Hello.
I am working on an ATtiny power / SWR meter, basically a smarter
replacement of the famous Tayloe SWR bridge. It will be a 1W to 5W (maybe a
bit higher, up to 10W) QRP resistive bridge with 6 LEDs analog-like
baragraph, with the intensity being interpolated between two successive
LEDs enhancing the baragraph resolution to at least 4x6 levels (I can
easily recognize 1 LED illuminated from 2 LEDs equally illuminated and 1
LED illuminated significantly stronger than the other). I have the
baragraph electronics and code prototyped, it works quite nicely.
The electronics (ATtiny13A driving the 6 LEDs) will be powered from a 3V
low drop low low quiescent current 16V max input linear regulator, and the
linear regulator will be powered with a 1:2 or 1:3 transformer from the
radio RF power, rectified by a single 1n5711 shottky diode (the diodes will
be used in the bridge anyway, so to reduce the BOM complexity). Look ma, no
batteries.
There will be a larger capacitor at the input side of the regulator, so the
microprocessor will have couple of seconds to perform additional tasks
after key up, that is to show the input power with quite a high accuracy.
So there will be no button: SWR will be shown during key down, and power
will be shown after a second or so after key up.
I feel quite confident with digital design and programming, but I am not
quite sure about the RF
side. Namely, what transformer should I use to feed the linear regulator?
These are my back of the envelope calculations:
On the transceiver output, when loaded with an open resistive bridge (all
three resistors 50 Ohm), the SWR is 2:1 and the voltage at the transceiver
output is 1.3333 * of Vpeak (assuming 50 Ohm output impedance of the
transceiver). Likely the maximum voltage will be lower as the output
impedance of the transceiver PA is not resistive. These are the maxima:
10W -> 42.16V Vpeak at SWR 2:1
7W -> 35.28V Vpeak at SWR 2:1
6W -> 32.65V Vpeak at SWR 2:1
5W -> 29.81V Vpeak at SWR 2:1
The minimum Vpeak at 1W transceiver output with the resistive bridge
shorted (again producing SWR 2:1 at the transceiver) will be minimum 8V
assuming the transceiver PA output is resistive. If not, the output voltage
will be closer to 10V peak.
Now the digital circuit requires a minimum 3.1V at 1.5mA DC before the
linear stabilizer, and the power will be rectified with a single diode, but
the micro will likely work happily down to 2V even if the linear regulator
just passes the voltage unregulated, as the micro may use its internal 1.1V
voltage reference for measurements. Operating the micro unregulated sounds
unusual, but it may be a viable option if tested, especially if the input
voltage will be stable enough due to the large capacitor before the
regulator.
Now my question is:
1) Should I use a 1:2 or 1:3 transformer? Using the 1:2 transformer may
require additional zener diode and likely a resistor before the linear
stabilizer if the input voltage could exceed 16V peak.
2) What will be the tiniest / cheapest ferrite and how many widings will be
optimal? I suppose this decision will be strongly dependent on the bands
supported. My personal goal is a 40m-20m EFHW tuner.
And also:
Will there be an interest for a kit?
Through hole or SMD? It may be a nice "introduction to SMD" kit as the
number of parts will be pretty low.
What bands?
What power range?
Should it contain an EFHW tuner? My personal preference would be a "NJQRP
Rainbow Tuner" like setup covering 40m-20m with a T50-6 transformer, mica
compression capacitor trimmer, jumpers to select the primary winding with
another jumper to add a capacitance for 40m coverage.
Thanks and 73,
Vojtech OK1IAK (former AB2ZA)
