Re: K3 - SWR Indication

Posted by N8LP on
URL: http://elecraft.85.s1.nabble.com/K3-SWR-Indication-tp2784793p2808130.html

Several interesting topics have arisen in this discussion. Here are a couple comments...

1) Most wattmeters use a diode peak detector for each of the coupler ports (FWD, REF). The forward voltage drop of the diodes becomes an increasing source of error as power is lowered. The means than for fairly low SWRs, there is more error in the REF voltage sample, which is lower to begin with. Squaring the voltage to calculate power exaggerates the error. Very few commercial meters try to compensate for the diode drop. The LP-100A does, using a circuit similar to the one in the Tandem Match design, as well as using a dual Schottky diode on a common substrate to minimize temp variations between the detector diode and the correction diode. It is possible to get a fairly good correction for static voltage drop errors, but errors due to the dynamic inverse impedance of the diode, which is frequency dependent, can't be easily compensated for. Still, it's much better than nothing.

2) Most wattmeters use a simple coupler where samples of line current and voltage across the load are combined to obtain FWD and REF voltage samples. There are some inherent phase errors which are relative to frequency in this simple design, and magnitude errors related to parasitic coupling which limit the directivity of the coupler. Directivity determines the ultimate limit of SWR measurement error. As mentioned by K8ZOA, 30dB is an excellent broadband directivity number, but rarely achieved with high power couplers from 2-54 MHz. The LP-100A feeds the raw current and voltage samples to a gain/phase detector chip for determination of SWR, instead of combining them in the coupler. No diodes are involved, and no FWD or REF voltage samples. The meter has a frequency counter, and any phase or gain errors in the coupler samples can be calibrated out, indexed to frequency. This allows the meter to achieve >40dB directivity from 2-54 MHz.

3) Power circuitry and calculations are completely independent of SWR circuitry and calculations in the LP-100A. As long as there is about 1-2W of power, the gain/phase detector will provide the same result at any power level, with slightly reduced accuracy down to 50mW. The LP-100A uses the differential magnitude and phase samples to calculate the complex reflection coefficient, from which all other impedance and SWR numbers are derived. The meter can display REF power, but it is calculated from FWD power and reflection coefficient, not measured directly.

73,
Larry N8LP

Jack Smith-6 wrote

A wattmeter built around a directional coupler always has to deal with
finite coupler directivity. Making the problem more difficult is that we
expect a wattmeter to be accurate over a rather wide frequency range,
1.8  to 30 or even 50 MHz. This places an even greater burden upon the
directional coupler.

If the directional coupler has 30 dB directivity--a very good number to
be maintained over a wide frequency range--then 1 KW forward power into
a perfect load will show 1 watt reflected power, corresponding to an SWR
of 1.065:1 instead of the expected 1.0000... for the theoretically
perfect load.

It is possible to measure the phase and amplitude of  the coupled signal
to "calibrate out" coupler imperfections. This is what is done with a
vector network analyzer when the standard "open/short/load" calibration
is applied. The VNA measures the phase and amplitude of the coupled
signal when the through port is operated into an open circuit, a short
circuit and a known value (resistance and stray L & C known) termination
for each test frequency. The VNA then computes and applies an
appropriate correction factor to correct for coupler errors. O/S/L
calibration has been supplemented by more advanced techniques in newer
VNAs. (There's a very good Application Note AN 1287-3 from Agilent on
this subject available at
http://cp.literature.agilent.com/litweb/pdf/5965-7709E.pdf. Bird
Electric has a less technical Application Note on the effect of coupler
directivity on SWR at
http://www.bird-technologies.com/techapps/app_notes/StraightTalkAboutDirectivity.pdf)

A wattmeter using diodes to measure RF voltage used with a directional
coupler cannot apply sophisticated error correction to compensate for
finite coupler directivity. At most, one can tweak a balance pot or
trimmer cap to null the reflected signal at a single frequency and power
level. Further complications result from the forward and reverse diode
detectors being operated at different points on their sensitivity curve,
etc.

Hence, it is far from surprising that different wattmeters will show
different SWR under ostensibly identical test conditions.

Larry's LP-100 wattmeter operates with a different methodology and I'll
leave it to him to explain the differences and how coupler directivity
is considered.


Jack K8ZOA
www.cliftonlaboratories.com



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