Hello Flex Forum, Subject: Ideas for providing an ALC input from Linear Amplifiers for the Flex 5000 and other FLEX radio products.
“Normally”, ALC (Automatic Level Control) for a ham radio linear amplifier has been provided by having an analog output of the linear amplifier and a mated (similarly calibrated) analog input to the HF transceiver. Typically, when the Linear Amp (LA) draws more that some desired amount of grid current, the ALC output goes negative and this signal is used in the transceiver to reduce the input power to the LA. The Flex 5000 currently has no such capability and I personally would like to add it to my own unit. At first glance, there appear to be a number of ways to provide such the ALC feature in the existing system. 1) If there were an A/D converter input available, the ALC signal from the amplifier could be input to the Analog input and the digital measurement could be fed to the SDR software in the PC. If the level was above some threshold, the RF output of the Flex Radio would be turned down with about a time constant of perhaps a half second. Then, whenever the radio was transmitting, the SDR software could adjust the drive level slowly in the Upward direction when the peak level was below a set threshold. The “up” time constant could be in the range of many seconds and with a small guard band to eliminate “pumping” of the RF level and yet eliminate overdrive of the linear amplifier. This appears to be the ideal approach as it is in keeping with maximum SDR flexibility and programmability. But: There is (as far as I know) not an A/D converter input available in the Flex 5000A hardware. 2) Another possibility would be to find a suitable low level transmit RF spot in the Flex 5000A transmit circuitry and insert a wideband electronically adjustable attenuator/amp such as the Analog Devices AD8367 (see: http://www.analog.com/library/analogDialogue/archives/36-01/xAmp/index.html) This converter/attenuator chip has +5db to about –40db gain from below 1mhz to over 500mhz. The voltage output range is 50mv to 950mv. Cost is about $5 in quantity for this IC, but perhaps there are lower cost circuits available. So the question is: Is there a suitable place in the Flex 5000A transmitter circuit to place such a device? In fact, the ALC’s “authority” could be limited to less than maybe 10db max and if the ALC was “maxed out” the user would simply turn down the Flex 5000’s basic transmit power setting. Care would have to be taken to put the attenuator in a spot where the Flex 5000 Circuitry did not try to bring the power level UP when the ALC turned it DOWN! 3) There may be a spot in the transmit circuitry of the Flex 5000 where there is already an ALC channel which can be modified. If there IS an existing analog ALC channel in the Flex 5000, it may be possible to sum in another signal (with the proper levels and time constants) from the HF amp’s ALC system to keep the Flex RF output where we want it for the LA. 4) Another (less clean) possibility is the use of the already available “buttons” on the Shuttle Pro control. I think it is possible to set one button for “turn up the power level” and another for “turn down the power level”. It would be possible to set up a couple of comparators to measure the ALC output of the LA and if it was too high an optocoupler could “press the DOWN POWER button and if too low could press the UP POWER button. This circuit would take about 7ICs and cost maybe $5 in parts plus a tiny PCB. It would need 3V or 5V power. Now.. I am off to peruse the schematics. But if some of you experts have any comments and especially on numbers 1, 2, and 3 above, please do speak up. There is no desire on my part to open up my box and start making measurements and trying things if “you guys” have already explored the territory and have suggestions! Thanks for any constructive comments! Joe Mehaffey W2JO 404-275-9830 See Product reviews of GPS Car Navigators, Geocaching, Hiking and other equipment at http://gpsinformation.net _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/
