Connector pin recession and network analyser accuracy

The apparent effect of a recessed connector on network analyser measurements is often talked about, but without any quantitative measurements it is very hard to either predict or correct for the resulting errors.

This article outlines an experiment undertaken to assess the impact on the measurement of reflection coefficient when using 3.5 mm connectors.

The ideal connector would not produce any discontinuities in the transmission line.

In reality, as a result of their dimensional tolerances, there will normally be some small gap between mated connectors.

This gap is often referred to as recession and it may be both or just one of the mated connectors which are recessed. The opposite of recession, protrusion, is likely to cause mechanical damage to 3.5 mm devices unless the mating connector is at least equally recessed and is undesirable.

The electrical effect of recession is to produce a very short section of line having different impedance characteristics due to the difference in diameter of the centre conductor and the exposed part of the connecting male pin.

The theoretical effect of this change in diameter could be calculated but because there are many more effects in operation with such a short length of line, it is simpler to assess practically.

The measurements were performed using an HP8722D vector network analyser over the range 3 to 27 GHz at 1601 trace points and with the bandwidth set to 300 Hz.

Calibration (network analyser error correction) was performed using an HP85052B precision-grade calibration kit.

Three separate calibrations were performed; first with the sliding load set to zero recession, another with it locked at 2.5 thousandths of an inch recession, and finally with 0.005 inch recession.

It was important that the characteristics of the item to be measured remained stable throughout the test, even though the connector's pin recession was to be varied.

The item selected for this task was another 3.5 mm sliding load (HP911D); the type where the connector recession can be adjusted using a small screwdriver.

This enabled the connector recession to be precisely incremented from zero to 0.005 inch in steps of one thousandth of an inch without damaging any of the components involved in the tests.

When the significance of recessed connectors was more generally recognised, it was suggested that if the test port of the network analyser were also recessed and the calibrating and measured devices had equal recession, then a 'good' measurement could still be performed.

While it is no easy task to recess the network analyser's test port connector, it is possible to recess the calibration and measured devices.

The small deviations seen (about 0.0025 VRC) could be accounted for by the unrepeatability of the connectors and possibly because the reference short and open circuits were not recessed by the same amount as the sliding load.

The results are modified by about the same degree but in the opposite direction depending whether the calibrating, or measured, sliding load connector is recessed.

The small differences are possibly due to the open and short not being recessed. In addition, the uncertainty due to use of the connector dial indicator gauge to set the recession has an effect on measured values.

• Recession reduces effective directivity by about 0.005 VRC per 0.001 inch;
• That is equivalent to a typical directivity of 46 dB being reduced to 40 dB with 0.001 inch recession, or to 30 dB with 0.005 inch recession.