Implementation details for the passive phased array controller

The nifty little computerized antenna tuner from LDG has all the smarts in it to automatically configure an L network to minimize the SWR. For this application, though, we essentially want to convert it to a smart peripheral for a controlling PC. Fortunately, the LDG design is clean and the 68HC11 has the right kind of interfaces for our application. I've used the QRP version, because it is smaller, and the 100W from my transmitter will be divided down so that no one tuner has to take the full power. The details of the modifications would be different for the AT-11, but the logic will work the same.

The modifications are described in more detail here.

One of the things we can do is always load the software into the tuner from the master computer, each time the system is powered up.

Here are the basic changes to be made:

New Sensors

Added sensors for current, voltage, and phase information. Fortunately, the 68HC11 has 8 A/D inputs, only two of which were used for the LDG tuner.  We'll actually put the SWR bridge on the output side of the network (on the feed to the antenna).

4 Aug 2002: Some form of phase detection would be really nice. I'll have to find some low power input (i.e. hi-Z input ) phase detector. Probably a suitable CMOS flipflop (that can work at 30 MHz) will do, followed by a RC integrator. Then the phase signal can be fed to a spare A/D input.

Another handy input might be a "input voltage", since the SWR bridge will be on the antenna side. High Z diode detector will do here.

New software requirements for the microcontroller

The master computer needs to send L and C configuration data to the microcontroller and read back the values from the A/D. For convenience, we'd like to talk to a bunch of LDG boards with one serial or parallel computer port. There are several ways to do this:

  1. Use the standard Asynchronous (SCI) interface and chain the data in from one to the data out to the next
  2. Use the standard Async interface (SCI) and bus the units together, making the TxD open collector (really open drain)
  3. Uuse the SPI interface.

In any case, it is convenient if we can set the address of each board, without having to burn a separate eeprom. Fortunately, we don't need the push button switch interface for the board anymore (it isn't an automatic tuner),so we can use the cap and inductor up/down bits to set up a 4 bit unit number with jumpers.

Commands to the LDG:
    Set L&C
    Read AD #

Data back from LDG
    AD value
 

Strategies for software

Alternative 1 - Use the special boot loader mode and load the software each time.  This doesn't require reprogramming the chip, however, it may require some special interface logic, and may not lend itself to the daisy chain communications architecture, because it requires responding to a byte from the processor at the right time, and it is uncertain whether you could rely on all the processors to be synchronized sufficiently so that you could just hook them in parallel. It also requires communication to the chip at a weird rate (8789 bps) which the run of the mill PC may have trouble doing, particularly under Windows, where you can't just go program the UART registers. However, replacing the existing 4.5 MHz crystal with a 4 MHz crystal will work at 4800 bps.

Alternative 2 - Write new software and burn it into the EEPROM on the chip. In the long run, this is probably the cleanest approach.

Alternative 3 - Get the version of the chip programmed with Motorola's debugger. Then, use debugger commands to load the necessary program (or, for that matter, to set the i/o port values). The latter strategy may not work very well in a multi unit environment  because the debugger doesn't have any inherent concept of a unit number, so you would need some sort of mux at the PC (if running RS232) or you'd have to burn a lot of bits on the printer port.

Strategies for the PC/LDG interface

Alternative 1 - Serial RS-232 - most computers have serial ports but, there may not be a free one. Also, it requires some level shifting to get from RS232 levels to the TTL levels the 'HC11 wants. The interface is fairly standardized and can easily be used from high level languages without getting into gory driver problems when running Win NT. This is the approach I have taken, with some of the modem control lines being used to reset the processors on the HC11s.

Alternative 2 - Printer port - Already TTL levels, but talking to the printer port directly from a windows program is difficult particularly if you are running Win NT.

Alternative 3 - Ethernet - In the long run, this would be the ideal approach: Put an embedded single board computer into the tuner box that does a lot of the computational work, and talk to it via the network (it could look, to the outside world, like an IP address, or a web server, or whatever). Dedicating a single board computer to the job of talking to all the hardware is attractive, however it makes software development much more complex because you don't have all the supporting infrastructure and you have to deal with a much more complex communications protocol. This approach would probably be more suitable for a Mark II version. It does lend itself to a "standalone" box, though, with a knob (or knobs) that sets the pointing direction....(Talk to me about my ideas for Ethernet interfaced receivers and transmitters in a discrete active phased array).

4 Aug 2002: the last alternative is looking really good.. Rabbit Semiconductor has a nice little SBC with a ethernet interface, and a pile of I/Os that can fan out to the LDGs

RS-232 Interface

A Maxim MAX207 or MAX208 is used to do the conversion between the PC RS-232 interface and the TTL levels inside the system. The signals used are:

 
TxD (from PC) DB-9 pin 3? RxD on tuners
RxD pin 2? TxD on tuners
DTR   Reset on tuners
RTS   Attention on tuners
Ground pin 5 ground

More on the RS232 Interface, including a schematic.

4 August 2002 - A bit of work on grounding loops and such, tells me that RS232 is horrid for the long interface to all these RF devices. Nope, now we're going to go for optoisolators and current loops. Still need the RS232 interface to the master computer though.

Packaging (revised)

Package each tuner in an independent little box, with optoisolated digital i/o. The boxes should be weatherproof(ish), although plastic baggies can do miracles) (Besides, I've used up most of those handy 5x7x2 chassis on other projects)

--------------------------------------Below has been Totally Obsoleted after thrashing around a bit ------------------------------

Packaging

Each LDG board is in its own 5x7x2 chassis for RF isolation. The power input and two data lines are brought out through filtered feed throughs. Suitable coax connectors are used to bring the RF in and out.
               
     

 

For 8 of the units (the ones connected directly to the antenna):

On one 5x7 end of the chassis, a BNC connector is used for the coax to the antenna. The 4 LEDs driven by the LDG board are also on this end. The odds that the impedance at this point is actually 50 ohms is quite low, but the BNC is easy to install and inexpensive. On the other end is a pair of BNC's (TNC?) one for the input to the tuner, the other for a phase reference signal. There are also feedthroughs (or a connector) for


implement.htm - revised 16 September 1999, Jim Lux
revised 4 Aug 2002
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