Make simple fixtures from coax cable

A fixed-geometry fixture produces consistent outcomes. As an example , you'll make fixed-geometry fixtures out of certain connectors. PCB-mounted coax cable are convenient for mounting typical bypass condenser sizes. These connectors have two distinct sides: the SMA connector side interfacing with the coaxial cable assemblies on your cables and therefore the PCB side that attaches to the PCB. The SMA male connector side comes with the selection of normal male or female, but there also are reverse-polarity SMA male connector and feminine connectors, so you would like to concentrate once you buy these parts. Common cables accompany regular male coaxial cable assemblies. Thus, you'll use regular female coax cable for your fixture. The PCB side also features a large sort of different geometries, hooked in to how you would like to mount the connector to the PCB. If the mounting is perpendicular to the board, then soldering the four posts at the corners to the board either as surface-mount or through-hole coax cable. If you would like to use the connector as edge-mount, the spacing between the posts let the connector straddle the board with its specified thickness.

You can build a fixture to live bypass capacitors using rg402, which are the last two connectors on the proper in Fig. 1. You must, though, consider the capacitor's base size. The narrow-base connectors in Fig. 1 have posts on a 5 mm grid and these connectors are best fitted to smaller-size components, like 0805, 0603 and 0402 capacitors. you'll also attach 1206 and 1210 size components to those fixtures. The wide-base edge-mount connector has posts on an 8 mm horizontal grid. You would like them for bigger components, like D-size (7.3 mm×4.3 mm) packages coax cable.

To create a fixture, solder two of those connectors back-to-back. you'll connect both ends to SMA male connectors and you attach the bypass condenser you would like to live to the center pin and ground frame the middle of the fixture.

Before making measurements, you want to calibrate the system. Up to about 30 MHz, an easy Response-Through calibration is typically sufficient coax cable. For the response-through calibration, you'll simply connect an empty fixture (no capacitor) between your vector network analyzer (VNA test cable)'s Port 1 and Port 2. After the calibration, you'll solder the DUT between the middle pin and ground coaxial cable assemblies frame of the fixture. you'll also create multiple fixtures with identical geometry, keeping one only for response through calibration and reserving the others for measuring DUTs. You measure the S21 transfer parameter with the network analyzer and from that you simply can calculate the coaxial cable assemblies complex impedance of the capacitor with rg402 this easy formula:

Where rg405 is that the port impedance of the VNA test cable, usually 50 Ω. The coax cable network analyzer used for these measurements has an option available that does this transformation inside the instrument

When you select the coaxial cable assemblies measurement's frequency range for the measurement, you face a the trade-off between the beginning and stop frequencies and therefore the nature of components you would like to live . If you would like to start out the sweep anywhere below 30 kHz and need to live components with low impedance at low frequencies, like low-ESR high-capacitance parts, you'll run up against the cable-braid loop error, described in Section 7.1.1 of [3]. Counting on how you reduce the rg405 cable-braid loop error, the chosen solution may accompany its own limitations at high frequencies. For this text , I used a home-made common-mode choke to scale back the effect of the cable braid resistance, with an upper bandwidth of roughly 50 MHz. Data was collected from 300 Hz to 30 MHz.

While these coaxial cable assemblies fixtures are still very simple to form and that they provide fixed and repeatable geometry, they lack a planar structure with power and ground planes. Coax cable do not represent our typical PCB applications. For this reason, you almost certainly got to ignore the inductance within the measured data. For applications where you're looking just for the capacitance information and possibly also for the equivalent series resistance (ESR) data, this easy fixture is a suitable solution. If you actually need the inductance to also represent that of your application, the simplest you'll do is to make fixtures with an equivalent or similar stack-up because the final application and connect the DUT with the coax cable escape patterns that you simply plan on using on your board. These fixtures are tailored to our PCB geometry and thus have more limited applications, but will best represent the DUT’s performance in real application over a good frequency range. For such coaxial cable assemblies, see Figure 7.13 on page 206 of Reference 3.