Thursday, August 18, 2016

LDMOS Amplifier ~ Next Steps

More Revelations about the LDMOS Control System.

Rev 8/19 Added a second video of the three light sequence.


My efforts over the past week or so were to further refine and explore the control system functionality. It is has truly been an exercise in "non-linear" thinking which by my definition is not a simple straight line from step A to B to C and so on. Instead it is the need to see that it is A to B & C followed by D and then perhaps and E & F & G. What may seem straight forward and direct often is not that simple based on the complexity of what you want to happen.
 
Above all, the complexity of the control systems lies in the three basic functions of the Arduino control system those being: 1) Pure Control, 2) Supervisory Oversight and 3) Hardware Protection. The Supervisory and Protection aspects frequently drive and override the Control functionality. Overlaying this is the speed at which events must occur to failsafe the very expensive RF device.
 
I cannot overstress the value of having built a breadboard of the Arduino Control system as this has been an invaluable tool. From this effort I uncovered that the 48 VDC power supply had to be essentially shut off immediately through the use of a FET power switch.
 
Using the on board LED indicators helped in "physically seeing" sequenced actions. To this end there is a sequence where the output of the Low Pass Filter banks are connected to the antenna (step one), a bias is applied to the LDMOS amp (step 2) and finally the transceiver output is connected to the LDMOS amplifier input (step 3). These steps are reversed when going back from transmit to receive. The Arduino Code has the timing (time delays) and forward/reverse sequencing built into sketch. However the time delays were more or less randomly picked based on a notional gut feel. When the hardware was built and the LED's sequenced I could see one of the delays was too short and the other too long.
 
Exercising the keypad buttons has shown where conditions could exist to essentially cause an unexpected event to occur. This is valuable in that such events are discovered and corrected before any expensive hardware is actually connected.
 
To date we have the following in operation:
 
  1. Basic Start and Stopping of the 48 VDC Power Supply
  2. Low Pass Filter Selection
  3. TR Circuit Tied to LPF Selection ( No Filter selected, no TR)
  4. Sequencing of the TR Relays and Bias Circuit.
  5. The FET Power Switch is built (awaiting the heat sink)

 
 
 
See the latest progress as of 8/18/2016
 
73's
Pete N6QW


2 comments:

  1. If you have a power supply that is 'turned off' for protection (an emergency shutdown so to speak) but is too slow decaying, you may consider a crowbar type circuit. The peak current may be high but the energy dissipated would limited to the stored energy in the circuit. The thermal mass and temperature rating of the crowbar switching device may be adequate to require little to no heat-sinking.

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  2. Hi David,

    Thank you for your posting and suggestion. I have the FET switch built but awaiting the heat sink. Probably could test it without the heat sink but do not want to tempt fate. Predicated on that testing I may very well have to utilize your suggestion. I have built crowbar protective devices previously and typically the triggering signal shunts the output to ground and contained in the circuitry is fuse that blows. That would be quite exciting 48 volts at 30 amps shunted to ground -- but what ever it takes to protect the RF device.

    Thanks again I am always grateful for input and suggestions.

    73's
    Pete N6QW

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