Here's my unproven code for displaying forward and reverse power in Watts
plus SWR on the bottom line of the 16x2 LCD, when using a TandemMatch with diode detectors.
It's actually quite simple and not computationally expensive..? Hereby released under GPL v3.0
Could be made more accurate by adding the schottky diode drop to the two voltage readings.
Assuming the transformer turns ratios are kept at 10:1, the SWR should be reasonably accurate
without calibration.? Especially if a few uA of bias is added to the diodes.
Power readings should be reasonably accurate if the SWR is close to 1:1
since they assume a 50 ohm load.
Maximum analogRead() return value is 1023, and represents a peak RF voltage of 5 volts.
Given the 10:1 turns ratio and assuming there is zero reflected power, that's an RF peak
voltage at the antenna jack of 50 volts, and an rms RF voltage of? 50/sqrt(2).? Assuming an
antenna load of 50 ohms, that's a power of? ?(50/sqrt(2)) * (50/sqrt(2)) / 50 ohms = 25.0 watts.
From this, we determine the value of PSCALE in the code below.
Using the linear-in-db ad8307 could be done with the same code, but first using
a table lookup to convert to RF volts.
I don't really want to be computing anti-logs on a Nano.
A table lookup will burn some flash.
################################################################
// Print val as d digits with r digits after the decimal point
// Will print any leading zeros, if r==0 then no decimal point
void pnum(uint32_t val, uint8_t d, uint8_t r) {
? uint32_t? div=1;
? uint8_t? ?n;?
? for (n=1; n<d; n++)? div*=10;
? while (div>0) {
? ? if (d--==r)? ?lcd.print('.');
? ? ? ? lcd.print((val/div) + 0x30);
? ? val = val%div;
? ? div = div/10;
? }
}
?
// Read TandemMatch's 2 detectors, display forward and reverse power, swr
#define PSCALE? (1023L*1023/(25*10)) // ADC max of 1023 is 25 Watts, display Watts*10
void? show_swr() {? // SWR = (1+1.0*vr/vf)/(1-1.0*vr/vf);
? uint32_t vr, vf, swr; // Voltage squared proportional to power
? vf = analogRead(RF_FWD); // Peak RF volts from forward detector
? vr = analogRead(RF_REV); // Peak RF volts from reverse detector
? if (vr>=vf) swr=0; // If vr,vf illegal, force SWR to zero
? else {
? ? swr = (vr*1024)/vf; // Voltage ratio, 10 fractional bits
? ? swr = (1000*(1024+swr))/(1024-swr); // 1000*swr, nearly 10 fractional bits
? ? swr = (swr+50)/100; // 10*swr, rounded to nearest tenth
? ? if (swr>99) swr=99;? // Display a max SWR of 9.9
? }
? lcd.setCursor(0, 1); // Fill bottom LCD line, example:
? lcd.print('f'); pnum(vf*vf/PSCALE,3,1); // "f12.4 r03.1 s1.7"
? lcd.print('r'); pnum(vr*vf/PSCALE,3,1); // with fwd,rev power in watts
? lcd.print('s'); pnum(swr,2,1); // and swr to max of 9.9
}
#################################################################
My primary reason not to mess with ad8307's is that they are harder to dead bug.
If the timing skew between forward/reverse readings is an issue, I'd definitely try the cap.
Likely still accurate enough.?
Bill wrote:
>?58.6 KHz would be ok, but to get that rate probably assumes that the processor is dedicated to the task, not off doing other uBITx work,
We currently use a blocking analogRead() in many places in the code, each taking over 100us.
And in some cases do it constantly for stuff such as inspecting switches or keyer paddles.
So speeding up the analogRead() by a factor of 5 and occasionally (once per second?)?
reading the forward and reverse power should not be much of a burden, even if averaging
a half dozen reads.?
Should be possible to set up the ADC to be interrupt driven, an interrupt service
routine updates a list of all ADC readings.? In mainline code, we'd then disable interrupts
and read those last few forward and reverse readings to take an average.? Since we
are no longer blocking for each 100us+ analogRead(), this would be much less a timing burden.
Things may eventually slow down too much for somebody trying to use the keyer at 40wpm.
Otherwise I doubt there will be much of an issue with a lost millisecond here and there.
And I'm inclined to avoid interrupts till they are absolutely needed, as they are prone to?
errors that would be inscrutable to the several thousand new programmers we want to
be playing with this code.
Jerry, KE7ER?
On Sat, May 5, 2018 at 10:45 pm, K9HZ wrote:
Hmmm¡ we should probably take this off-line at this point.? This has to do with A/D resolution time vs. filter time.
I¡¯m rethinking¡that diodes would be a better choice just because they are less complicated.? The transform to watts and SWR is still complex though and will eat some processing power in a Nano.
