Ten-Tec Orion S-meter & Hamlib

We have been making some progress with the Ten-Tec Orion backend software in the "Hamlib" package, an open-source project to provide a device-independent programming interface for amateur radio equipment in the Linux/POSIX world.

Here's a tid-bit you won't find everywhere: the Orion S-meter calibration curve.

#define TT565_STR_CAL { 15,  {
{  10, -45 }, /* S 1.5 min meter indication */
{  13, -42 },
{  18, -36 },
{  22, -30 },         
{  27, -24 },
{  30, -18 },
{  34, -12 },
{  38,  -6 },
{  43,   0 }, /* S9 */
{  48,  10 },
{  55,  20 },
{  62,  30 },
{  70,  40 },
{  78,  48 }, /* severe dsp quantization error */
{ 101,  65 }, /* at high end of scale */
} }

The first column is the Orion's response to the "?S" command in computer units. The second column is indicated decibels (dB) as read on the actual S-meter.

(Literary event: I just defined "S meter" in the Wikipedia! See my article.)

The Orion has a couple of peculiarities when measuring power. The minimum reading (on my unit) is about S 1.5 on all bands when looking at a dummy load. What does that mean? If S9 = 50 µV, then S1.5 is 6 dB x 7.5 S-units or 46 dB down in power = 2.5 x 10^-5. The voltage ratio is the square root of power, or about 5 x 10^-3. So the equivalent RF voltage level is 50 x 5 x 10^-3 = 0.25 µV. The Orion's sensitivity spec (main receiver) is "< 0.18 µV" says that the receiver's internal noise will be about this level, which is consistent with the meter reading. In other words, Ten-tec seems to have this right.

(On the other hand, the tried and true RST system says that S1 means "Faint signals barely perceptible". That's an argument for using 5 dB per S-unit, as some have defined it, at least with our current generation of receivers.)

The other feature of note on the S-meter is the behavior at very high signal levels. As you can see from the calibration table above, the raw S-meter units are spread out roughly 8 units per 10 dB. However, the meter reading is highly quantized. You only see values of S9+48 and S9+65, nothing in between. This indicates that the software computation of received power has a quantization problem. Of course, it's hard to complain too much, because such power levels are almost never seen in real life. (If they are, better switch in some attenuation!)