peak-riding source for the AGC control line. Whether or not it follows fast "attack" conditions depends on the source impedance capabilities of the final IF stage. If that is too high then the "attack" time is slowed from the necessity to build up a charge on the diode's load capacitance; that can be seen on examining an ordinary AC rectifier circuit in response to a step transient of AC input through various values of AC source resistors. The peak-riding capability is usually distorted on the leading edge...which then reflects on the AGC control characteristics (when loop is closed) in trying to hold the received signal constant at the detector. Thought of as a servo-control loop, the AGC subsystem can get rather involved and complex, affected by a number of different factors, ALL of which are important insofar as AGC instability is concerned. "BFO" level is just one item and I will disagree that it is a very important. It is no more important than anything else in that loop in my experience. As a suggestion to anyone else, I would recommend first either measuring or calculating the AGC control line versus both the antenna input level and the IF level at the AGC detector input. That yields a DC baseline datum on the controllable level of the receiving chain. From that, one can "back-track" calculate how well the closed-loop AGC action behaves; i.e., the antenna input RF level versus the peak audio output with AGC on. If that is using old-style "variable-mu" pentode tubes, then the control characteristics will show whatever non-linearity it has steady-state. That can be used as a special controlled- gain model baseline for a Spice analysis of the AGC loop. Differing time-constants IN the AGC control feedback can be set to observe closed-loop response with transient signal input to the antenna. The last AGC circuit I did was very conventional, and it's the sweetest operating one I've ever used. But I went to great pains to keep the BFO out of it, and feel that was one of the essential ingredients in getting it to operate so well. Having had a National NC-57 receiver since 1948, I decided to "play" with it in 1959 and "improve" its performance, such as increasing IF gain. The first IF stage as well as the RF stage were AGC-controlled. Not knowing enough about Control Theory then, nor considering the low-frequency characteristics of the AGC control voltage line R-C decoupling, that modification became a disaster for anything but manual RF gain control. The motorboating (very low-frequency oscillation) extended to having the VR-150 screen supply regulator (gaseous shunt regulator to those of solid-state era times) going on and off. It was restored to its original components and not played with for over a decade. Much later, on having had to get into Control Theory and servo control loops, I could analyze how bad it was and see what I SHOULD have done. The control was too "tight" in trying to hold the audio output too constant over a wide signal input range. There was low-frequency phase shift in the AGC voltage control decoupling that was responsible for most of the motor- boating; the VR-150 shunt regulator control range was a bit too narrow so naturally it had dropped out of regulation and added the final insult to the original "mod." [forty somethings and younger may not be familiar with such relaxation oscillator circuits :-) ] National Radio Company had made an acceptible product in the NC-57 but it was a low-end item in their product line. It worked well enough as a single-conversion HF receiver but it wasn't optimum in design and no doubt stock logistics at the factory probably accounted for some of the parts values. Several passive components seemed to be rather arbitrary in value choices. I had learned (or should say re-learned) that NO product is an example is "what something should be" as a design example. There just aren't any "easy" answers for some things in electronics. But, they can be WONDERFUL, challenging "cross-word puzzle" kinds of thing to solve! :-) |
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"Roy Lewallen" wrote in message
... Instead of solving the fundamental problems, increasingly complex circuits are developed until one accidentally works correctly, then the improvement is credited to the complex circuit rather than its accidental relative immunity to the results of poor fundamental design. I prefer the solution of, "Hmm... there's already a CPU in this radio anyway... and we've got an ADC around... hey, let's make it the software guy's problem!" :-) :-) |
From: Roy Lewallen on May 27, 6:49 pm
wrote: I apologize for not being more precise in my nomenclature. No problem to me...I fear I got off on a "lecture mode" again, but was speaking in generalities to other readers about receiver back-ends. By "BFO" I mean the oscillator used for product detection when receiving SSB and CW signals. No AM detector is involved. The AGC pickoff is of course done from the IF preceding the product detector, and doesn't intentionally use the BFO or product detector in any way. The problem I was alluding to is that the BFO produces a large signal which is very near the IF, and therefore can get into the AGC circuitry unless some care is taken to prevent it. This produces a DC bias among other problems, which can interfere with AGC circuit operation. I found it necessary to completely shield the BFO, use a good doubly balanced detector, and use differential amplifiers in the AGC chain in order to reduce the BFO crosstalk to a tolerable level. Sounds good to me. Separated, isolated detectors allow one to concentrate on the particulars of each, makes it a lot easier to work with. For what it's worth on the audio-output part, I'm more fond of rather high levels of IF into the detector to get around the "square-law" response...looking for a better AM envelope reproduction. While that results in better audio, it also makes decoupling more difficult to avoid feeding the strong IF back to the input. Different problem, same cuss-words on the bench, though. :-) I strongly suspect that a number of the complicated AGC circuits evolved because a simpler AGC circuit was poorly designed and/or subject to problems like crosstalk from the BFO. Instead of solving the fundamental problems, increasingly complex circuits are developed until one accidentally works correctly, then the improvement is credited to the complex circuit rather than its accidental relative immunity to the results of poor fundamental design. This isn't of course universally true, but it happens pretty often. I agree with you there. At least for voice-band detection receivers (of which I've only built two in a half century from my own design). Discounting copies of "All-American Five" table-model cheapies using a single diode for both audio rectification and (low-pass filtered) for AGC voltage to a single controlled variable-mu amplifier. Ultimate simplicity for reasons of price over the counter. One CAN put a BFO on those (Hallicrafters did back in the late 40s) but the performance is not the best. Separating the "detectors" by function is best. The audio "detector" (I still think of them as 'rectifiers') can be optimized for best sound. The AGC detector can be optimized for its action separately...and its response versus IF input and overall receive chain amplification tailored for the AGC control-loop "gain." Filtering-decoupling that follows can be figured out to keep the low-frequency phase response from upsetting the closed-loop AGC control. Separate AGC and voice detectors lets one play around with "attack" and "decay" time-constants with no more than a single dual op-amp shaping circuit...multiple time-constants under manual control if desired, that won't interfere with the audio detection part. AGC detector input would have to be the fastest-responding (to desired time-constant) with a relatively simple op-amp doing the time-stretching. Some folks might consider that op-amp addition "complicated." Won't blame them if they do. From my experience, a "complicated" AGC subsystem is having to AGC on a 1 uSec pulse with a time gate in the presence of other assynchronous 1 uSec pulse sidebands located on 1 MHz intervals (up to 3) on either side...with a decay to attack time ratio of about 1000:1. :-) Did that for an R&D airborne system at RCA...was somewhat too much but that allowed a greater simplification for a following generation of airborne equipment. A lesson there can be to "cover all bases possible" the first time around, then investigate to see what can be simplified for something less complicated. AGC, in the basic consideration, should begin as a control loop. From there on its a matter of choice of circuits. |
Roy Lewallen wrote:
I strongly suspect that a number of the complicated AGC circuits evolved because a simpler AGC circuit was poorly designed and/or subject to problems like crosstalk from the BFO. Instead of solving the fundamental problems, increasingly complex circuits are developed until one accidentally works correctly, then the improvement is credited to the complex circuit rather than its accidental relative immunity to the results of poor fundamental design. This isn't of course universally true, but it happens pretty often. All too many software wannabees work this way too. -- I miss my .signature. |
Thanks to all for the constructive and informative discussion. I wish I
could say that it has helped to solve my problem but I'm still wrestling with it. The low frequency circuit analysis by Spice, Bode and Nyquist plot are beyond my capacity but I have had some excellent help from Dave, off-list, who eye-balled the DX-394 schematic and my mod. We identified decoupling networks and what I assume to be AGC 'delay' networks and tackled it on the basis that reducing the time constants of the larger ones should reduce the low frequency phase shift that could be contributing to the problem. The opposite occurred - increasing the 'delayed AGC' time constant reduced the instability and effectively slowed the attack, especially on the RF front-end. More or less the same effect was obtained by slowing the attack in the attack network that affects all stages. I believe 'delayed AGC' means a slower or delayed attack at the RF stages; in the DX-394, there is a R-C network adding maybe 15 ms to the attack on the AGC line affecting both the 1st mixer and the drain-source current of the RF preamp and a second network adding maybe 10 ms on top of this affecting the AGC gate of the RF preamp. I've doubled that first time constant and doubled what I would like in my attack and release networks in order to get the fastest stable speeds which I guess would be on the order of 20-40 ms attack and 50-80 ms release. My mod has a FET amp/buffer at IF driving the heck out of the diodes so that should be fairly linear. I have adjustable gain at the output of the detector/attack filter. I don't think BFO interference is an issue; I don't notice any great difference in stability with it on or off - there are separate envelope and product detectors. I'd welcome any more input. The base DX-394 schematic is at http://www.monitor.co.uk/radio-mods/dx-394/dx-394.htm and I'd be happy to send anyone the schematic of my mod. 73, Tom |
In message , Tom Holden
writes Thanks to all for the constructive and informative discussion. I believe 'delayed AGC' means a slower or delayed attack at the RF stages; in the DX-394, there is a R-C network adding maybe 15 ms to the attack on the AGC line affecting both the 1st mixer and the drain-source current of the RF preamp and a second network adding maybe 10 ms on top of this affecting the AGC gate of the RF preamp. I've doubled that first time constant and doubled what I would like in my attack and release networks in order to get the fastest stable speeds which I guess would be on the order of 20-40 ms attack and 50-80 ms release. 73, Tom Tom, I haven't been following this thread. However, in my understanding, 'Delayed AGC' doesn't refer to a time delay. It normally means that the AGC in the RF stage doesn't cut in until a certain level of signal is reached. AGC is applied as 'normal' to the IF stages but the RF stage is held at maximum gain until the input signal is higher. The effect is to obtain a better signal-to-noise ratio with low-level signals. Ian. -- |
Thanks for the reference, Bill. I did learn something of value from it but
the devices are clearly intended for audio frequency although one might actually support 0dB gain at 455kHz! However, they are a closed loop system and it's not obvious that one could bring out the required control voltage to drive the receiver AGC. Regards, Tom "Netgeek" wrote in message ... Hi Tom, You might find some good info from reading the description and data sheets for the Analog Devices SSM2165 and/or SSM2166 at www.analog.com. They address the issues of having a threshold which can be adjusted and then varying the amount of compression or limiting asymetrically. Perhaps you could modify your circuit to emulate some of these features - or perhaps just use the devices described? Bill wrote in message oups.com... This sounds like a classic negative feedback oscillation. You sense the signal is too large, so you send a signal to kill the gain, and then you sense the signal is too small, so you send a signal to increase the gain. Having different attack and release time means you have two different time constants My guess is the quick attack leads to the instability, since it is the lesser damped system. If this is true, then you should concentrate on the attack time, i.e find how slow it has to be for the sytem to be stable. Of course this is really had to do without seeing the circuitry in action. |
I have struggled with this in the past
It is a function of the behaviour of the servo loop at low freq and I dont have the theory to analyse it properly. However, the most successful AGC system I used in a DC receiver had a T attenuator as the control element, consisting of resistors on the horizontal arms of the T with a Darlington pair to ground as the control element. This was driven by Opamp - rectifier (BE junction of a 2N3904) driving an emitter follwer driving a conventional RC circuit. It seemed that when you got rid of any DC shift in the system this fixed the problem. As a bonus you got a surprisingly accurate log detector(over about 60bB range) for an S meter. The difficult part in all systems seems to be the control element. Richard Tom Holden wrote: I'm looking for some advice/guidance on the design of AGC detection and timing circuits, prompted by some level of frustration with a modification I have been doing to a DX-394 SW radio. My questions, though, probably apply to receiver design generally. I have a problem with stability - the receiver gain oscillates at medium and fast release speeds. Previously I had done a mod that pretty successfully provided 3 release speeds for the DX-394 but fell short of what I thought was the ideal: an attack time of ~1 millisecond, independent of the release time. That was based on a survey of receivers from which I concluded that the attack should be less than 13 ms and that 1 ms seemed to be the goal. Release speeds should probably be on the order of 30ms, 300ms and 3 seconds, for fast, medium and slow, respectively, although there seems to be lots of scope for subjective preference. My mod required a rather large capacitor for Slow release so my Slow was more like 1.2 seconds and the attack was slowed to maybe 50-70 ms for the slow release.. The objectives of the enhanced mod are to: a) improve the attack speed to better less than 13ms for all release speeds b) extend the Slow release using smaller cap c) reduce the loading of the AGC detector on the output of the 2nd IF amp and also possible distortion due to the AGC and AM/Product detectors fed in parallel I used a JFET to buffer between the IF amp and the diode detector and an emitter follower between the attack R-C circuit and the release R-C circuit, dc coupled to the stock AGC amplifier. On the release side, about 1/10 the capacitance vs the earlier mod is required for slow release and the attack does seem to be similarly less affected by the release network. However, at the fast and medium release settings, the receiver gain literally oscillates at a rate that seems to be a function of attack and release time constants, manual RF/IF gain setting, AGC gain setting and signal strength. The depth of this gain modulation is affected by AGC and RF gain. In order to get stability, it seems that I have to slow down the attack (and/or release) time constant and carefully tweak the AGC gain between the onset of oscillation and receiver peak distortion caused by not enough gain reduction. Have I completely misunderstood the meaning of attack/release speeds? My 'ideal' attack circuit has a R-C time constant of 1 ms, which means it will even respond substantially to 1kHz modulation. That seems high. The R-C time constant for my target fast release of 30 ms means that it will substantially follow a 30Hz signal. I have had to pad these out to ~20ms attack, 50ms release for stability or tolerably low gain oscillation depth at medium and lower signal strengths. With this slower attack, stability is much improved with the 500ms medium release speed. The target attack/release of 1ms/30ms is not good for AM reception anyway as it causes considerable distortion on heavy bass modulation - it is for data services on steady carriers, e.g., PSK, FSK, DRM. But if the AGC causes oscillation, then that's interference of another kind that would adversely affect error rates. Several, including myself, have noted that DRM SNR is improved by defeating AGC, on a wide variety of receivers. Is this a typical problem for receiver design? Would 'hang' AGC stabilise the AGC loop? Are my design objectives reasonable? Comments from experienced radio designers/builders/experimenters much appreciated. Tom |
From: Richard Hosking on Tues 31 May 2005 20:05
I have struggled with this in the past It is a function of the behaviour of the servo loop at low freq and I dont have the theory to analyse it properly. The "theory" part should be evident to anyone who has made a negative-feedback amplifier, single transistor to op-amp. Getting to know op-amp responses both open-loop and closed- loop (with negative feedback) can be helpful. Note that op-amp designers actually build in open-loop phase shifts at high frequencies to avoid oscillation with feedback. The only difficult part is in MEASURING the PHASE at low frequencies in the 0.1 to 10 Hz range...especially that of the AGC control-line (feedback) circuitry. If not, some dog-work on analyzing the magnitude and phase response of that circuit will show that. [ability to handle complex quantities is preferred there] If the phase response is 0/360 degrees between AGC control- line input and output (to the gain-controlled stages), AND the closed-loop gain of the system is greater than unity, there be troubles there! :-( To get a view into AGC behavior with any general receiver, disconnect the AGC control-line from the gain-controlled stage and substitute a small variable DC source for the AGC control-line input to that gain-controlled stage. Using a reasonably-well-calibrated RF source, pick some RF levels over the expected dynamic range of receiver input. At each input level, adjust the DC control-line substitute to be the same as the value of the disconnected AGC control- line. Measure the DC value of both the input and output of that AGC control-line circuitry. The reason for doing that is to remove any phase effects at low frequencies. That's a baseline value set that SIMULATES the closed-loop control range of the AGC. With enough RF input signal levels, the characteristic curve of the closed-loop AGC action at DC can be seen...from no AGC (maximum receiver gain) to high values of AGC control (essentially minimum receiver gain). That will show the delta of tiny AGC-control line variations which is the equivalent of the "feedback percentage" of a negative- feedback amplifier simple formula. Alternately, one can do an open-loop gain measurement using a series of AGC control-line value increments from minimum to estimated maximum. Setting the simple DC supply (substitute for the AGC control-line) to those increments will do it nicely. Log the RF input level for those DC increments and measure the input to the AGC control-line feedback circuitry (even though it is disconnected from the controlled stages). That will result in the same input signal characteristic curve. Either way will result in "seeing" what the RF input signal characteristics are, allow one to use the AGC line for things like an S-Meter indicating circuit, squelch control, etc. Note: The alternate method can also be done analytically on paper if the controlled stages' gain v. control line is known. A "gain budget" can be tabulated of the total receiver sensitivity to various RF input levels that produce various AGC control-line values. That takes part of an afternoon's bench data logging, dreary though that may be. It will establish THE characteristics of that receiver, valuable reference for later work on it. That curve will be no different than that of a single amplifier stage with varying amounts of negative feedback. Next is to either measure or calculate the low-frequency magnitude and phase characteristics of the AGC control-line circuitry (ALL of it, even to bypass caps at the controlled stage input connection). Magnitude alone will yield the feedback percentage of the equivalent negative-feedback amplifier. The phase response at various low frequencies has to be compared to the "attack" and "decay" times as desired. THAT is not intuitive but must be examined to see if the low-frequency AGC control characteristics will result in a negative-feedback or positive-feedback (oscillatory, motorboating) amplifier equivalent. A stable receiver WITH AGC should ALWAYS have some error. If actual low-frequency oscillation occurs, one cure is to attenuate the AGC control-line range. A voltage divider if the AGC control is through voltage does that. Such attenuation works on the magnitude of the AGC control but will also affect the phase. I hope this simplified explanation is a help for all who aren't familiar with Control System theory. Control Systems aren't as intuitive as many instructors on the subject claim so there isn't a lot of literature on it in popular publications for hobbyists. Those usually present some very simple analogue such as the ball governor valve on a steam engine of old and let it go at that. :-( |
wrote in message oups.com... From: Richard Hosking on Tues 31 May 2005 20:05 I have struggled with this in the past It is a function of the behaviour of the servo loop at low freq and I dont have the theory to analyse it properly. The "theory" part should be evident to anyone who has made a negative-feedback amplifier, single transistor to op-amp. Getting to know op-amp responses both open-loop and closed- loop (with negative feedback) can be helpful. Note that op-amp designers actually build in open-loop phase shifts at high frequencies to avoid oscillation with feedback. The only difficult part is in MEASURING the PHASE at low frequencies in the 0.1 to 10 Hz range...especially that of the AGC control-line (feedback) circuitry. If not, some dog-work on analyzing the magnitude and phase response of that circuit will show that. [ability to handle complex quantities is preferred there] Lacking a calibrated RF source and much other critical equipment, I do have a 45 year old Eico scope that once belonged to the famous Bach pianist Glenn Gould, and could cobble together a variable dc source and a low freq oscillator. To observe phase response of the open loop system, I'm thinking that the loop could be broken between the AGC detector and the AGC time constant/buffer. Drive the latter and the X input of the scope with the dc supply and superposed low freq signal, feed the receiver with steady state RF carrier and take the output of the AGC detector to the scope's Y input. The variation of the input to the AGC system will cause variation in the receiver gain and the output of the AGC detector. If in phase, the scope would show a line with positive slope; if antiphase, a line with negative slope; if in-between, an ellipse or some open shape subject to time constants and non-linearities. This arrangement would leave the receiver's RF gain control intact and its effect on time constant and phase observable; it appears to modify the discharge resistance seen by a 1uF cap at the RF and 1st Mixer in addition to pulling down the AGC voltage applied to them. Does that seem to be a practical approach, Len? Tom |
From: "Tom Holden" on Tues 31 May 2005 22:08
wrote in message roups.com... From: Richard Hosking on Tues 31 May 2005 20:05 Lacking a calibrated RF source and much other critical equipment, I do have a 45 year old Eico scope that once belonged to the famous Bach pianist Glenn Gould, and could cobble together a variable dc source and a low freq oscillator. To observe phase response of the open loop system, I'm thinking that the loop could be broken between the AGC detector and the AGC time constant/buffer. Drive the latter and the X input of the scope with the dc supply and superposed low freq signal, feed the receiver with steady state RF carrier and take the output of the AGC detector to the scope's Y input. The variation of the input to the AGC system will cause variation in the receiver gain and the output of the AGC detector. If in phase, the scope would show a line with positive slope; if antiphase, a line with negative slope; if in-between, an ellipse or some open shape subject to time constants and non-linearities. This arrangement would leave the receiver's RF gain control intact and its effect on time constant and phase observable; it appears to modify the discharge resistance seen by a 1uF cap at the RF and 1st Mixer in addition to pulling down the AGC voltage applied to them. Does that seem to be a practical approach, Len? If that tells you what you want to know, it is practical. However, the phase information from that Lissajous display is rather gross. If, with a closed-loop condition, there is marginal stability, then a better handle on phase response would be necessary...or just reducing the AGC control-line magnitude (which would offer less AGC action). I'll have to presume the Eico scope doesn't have a slow sweep rate. If that scope has a DC input on both horizontal and vertical, then the cobbled-together low-frequency source could be built with a ramp output that would act as the horizontal sweep; the display would then be just one cycle but that would indicate the phase difference. Suggestion for source: Exar XR-8038 DIP which has both square-wave and sine outputs. A "bounce-less" switch circuit can be put together out of two NAND gates connected as an R-S flip-flop, an SPDT switch grounding/earthing one input on each NAND gate. That simulates a very extreme "attack" situation to check the response of the AGC control-line circuit. It's a bit much to infer anything of numerical value out of that, though, since the amount of analysis of the waveform out of the AGC control-line is lengthy and probably more time than it's worth. I'll have to remind all that a reasonably-calibrated RF signal source is also necessary. That will yield both the open-loop gain and the closed-loop gain...which can then be applied to a standard negative-feedback amplifier formula. Even with a "cheap" RF signal source, an RF output voltage meter circuit (even if a 1N34 diode rectifier is used, good to ~ 30 MHz) will provide a maximum RF output level. Resistor Tee or Pi pads built on DPDT switches (cheap slide switches work out best due to least internal inductance) external to the RF generator are effective although not to the wideband accuracy of the waveguide-below-cutoff type used in older commercial RF generators. A sequence of 1, 2, 3, 5, 10, 20, 40 etc db pads would do well enough. If needs be to make the pads the most accurate, a spoiler pad of around 10 db at the start of this chain of pads would insure a good source impedance. While not of greatest metrology quality, those would be better than nothing at all. Note on the above: The RF signal generator meter would determine the signal level into the attenuator chain. The chain's output would then be that value minus the total db of the attenuators switched-in. Making the attenuator- switch mountings in-line in an outboard long metal box having 1:2 ratio of width to height will reduce most of the RF feed-around across switched-in attenuators; if that is 1 x 2 inches it is roughly high C-Band waveguide size and a maximum of 30 MHz RF input would certainly be below cutoff frequency of that "waveguide." Attenuation through that long metal box would be a linear relationship of db v. length. I did just that with an old Heathkit RF generator (meter calibration set against lab equipment) and outboard switched attenuators...until I lucked-out and obtained a pair of HP 355 step attenuators (wideband to 500 MHz, easier to use). |
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I think you are onto something here regarding the quick hit, i.e.
closed loop step response. Use one of those RF detecting scope probes or just look at the envelope detector.If the envelope is ringing, you need more delay. The step can be off to on for attack and on to off for decay. It seems to me it not a matter of what you want, but rather what is stable. |
On Sat, 28 May 2005 22:25:18 -0400, "Tom Holden"
wrote: Thanks to all for the constructive and informative discussion. I wish I could say that it has helped to solve my problem but I'm still wrestling with it. The low frequency circuit analysis by Spice, Bode and Nyquist plot are beyond my capacity but I have had some excellent help from Dave, off-list, who eye-balled the DX-394 schematic and my mod. We identified decoupling networks and what I assume to be AGC 'delay' networks and tackled it on the basis that reducing the time constants of the larger ones should reduce the low frequency phase shift that could be contributing to the problem. The opposite occurred - increasing the 'delayed AGC' time constant reduced the instability and effectively slowed the attack, especially on the RF front-end. More or less the same effect was obtained by slowing the attack in the attack network that affects all stages. I believe 'delayed AGC' means a slower or delayed attack at the RF stages; in the DX-394, there is a R-C network adding maybe 15 ms to the attack on the AGC line affecting both the 1st mixer and the drain-source current of the RF preamp and a second network adding maybe 10 ms on top of this affecting the AGC gate of the RF preamp. I've doubled that first time constant and doubled what I would like in my attack and release networks in order to get the fastest stable speeds which I guess would be on the order of 20-40 ms attack and 50-80 ms release. My mod has a FET amp/buffer at IF driving the heck out of the diodes so that should be fairly linear. I have adjustable gain at the output of the detector/attack filter. I don't think BFO interference is an issue; I don't notice any great difference in stability with it on or off - there are separate envelope and product detectors. I'd welcome any more input. The base DX-394 schematic is at http://www.monitor.co.uk/radio-mods/dx-394/dx-394.htm and I'd be happy to send anyone the schematic of my mod. 73, Tom Tom, You might try an rf choke in the agc line near the agc detector. With fast attack that usually means small filter capacitors on the agc line. Rf can be coupled into the agc line that larger time constant capacitors would filter out. I have also found that decreasing the capacitors at each controlled stage to try and speed up the attack times resulted in instability of stages due to not enough rf decoupling. As another poster mentioned, using a low source impedance on the agc driver amplifier will allow you to use larger capacitors on the agc line. That can solve most rf on the agc line problems. Regards Gary K4FMX |
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