Coax connectors make low-cost test pieces

Richard M Kurzrok, RMK Consultants, Queens Village, NY -- EDN, 6/22/2000

You can readily construct low-cost test pieces using coaxial panel jacks without pc boards or enclosures (Figure 1). Some engineers and technicians occasionally use this construction technique, but the versatility of the technique is not well-known. You can design and construct these low-cost test pieces for a variety of passive circuits using tee, pi, el, or bridged-tee networks. Other simple circuits are also amenable to this type of construction. These circuits include highpass filters, minimum-loss pads, transformers, amplitude equalizers, and feedthrough terminations. The technique applies to all commercial, industrial, and educational breadboard units that require a quick-and-dirty implementation. The test pieces are useful for low-cost laboratory experiments on passive circuits at reasonably high frequencies. The quasi-open "enclosure" for the test pieces provides good visibility of components and solder joints.

The 50W BNC is a popular coaxial panel connector, and significant cost differences exist between military and commercial versions. Some panel jacks come with number 3-48 tapped holes. You can drill these holes out for number 4-40 clearance. This application accommodates minor differences in panel-jack hole spacing.

Using a single panel jack with a solder lug, you can construct a one-port circuit, such as a termination or standard mismatch. You can fabricate a two-port circuit using two panel jacks mechanically secured to each other with four 4-40 stainless-steel machine screws and ancillary hardware, such as hex nuts, lock washers, and solder lugs. The four screws provide electrical-ground continuity between the two panel jacks. You create simple passive circuits by soldering components, such as resistors, inductors, and capacitors, to the panel jacks' center conductors and the ground lugs. You assemble three screws to the panel jacks before soldering and add the fourth screw after soldering.

The test piece in Figure 1 is a pi-section fixed attenuator. This 6-dB, 50W attenuator uses ¼W composition and carbon-film resistors with 5% tolerances. The nominal value of series resistor R₂ is 36W, and the shunt resistors, R₁ and R₃, are nominally 150W. From 2 to 150 MHz, the measured attenuation is 6±0.2 dB. At 50 MHz, the measured attenuation is 6.4 dB.

You can also create a dc block using this construction technique by placing a series capacitor between the panel-jack center conductors. The 0.1-µF CK05 capacitor has a tolerance of 10%. From 300 kHz to 30 MHz at a 50W impedance, the measured insertion loss is less than or equal to 0.1 dB. At 50 MHz, the measured insertion loss is 0.2 dB.

You can also achieve a pi-section lowpass filter with a 9.6-MHz cutoff frequency at a 50W impedance level using a series inductor and two shunt capacitors. You can characterize the filter as a single constant-K section using image parameters or as a three-pole Butterworth unit using modern network theory. Shunt input and output capacitors are 330-pF polypropylene units with 5% tolerances. A 1.66-µH series inductor comprises 18 turns of number 26 magnet wire wound on a Micro Metals T37-2 toroid. The estimate of the inductor's unloaded Q exceeds 100. Table 1 shows the measured test data.

When the frequency exceeds 50 MHz, alternative design techniques are more appropriate (Reference 1). Similar designs are achievable using other panel jacks, such as threaded-N-connector and N types. The use of nut plates can extend the technique to three- and four-port circuits. When radiation is a problem, wrapping copper-foil adhesive tape around the four machine screws provides partial shielding. (DI #2548)

REFERENCE

1. Kurzrok, RM, "Low-cost prototyping for VHF Circuits," Electronic Design, Oct 18, 1999, pg 91.