Design objectives
- The primary objective of the Picswr project was to use the 12-LED S-meter display on transmit as a wattmeter and SWR meter.
- A secondary objective was to display the same information using the status board driven analog meter, if installed.
- These objectives should be achieved with minimal disruption of existing PicAStar wiring.
The DSP unit sends 9600 baud serial S-meter telemetry to the PIC16F27 on the status board. This PIC drives the 12-LED display and the analogue meter.
On transmit, Picswr needs to optionally take over control of the display by generating this telemetry from forward and reflected voltages from an SWR bridge.
The minimum installation requirements are therefore:
- DSP generated S-meter telemetry must be diverted through the Picswr unit.
- An SWR bridge must be fitted in the Tx output or antenna coax.
The unit also needs a 12v supply and an RS232 PC-link socket on the rear panel - You now have three of these rather than two. Picswr uses the same RS232 - jack plug cable as the PicAStar PC link.
- Picswr's LED display should have a linear watts/LED calibration.
- Picswr's analogue display should have a linear watts/mA calibration
The output of SWR bridges is non linear and may differ slightly between forward and reflected. Picswr therefore needs to store calibration profiles. These are generated using the PC based Calswr program.
- Picswr should optionally display computed SWR both on the LEDs and on the analogue meter.
The PIC16F876 software computes SWR. The LED display calibration is : 1 LED = 1.0, 5 LEDs = 2.0, 9 LEDs = 3.0.
- Picswr's calibration must be tailored to suit the installed PA. For example: power ranges of 0-20w, 0-100w etc
Calswr allows you set the LED power range in watts per LED. There are 12 LEDs.
For a 20w PA you would set 2w/LED giving a range 0 - 24w.
For a 100w PA you would set 10w / LED giving a range 0 - 120w.
The forward and reflected watts per LED don't have to be the same.
The analogue meter calibration is in watts per 'dB over S0' where dBs refer to the existing S-meter callibration - eg: 54dB = S9 = 75% full scale.
- To provide some degree of mismatch protection.
Many transceivers use the ALC facility to progressively reduce the drive with worsening mismatch. PicAStar has no such ALC system and doesn't need it.
Picswr provides the facility to shut down the PA if reflected power is above a specified level for a specified time interval. The power level is specified in the Calswr calibration program. The time interval is determined by R13 / C7 on the Picswr circuit board. My default provision is > 4w for approx 8 seconds. This arrangement allows you to tune at full power for eight seconds or more. The 'shut down' output is an open collector NPN transistor.
- Control requirements on the front panel should be very simple.
The front panel hardware is one tricolour LED (R,G,Y) and an adjacent miniature pushbutton (push to make).
Each press of the button steps through a cycle of five transmit display modes. (Mode can be changed on receive or on transmit):
- Original DSP transmit display - LED is off
- Forward power - LED is green (startup default)
- Reflected power - LED is red
- Reflected power 10x - LED is red flashing.
- SWR - LED is yellow
The 'reflected x10' mode is 10 x more sensitive setting for fine tuning with low reflected power levels.
The same pushbutton is also used to receive calibration information from the PC based Calswr program . (Hold for 4 seconds unit it flashes green and then click 'Send to Star')
- PC monitoring of forward and reflected power and SWR should be provided.
The main window of Calswr has four vertical bar indictors each of twelve segments (four green, four yellow and four red) which behave according to the current calibration and so correspond to the to 12-LED display in PicAStar (providing that the calibration has been sent to PicAStar !!! ) There are also numeric displays of raw ADC output (0-1023) and of forward, reflected power and SWR. (I thought that designing a graphical cross needle display was a bit heavy and unnecessary at this stage !!)
- Calibration procedures should be easy to carry out
- The Picswr module should be small, simple and easily constructed.
The Picswr module measures 4.8 x 6.8cm.
It contains:
- a 28 pin DIP PIC16F876-20 processor (as used in PicNMix).
- an ST232N RS232 interface device (as used on the DSP motherboard).
- a 78L05 regulator
- a 20Mhz crystal
- a multiturn preset pot.
- Rs and Cs, and two diodes
- Connectors - including a connector for in situ programming - identical to those in PicAStar.