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#21
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#22
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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. |
#23
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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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