PicoPlot Completion Information
Physical Properties
The PicoPlot DF comes as an assembled / tested PC board, but requires additional "completion" work by the user, before it can be employed as a functional DF. ( completion work is described on this page ) The PC board has a DB15 ( male ) connector for connections to/from the external equipment. The PC board can be mounted however the user desires, but it is designed to fit perfectly in a Hammond model 1593QBK or 1593QGY plastic box, which has space for a 9V battery. ( for portable operation )
The figure below shows the dimensions and locations of the mounting holes. If desired, the board is sufficiently rigid and sturdy so that it can be secured with the two DB15 panel jackscrews, alone. The RSSI calibration trimpots can come "pre-adjusted" if the user provides information about the maximum and minimum RSSI voltages, for the DF receiver that will be employed. This information must be provided by the user, prior to shipment. Otherwise, the trimpots will require user-performed adjustments ( once, when installed ) so consider this when installing the board. ( provide screwdriver access to the trimpots ) As an alternative, perform the trimpot adjustment procedure ( described below ) before installing the board.
Some wire straps also must be installed on the backside of the board, to select the desired mode of operation. Assuming the user does not intend to change these straps, this can be done before installation. Otherwise, ( if the straps settings will change ) the user must contrive some connections to the M0 and M1 mode select pads, to allow this. ( these pads are not available on the DB15 connector, not enough spare pins )
Install Connections
The image below shows the external connections required to operate the PicoPlot DF. Some of the connections shown are required in all modes, others are optional.
Strictly speaking, the IBM PC display computer is not absolutely necessary because the PicoPlot can be used with the SquealTone output, alone. ( mostly intended for portable operation ) The numbers on the image refer to the DB15 connector pin numbers, also shown below :
Getting RSSI Signals
The PicoPlot DF is NOT suitable for a technical "beginner". Most receivers do NOT provide an RSSI output and therefore will require some ( non-trivial ) internal modifications to provide this signal. In some cases, these modifications may be impractical or impossible, and will require the use of a different receiver... a lot of the radios available today are simply "too tiny" to safely permit someone "tinkering around" inside, looking for an analog RSSI signal.
It is the users responsibility to identify and perform the required mods to obtain a suitable RSSI signal, there are simply too many different receiver designs to provide a detailed description for each receiver, here. If you are not confident about your ability to do this, probably you should NOT attempt it, and should not buy a PicoPlot DF. ( or else get some serious technical help from a friend )
RSSI Requirements
The PicoPlot DF is designed to directly accept RSSI voltages of positive polarity, ranging from 0.05 to +5.0 volts DC. Higher voltages can be accommodated with external resistor networks. The DC difference between maximum and minimum RSSI voltage should be at least 400 mV, to achieve the "full range" of RS232 output values available from the PicoPlot DF. Trimpots are provided to compensate for any DC "bias" in the RSSI signal, as well as variations of RSSI range. ( 0.4 to 5.0 volts of RSSI "delta" from max signal to min signal ) The PicoPlot input impedance for the RSSI signal is 2 Megohms, so circuit loading should not be a problem.
The PicoPlot DF can accept RSSI signals with either a positive or a negative co-efficient.
A positive co-efficient means the RSSI voltage increases with increasing signal strength. ( higher positive voltage for stronger signals )
A negative co-efficient means the RSSI voltage decreases with increasing signal strength. ( lower positive voltage for stronger signals )
The selection of the proper RSSI co-efficient is performed in the WinPlot display progam, on the OPTIONS page, where a button is provided for it, but the SquealTone audio output assumes a positive co-efficient. This can be reversed ( negative co-efficient ) by shorting pins 22 and 23 on the PIC micro, with a blob of solder. This ( crude ) "jumper" is only examined when DC power is applied.
Best results occur with an RSSI signal that follows a logarithmic curve, so the RSSI voltage is proportional to the "decibels" of the signal strength. ( decibels = logarithmic ) Most modern recievers employ an I/F amplifer "chip" with such an RSSI output signal, and the signal usually spans 60 db ( or more ) of dynamic range. The PicoPlot DF will work properly with other RSSI signals, but this is the "preferred" signal that will yield the best results, across the widest range of signals.
RSSI Coefficient Polarity
The PicoPlot can accept RSSI signals of either positive or negative coefficient polarity. A positive RSSI co-efficient simply means that stronger signals yield higher RSSI voltages, ( = more positive ) and a negative coefficient means stronger signals yield lower RSSI voltages. ( = less positive )
For example, a reciever that generates RSSI voltage = +2.4 for a zero-strength signal, and +2.8 volts for a full-strength signal would have a positive coefficient. Conversely, a receiver that generates +2.8 volts for a zero-strength signal and +2.4 volts for a full-strength signal would exhibit a negative coefficient.
The calibration process is slightly different for positive and negative RSSI coefficients, so the user must identify the "coefficient polarity" before performing the calibration procedure.
Notes on RSSI Calibration
The PicoPlot DF can be shipped with the RSSI trimpots "pre-set" to their proper values, if the user provides information about the maximum and minimum RSSI voltages, ( for the DF reciever ) prior to shipment. Otherwise, the user must perform this calibration process. A detailed procedure for this is provided below, ( for both positive and negative RSSI coefficients ) but the following "general notes" are worth reading, first :
"CW" refers to clockwise rotation of a trimpot adjustment
"CCW" refers to counter-clockwise rotation of a trimpot adjustment.
CW rotation of the BIAS trimpots ( either one ) will cause the reported RSSI voltage to increase.
CW rotation of the GAIN trimpots ( either one ) will cause the reported RSSI voltage to increase.
The "authority" of the COARSE trimpots is about 20x the authority of the FINE trimpots. ( effect = 20x greater )
The trimpots have a total adjustment range of 12 turns.
There are no mechanical "stops" to indicate when the end-of-range has been reached. The only way to ensure a pot has reached the end of its adjustment range is to turn it at least 12 turns in one direction.
The MAX LED = ON whenever the RSSI reading is 95% of full range, or greater.
The MIN LED = ON whenever the RSSI reading = 5% of full range, or less.
The calibration process can be performed with the MAX/MIN LEDs alone, but it is generally somewhat easier if the SquealTone is also monitored, while the trimpots are adjusted.
If desired, the input to the microcomputer ADC can also be observed with a DVM or a scope, while the adjustment process is performed. ( trimpot settings will affect this reading ) This signal can be observed at the junction of resistors R12 and R13, near pin 1 on the micro. For this signal, +5 volts = RS232 reported "8191" and 0 volts = RS232 reported "0000". The RS232 messages can also be directly observed using the Windows HyperTerminal program, usually found in the START / PROGRAMS / ACCESSORIES menu. Set it for 9600 baud, 8 data bits, no handshaking, and 1 stop bit.
Positive RSSI Cal
Use this procedure if the reciever exhibits a positive RSSI co-efficient. ( see RSSI COEFFICIENT POLARITY above, for explanation ) A transmitter ( or equivalent signal source ) will be required, to generate a "full-quieting" signal. ( = max RSSI voltage )
Select the Portable SteadyScan mode : strap M0 = grounded, strap M1 = open-circuit. Turn all four trimpots at least 15 turns CCW. Then turn each of the FINE trimpots 6 turns CW, to preset them at mid-range.
Hook up the reciever that will be employed, turn it on, and tune it to an empty channel. ( no signal = minimum RSSI voltage )
Turn on the PicoPlot DF, and close the ENABLE switch. ( switch must remain closed during this process )
STEP 1 : Provide RSSI signal = maximum voltage ( key the TX to make a full-quieting signal )
STEP 2 : Adjust the GAIN COARSE trimpot CW until the MAX LED reaches the ON / OFF threshold.
STEP 3 : Finish the adjustment with the GAIN FINE trimpot. ( MAX LED = ON / OFF threshold )
STEP 4 : Provide RSSI signal = minimum voltage ( unkey the TX, confirm the channel is silent )
STEP 5 : Adjust the BIAS COARSE trimpot until the MIN LED reaches the ON / OFF threshold.
STEP 6 : Finish the adjustment with the BIAS FINE trimpot. ( MIN LED = ON / OFF threshold )
STEP 7 : Repeat steps 1 through 6 until the successive adjusments required on each COARSE trimpot is ½ turn or less.
NOTE : The trimpot settings "interact" to some degree, so properly adjusting the BIAS trimpots will "mis-adjust" the GAIN trimpots, and vice versa. Therefore, a "back and forth" adjustment process is required, until the trimpot changes are small enough to ignore... if the corrections require ½ turn or less on the COARSE trimpots, there is negligible benefit to continuing the process... you are done.
Negative RSSI Cal
Use this procedure if the reciever exhibits a negative RSSI co-efficient. ( see RSSI COEFFICIENT POLARITY above, for explanation ) A transmitter ( or equivalent signal source ) will be required, to generate a "full-quieting" signal. ( = max RSSI value )
Select the Portable SteadyScan mode : strap M0 = grounded, strap M1 = open-circuit. Turn all four trimpots at least 15 turns CCW. Then turn each of the FINE trimpots 6 turns CW, to preset them at mid-range.
Hook up the reciever that will be employed, turn it on, and tune it to an empty channel. ( no signal = minimum RSSI value )
Turn on the PicoPlot DF, and close the ENABLE switch. ( switch must remain closed during this process )
STEP 1 : Provide RSSI signal = maximum voltage ( confirm the channel is silent, = no signal )
STEP 2 : Adjust the GAIN COARSE trimpot CW until the MAX LED reaches the ON / OFF threshold
STEP 3 : Finish the adjustment with the GAIN FINE trimpot. ( MAX LED = ON / OFF threshold )
STEP 4 : Provide RSSI = minimum voltage ( key the TX to make a full-quieting signal )
STEP 5 : Adjust the BIAS COARSE trimpot until the MIN LED reaches the ON / OFF threshold.
STEP 6 : Finish the adjustment with the BIAS FINE trimpot. ( MAX LED = ON / OFF threshold )
STEP 7 : Repeat steps 1 through 6 until the successive adjusments required on each COARSE trimpot is ½ turn or less.
NOTE : The trimpot settings "interact" to some degree, so properly adjusting the BIAS trimpots will "mis-adjust" the GAIN trimpots, and vice versa. Therefore, a "back and forth" adjustment process is required, until the trimpot changes are small enough to ignore... if the corrections require ½ turn or less on the COARSE trimpots, there is negligible benefit to continuing the process... you are done.
Mode Selection
The PicoPlot DF has 3 DF modes and one SelfTest mode, selected with two wire straps that are installed ( or not installed ) on the backside of the PicoPlot PC board. Once set, these straps usually won't be changed for a specific DF, but while the PicoPlot is running, they are examined "in realtime", so that the SelfTest mode can ( if desired ) be invoked, at will. They are NOT available through the DB15 connector, there were not enough spare pins. ( sorry ... )
TachScan Prescaler Straps
The TachScan prescaler straps are only employed in the TachScan mode, and they are only examined when DC power is applied. They are used to "scale down" the TACH clock rate, in case the TACH signal operates at a frequency that is too fast for the PicoPlot to "keep up" with it. In the TachScan mode, PicoPlot sends RS232 messages at a rate based on the TACH signal, and the message rate must be limited to a value of 40 messages per second or less.
The TACH prescaler straps ( and their settings ) are shown in the image in the preceeding section.
The prescaler straps ( three, total ) allow the user to adjust the number of TACH pulses required to "trigger" an RS232 message, ranging from one TACH pulse per message, up to 128 TACH pulses per message. A precise prescaler ratio is not required because the WinPlot display program will "figure out" the exact rate, based in the number of RS232 messages recieved during each 360 scan.
In practice, it is desirable to use the lowest prescaler division ratio ( = highest message rate ) that still yields less than 40 messages per second, since this will provide the greatest angular precision, in the display.
The proper setting of the TACH straps can be found by calculation, if enough information is available. If the maximum TACH signal frequency is known, simply divide it by 40, and pick the nearest prescaler ratio that EXCEEDS the resulting value. ( to yield the fastest message rate that is less than 40 messages per second )
The proper setting of the TACH straps can also be found "experimentally", by starting with the lowest prescaler ratio, ( 1:1 ) and watching the two LEDs on the PC board, while rotating the antenna at maximum speed. ( = maximum TACH rate ) If the PicoPlot DF detects that the TACH pulses are arriving "too fast", it will "lock up" and turn on both LEDs. Once this occurs, no further RS232 messages will be sent, and ( in fact ) the PicoPlot DF will not respond to ANY external stimulation at all... The only way to escape from this mode is to interrupt the DC power and "re-boot" it. This might seem inconvenient, but the LEDs might not always be visible to the user when this condition occurs, so this method of "getting the user's attention" was chosen.
ENABLE Input
The ENABLE input is self-powered and expects a simple switch closure, to signal the start or end of RSSI reporting. ( switch closed = reporting ) This input is employed in all operating modes. ( input = required )
For Portable SteadyScan mode, this switch normally would be mounted at ( or near ) the "grip point" for the DF antenna, since it must be closed / opened when the antenna points at the " 12 o'clock" direction for the start / stop of a 360 scan. For other modes, it can be installed anywhere convenient for the operator.
HEADING Input
The HEADING input is only required in the Triggered SteadyScan and TachScan modes. ( mast-mounted antennas ) This input is self-powered and expects a simple switch closure, to signal the moment when the antenna position = 12 o'clock on the display. This input is edge-triggered, and the trigger occurs on the rising edge of the waveform. ( switch opens when antenna position = 12 o'clock position )
This signal can be provided by a magnetic-triggered ( reed ) switch, with a magnet mounted on the antenna mast. A cam-triggered microswitch will also work. Other methods can be employed, but some of them might require ( minor ) changes in the input circuits of the PicoPlot DF. ( not a problem, ask if you want to do this... )
TACH input
The TACH input requires a user-provided signal. This signal is only required in the TachScan mode. The signal voltage can range from +5 volts up to +100 volts. ( actually, somewhat more... ) Input protection is provided for this signal. Frequency should not exceed 5.12 KHz, the maximum allowed. Input impedance is 180K ohms, minimum.
The TACH signal should closely follow the antenna rotation rate, at every instant in time, for best display results. The TACH signal should generate at least 100 pulses per ( 360 degree ) antenna rotation, to get reasonably good angular resolution in the display. ( 100 pulses / rev = 3.6 degrees per TACH pulse ) Fewer pulse per revolution will work properly, but the display will appear more "jagged" and "grainy"... this can be mitigated to some degree by setting the SECTOR value in the WinPlot display program to a value that yields the desired image quality. ( it "smoothes out" the pattern )
The TACH input ( if employed ) eliminates the need for antenna rotation at a constant speed. Therefore, the antenna can be rotated manually, ( "by hand" ) which eliminates the need for a motor drive, and yields a simplified DF design.
RS232 Output
The RS232 output operates at 9600 baud, 8N1 format, no flow control or handshaking. If desired, the messages can be observed with the HyperTerminal utility program in Windows, which is usually in the START / ACCESSORIES menu.
A connection to the TX output from the host computer is required, but only to provide a source of negative DC voltage for the PicoPlot RS232 messages. This is basically a cheap ( circuit ) "trick", necessary because the ( formal ) RS232 specification requires a negative voltage for messages, and it is easier to "rob" this voltage from the host than it is to create a separate negative supply on the PicoPlot board, just for the RS232 output. The TX input is otherwise ignored.
SquealTone Output
The SquealTone audio output can drive a small 8-ohm speaker to a level of 100 mW. A trimpot on the PicoPlot PC board allows adjustment of the speaker level. The SquealTone audio is available at pin 9 on the DB15 connector. The SquealTone assumes the reciever has a positive RSSI coefficient, but this can be changed with a crude "strap", as described below.
The SquealTone is enabled whenever the ENABLE switch is closed, and muted at other times. The audio tone varies ( in pitch ) with the RSSI level, as observed by the microcomputer ADC input. ( the SquealTone input is connected to the output of the op-amp circuits, so the RSSI bias and gain pots will affect it. ( can be used to assist trimpot adjustments )
The SquealTone pitch varies logarithmically across 3 octaves of frequency, from about 200 Hz ( = zero RSSI level ) up to 1600 Hz, yielding a very smooth, useful and "natural sounding" tone response. The logarithmic response is achieved with a software "lookup table", which translates the RSSI measurements into the corresponding tone frequencies. Assuming an RSSI signal that varies across 60 decibels of range, the tone resolution is about ½ decibel per tone "step".
About 20 decibels of "bass boost" is provided at 200 Hz, to enhance the ( usually poor ) low-frequency performance of small speakers.
The SquealTone has two operating ranges, 1x ( normal range ) and 5x. ( magnified range ) The magnified range is only available in SelfTest and Portable SteadyScan modes. The magnified range is selected by grounding the HEADING switch input, since this input is not ( otherwise ) employed in the SelfTest or Portable SteadyScan modes.
In the normal mode, the SquealTone varies from 200 to 1600 Hz across the entire range of RSSI input voltages. In the magnified mode, the SquealTone varies from 200 to 1600 Hz not once, but five times, as the RSSI voltage varies across the same range. Whenever the tone reaches the end of one ( 3-octave ) range, it "up-ranges" ( or "down-ranges", as required ) to begin another 3-octave sweep.
The SquealTone assumes the reciever RSSI has a positive co-efficent, so ascending signal strength will yield an ascending SquealTone. If the receiver coefficient is negative, the SquealTone polarity can be reversed by shorting pins 22 and 23 of the PIC microcomputer, ( shown in the image below ) with a bit of solder to "bridge" the two pins together. These pins are only examined when DC power is applied.
IrDA Output
The IrDA output is for future use, since at this time, ( June 2007 ) I have not created a PalmOS PDA display program to use it. The PicoPlot DF generates IrDA messages that comply only with the "physical layer" of the official IrDA specification. ( sometimes called "raw IR" ) It does NOT implement the full protocol, which includes negotiations and transactional messages, required to establish a flexible / agile data link between two IrDA-compliant devices.
Details about the message that IS available at this output will be provided on request, in case someone wants to hack their own display program using a PalmOS ( or other ) display with IrDA capability.
More Information
All the information required to finish the assembly of a PicoPlot DF should be available on this webpage, but if questions remain, feel free to enquire by e-mail :
