High Frequency Performance

Background

Ever-increasing circuit performance challenges all aspects of test technology. Getting a clean and accurate signal from the tester electronics to the board under test is critical for high-speed testing. Fixture wiring can be a major contributor of distortion and noise to the signal transmission path. QA’s wireless X Probes and Double-Ended Sockets address the limitations of fixture wiring by eliminating the wire.

Scope

To better understand the possibilities of wireless fixturing, QA has examined the high frequency performance of wireless X Probes as well as Double-Ended Sockets and Probes. A network analyzer was used to measure the frequency response characteristics of a wide variety of probe configurations. Initial testing of Double-Ended Sockets utilized an RF network analyzer covering the frequency range of 300 KHz to 3 GHz. Subsequent testing using a newer microwave network analyzer covered the frequency range of 50 MHz to 20 GHz. For consistency, graphs of the more recent tests extrapolate data below 50 MHz and omit data above 10 GHz. A TDR oscilloscope was used to look at the impedance of the signal path through the test fixture. Time domain impedance information was also obtained by use of the time domain transform option of the microwave network analyzer.

Procedure

Wireless Test fixtures were constructed for conventional Double-Ended Socket products. These fixtures consisted of a .250 [6.35] G-10 socket mounting plate, a .062 [1.57] G-10 socket spacer plate and two electrical interface boards attached to the socket mounting plate with non-conducting standoffs. Test fixtures for the wireless X Probes were built up from numerous G10 plates totaling 1.562 [39.67] thick. This stack-up was then sandwiched between two electrical interface boards. In all the fixtures, the electrical interface boards provided the SMA connectors for the test equipment and copper traces to contact the various probe/socket configurations. Configurations consisted of different spacings for the ground and signal probes, multiple ground probes and arrangements to measure cross-talk where one pair of probes was “driven” and the “pick up” on an adjacent pair measured.

Results

Diagram A shows the frequency response of two X75 probes on 1.00 [25.4] centers. This might be representative of the signal probe to ground probe separation for an IC package. Note the bandwidth roll off below 100 MHz. This response is dominated by the separation between the signal and ground probe. Plots for the other wireless probe families tested on 1.00 [25.4] centers have very similar performance. In Diagram B, the probes are on their nominal .075 [1.91] centers. On these closer centers, a -1dB frequency response to over 400 MHz is achieved. This improvement results from the more closely-spaced probes providing a better match to the impedance of the 50 Ohm test environment.

The TDR option of the microwave network analyzer allows measurement of the impedance of a transmission line at any point along its length. Diagram C shows the impedance of two wireless .075 [1.91] X Probes on .075 [1.91] centers. In this TDR graph, the transmitted signal has an effective rise time of 50 picoseconds, which equates to a 7 GHz test frequency. The impedance extremes are exaggerated by the high bandwidth of the measurement; at lower frequencies the impedance differences would be less apparent. These high frequency measurements show three distinct physical regions: the termination pin, the transition from the termination pin to the X Probe and the X Probe itself. These changes of impedance are caused by the differing diameters of the termination pins and probes as well as the drilled clearances surrounding them. The nature of the dielectric material separating the probes also plays a critical role in determining the characteristic impedance of the transmission line.

Diagram D shows the performance of a three-probe in-line configuration on .075 [1.91] centers with the signal probe placed between two grounds. Although this configuration may not always be practical, its -1dB performance to greater than 1400 MHz is excellent. Diagram E shows the corresponding TDR plot for the same three-probe configuration.

Crosstalk in a conventional fixture is a complex function of many variables: the characteristics of the test signals, the length and type of wiring used, how the wiring is (or isn’t) dressed, and the relative locations of the probes themselves. Wiring problems are the reason for the existence of wireless probing solutions. Replacing fixture wiring with a translator board provides a more repeatable and controllable environment for routing test signals between the UUT and the test electronics. The test signals and probe locations are driven by the needs of the UUT. For reference purposes, a plot of the crosstalk between two pairs of .075 [1.91] wireless X Probes on .075 [1.91] centers appears in Diagram F.

Conclusions

A wireless probing solution is capable of delivering excellent high frequency performance. Signal-to ground probe spacing and the dielectric material separating the probes both play a major role in determining the impedance and the bandwidth of the transmission path. In general, a more constant probe diameter and consistent dielectric material separating the probes makes for fewer impedance changes in the signal path and better overall high frequency performance. Replacing fixture wiring with a translator board allows the test engineer greater control of length and impedance characteristics of the signal path to the unit under test. This results in cleaner, distortion-free test signals and higher performance testing.

X75-25 Series Wireless (1) frequency response
X75-25 Series Wireless (.075) frequency response
X75-25 Series Wireless (075) TDR
X75-25 Series Wireless (075x3) frequency response
X75-25 Series Wireless (075x3) TDR
X75-25 Series Wireless (.075) Cross Talk

.............. Two Probes on 1.00" Centers (Signal-Ground) Two Probes on Nominal Centers (Signal-Ground) Three Probes on Nominal Centers (Ground-Singal-Ground)
Wireless Probe Series -1dB (MHz)
-3dB (MHz)
-1dB (MHz)
-3dB (MHz)
-1dB (MHz)
-3dB (MHz)
039-25 100 197 481 3850 1550 2145
050-16 81 119 336 2930 2500 3000
050-25 92 185 585 3050 2250 3700
075-25 94 140 222 306 771 2420
100-25 84 125 321 352 771 2320
X31-25 85 165 473 4085 1450 5710
X39-25 87 170 540 4950 740 7550
X50-25 90 178 530 4750 1800 5450
X75-25 92 182 435 630 1450 5600