Anritsu HFE0503 Kurzrok

60 High Frequency Electronics

High Frequency Design

TEST ACCESSORIES

Simple Lab-Built Test Accessories for RF, IF, Baseband and Audio

By Richard Kurzrok RMK Consultants

very engineering lab bench needs a

supply of handy test accessories—cables, adapters, attenuators, couplers, combiners, filters, detectors, limiters, etc. This article describes several accessories that

can be built quickly (and cheaply), yet they provide sufficient performance from DC to 100 MHz. This frequency range includes most audio, baseband and intermediate frequency circuits, as well as many RF applications from VLF through VHF.

Connector-Mounted Accessories

Low cost tests pieces can be realized using BNC panel jacks and associated hardware, although improvements can be attained using UHF connectors and panel jacks, as well as the higher quality N and TNC connectors. SMA and similar microwave connectors can certainly be used, but most hand-built circuits will not require their microwave performance.

Low cost test pieces can be constructed readily, via machine screw and solder assembly, using four hole coaxial panel jacks without printed circuit boards or enclosures. This technique has been demonstrated for passive circuits using BNC connectors [1], and UHF connectors have been extensively used with low cost transmission line transformers [2].

Connector Performance

Some coaxial panel jack characteristics are shown in Table 1. With BNC hole spacing of 0.500 inch and #4-40 screws, the working cross section area of BNC and TNC connectors is 0.500 – 0.125 = 0.375 inch. The corresponding working cross section of N and UHF connectors is 0.718 – 0.138 = 0.580 inch. The additional area can permit construction of test pieces with larger components such as half inch diameter toroidal inductors.

Typical small quantity unit costs for commercial grade coaxial connectors from economical distributors are $1.25 for BNC panel jacks, $2.50 for TNC panel jacks, $2.50 for N panel jacks and $0.65 to $1.00 for UHF panel

Here are some practical tips for making simple test accessories to use at your

engineering workbench:

filters, detectors, attenua-

tors and return loss bridges

for audio and RF/IF

Connector Flange Size Hole C/L Hole Dia. Peak Volts Max. Freq.

BNC .687 or .750 .500 or .531 .125 500 4 GHz TNC .687 .500 .125 500 11 GHz

N 1.000 .718 .125 1,500 11 GHz

UHF 1.000 .718 .125 or .138 500 300 MHz

Notes: 1. BNC, TNC, and N connectors have 50 ohm impedance. UHF impedance is not specified.

2. Flange is square with rounded corners. Dimensions are in inches.

3. 0.125 holes provide clearance for #4-40 screws; 0.138 holes are for #6-32.

4. Some BNC and TNC connectors are tapped for #3-48 instead of 0.125 diameter. These holes must be drilled if larger hardware is desired.

Table 1 · Mechanical and electrical characteristics of common coaxial connectors.

From May 2003 High Frequency Electronics

62 High Frequency Electronics

High Frequency Design

TEST ACCESSORIES

jacks, depending on quality and manufacturer. Quantity discounts have not been considered. Since UHF connectors have the lowest unit cost and are readily available, this article will emphasize low cost test pieces using UHF connectors. These costs will change with time, and higher prices will be paid for military grade connectors, different quality commercial connectors, and for retail purchase.

UHF Panel Jack Performance

Figure 1 is a photo that shows two UHF connectors in a mounting that allows various circuits to be con-

structed with components supported by their leads. Also in this photo is an assembly of two BNC jacks mounted to a U-shaped bracket, which allows similar circuit assembly using these connectors.

The upper frequency limit for UHF connectors is 300 MHz and the impedance is not specified. To evaluate these connectors, a preliminary 50 ohm test piece was constructed using two UHF panel jacks interconnected back-to-back with no special concern for impedance matching. Insertion losses were less than 0.1 dB up to 90 MHz.

Practical Test Circuits

A pi-section fixed pad attenuator, using two UHF panel jacks and associated hardware, is illustrated in Figure 2. This simple circuit is displayed mainly for illustrative purposes—resistive pads are readily available and inexpensive, but custom attenuators with non-standard values can be made in this manner.

Another test piece with two UHF jacks separated by two inch long #632 screws was constructed and tested. This unit is a five pole low pass filter with nominal 0.02 dB passband ripples, three dB cutoff of 2.2 MHz, and 50 ohm impedance. The filter schematic is shown in Figure 3. All shunt capacitors are polypropylene units with two percent tolerance. The series inductors use 34 turns of #26 magnet wire on MicroMetals T50-2 toroids, which have a nominal 0.5 inch outside diameter. Measured test data is shown in Table 2.

Other Useful Circuits

As we have seen, low cost 50 ohm test pieces operating at typical frequencies from 100 kHz to 100 MHz can be readily constructed using UHF panel jacks and other connectors, soldered components, hand

Frequency Insertion Loss

(MHz) (dB)

0.3 <0.1

0.7 <0.1

1.0 0.1

1.3 0.2

1.6 0.3

1.8 0.4

2.0 1.1

2.2 3.5

2.4 7.2

2.9 18.0

3.5 27.0

4.0 34.3

10.0 >40

20.0 >40

Table 2 · Measured amplitude response for the 2.2 MHz lowpass filter mounted on UHF connectors.

Figure 1 · Back-to-back mounting of low-cost coaxial connectors allows easy construction of test circuits.

Figure 2 · Assembly diagram of a simple UHF connector-based pi-section attenuator (not to scale).

Figure 3 · Circuit diagram of a 2.2 MHz lowpass filter.

L2 – 6.02 µH

C1 1,500 pF

C4 1,500 pF

1,500 pF

L5 – 6.02 µH

1-3/16 inch typical with 1-1/2 in. screws

Solder lugs

#4-40 or 6-32 screws, lockwashers, nuts

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