Anritsu HFE0503_Kurzrok 电路图.pdf

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1、60High Frequency Electronics High Frequency Design TEST ACCESSORIES Simple Lab-Built Test Accessories for RF, IF, Baseband and Audio By Richard Kurzrok RMK Consultants E very engineering lab bench needs a supply of handy test accessoriescables, adapters,attenuators, couplers, combiners, fil- ters, d

2、etectors, 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 th

3、rough 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 certa

4、inly 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 assem- bly, using four hole coaxial panel jacks without printed circuit boards or enclosures.This tech- nique has been demonstrated f

5、or passive cir- cuits using BNC connectors 1, and UHF con- nectors 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 se

6、ction area of BNC and TNC connectors is 0.500 0.125 = 0.375 inch. The correspond- ing working cross section of N and UHF con- nectors is 0.718 0.138 = 0.580 inch.The addi- tional area can permit construction of test pieces with larger components such as half inch diameter toroidal inductors. Typical

7、 small quantity unit costs for com- mercial grade coaxial connectors from eco- nomical 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 engine

8、ering workbench: filters, detectors, attenua- tors and return loss bridges for audio and RF/IF ConnectorFlange SizeHole C/LHole Dia.Peak VoltsMax. Freq. BNC.687 or .750.500 or .531.1255004 GHz TNC.687.500.12550011 GHz N1.000.718.1251,50011 GHz UHF1.000.718.125 or .138500300 MHz Notes:1. BNC, TNC, an

9、d 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

10、 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 Copyright 2003 Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 62High Frequency Electronics High Frequency Design TEST AC

11、CESSORIES jacks, depending on quality and man- ufacturer. Quantity discounts have not been considered. Since UHF con- nectors 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 pric

12、es will be paid for military grade con- nectors, 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

13、 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 evalu- ate these connectors, a preliminary 50 ohm test piece was

14、constructed using two UHF panel jacks intercon- nected 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 asso- ciated hardware, is illustrated in F

15、igure 2. This simple circuit is dis- played mainly for illustrative purpos- esresistive pads are readily avail- able and inexpensive, but custom attenuators with non-standard val- ues can be made in this manner. Another test piece with two UHF jacks separated by two inch long #6- 32 screws was const

16、ructed and test- ed.This unit is a five pole low pass fil- ter 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 turn

17、s 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 fre- quencies from 100 kHz to 100 MHz can be readily constructed using UH

18、F panel jacks and other connec- tors,soldered components,hand FrequencyInsertion Loss (MHz)(dB) 0.30.1 0.740 20.040 Table 2 Measured amplitude response for the 2.2 MHz lowpass fil- ter mounted on UHF connectors. Figure 1 Back-to-back mounting of low-cost coaxial connectors allows easy construction o

19、f 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. R1R3 R2 L2 6.02 H C1 1,500 pF C4 1,500 pF C3 1,500 pF C6 1,500 pF L5 6.02 H 1-3/16 inch typical with 1-1/2 in. screws Solder lugs#4-4

20、0 or 6-32 screws, lockwashers, nuts RadioFans.CN 收音机爱 好者资料库 May 200363 tools, machine screws, ground lugs and other ordinary hardware. In addition to the example attenu- ator and lowpass filter, some other simple circuits include: RF detectorA Schottky diode detector can readily be constructed using

21、 this technique.The detector can be series or shunt, with or without a termination, as the user desires. AM broadcast highpass filter Communications facilities are often located near broadcast transmitters, and field measurements can be com- promised by their strong signals. A highpass filter with a

22、 cutoff of about 2 MHz will reduce AM band (Medium Wave) interference. FM or television band notch fil- tersThe FM radio band can be reduced with a band reject filter, while interference from either FM or TV transmitters can be reduced with notch filters tuned to the frequency of the offending stati

23、on. These low cost test pieces are applicable to low frequency commer- cial, industrial and educational bread- boards when “quick and dirty” imple- mentation is sufficient to solve a mea- surement problem.They are most use- ful for passive circuits with non-stan- dard specifications. Of course, thes

24、e lab-built test pieces are not meant to compete with standard manufactured products, which provide a much greater degree of precision and frequency coverage through microwave frequencies. Low Cost Return Loss Bridges Below 100 MHz, lumped-circuit passive return loss bridges can be implemented readi

25、ly using commer- cially available transformers and die cast aluminum boxes. Return loss is a more sensitive indicator of circuit performance than transmission behavior, such as inser- tion loss (used with passive circuits) or voltage transfer (used with active circuits). Return loss bridges are wide

26、ly used from audio frequencies well into the microwave region. Below 100 MHz, reasonable quality return loss bridges can be realized using commercial transformers and uncomplicated construction methods. Commercial grade coaxial connec- tors should conform to the impedance of the return loss bridge a

27、nd the desired level of accuracy. At audio frequencies, fifty ohm BNC connec- tors can be used with seventy-five and six hundred ohm bridges. Bridge Circuit The schematic of a typical return loss bridge, based on the classic Wheatstone Bridge circuit, is shown in Figure 4. The four port bridge cir-

28、cuit uses resistors R1 and R2 to con- nect the unbalanced bridge input port to the two test ports. The four port bridge maintains circuit symme- try. (Note: Some commercially avail- able bridges are three port units with an internal auxiliary termination.) The auxiliary test port is connected to an

29、external matched termination. The other test port is connected to the input or output of the unit under test (UUT).The transformer connects the test ports to a detector at the out- put port, where the magnitude of the imbalance is proportional to the return loss. Internal bridge resistors R1 and R2

30、are typically equal to the bridge impedance Z0. At audio frequencies, three return loss bridge engineering models were constructed. Audio bridge compo- nents are listed in Table 3. At lower RF and intermediate fre- quencies, two return loss engineering models were constructed. Compo- nents used in t

31、hese RF/IF bridge models are listed in Table 4. In both audio and IF units, the internal resistors R1 and R2 were nominal half-watt metal film units with one percent tolerances. Audio return loss bridges were constructed Bridge Z0Components Used 49.9 ohmsBNC connectors T1: Mouser Electron- ics P/N 4

32、2TM030 75.0 ohmsType F connectors T1: Mouser Electron- ics P/N 42TM030 604 ohmsBNC connectors T1: Mouser Electron- ics P/N 42TM016 Bridge Z0Components Used 49.9 ohmsBNC connectors T1: Mini-Circuits model T1-1 75.0 ohmsType F connectors T1: Mini-Circuits model T1-1 Table 3 Audio return loss bridge co

33、mponents. Table 4 RF/IF return loss bridge components. Figure 4 Return loss bridge schematic. UUT Test Port T1 R1 R2 Output Port (Detector) Input Port (Signal Source) Aux. Test Port (Termination) RadioFans.CN 收音机爱 好者资料库 64High Frequency Electronics High Frequency Design TEST ACCESSORIES in Hammond 1

34、590LB (1.99 1.99 1.06 inch w/o lid) die cast aluminum enclosures. The RF/IF return loss bridges were constructed in Hammond 1590H (2.07 1.50 1.06 inch w/o lid) die cast aluminum enclo- sures. Transformers were turned upside down and affixed to the bot- tom interiors of the enclosures. Point- to-poin

35、t wiring was used throughout with leaded components and no printed circuit boards. For the audio bridges, the trans- formers had small quantity unit prices less than two dollars, while the transformers used in the RF/IF bridges had prices less than four dol- lars each. The enclosures were priced at

36、less than seven dollars each. Bridge Performance Performance of the audio return loss bridge is tabulated in Table 5 where L is insertion loss from gener- ator port to output port and D is directivity in dB. The IF return loss bridge perfor- mance is tabulated in Table 6. Again, L is insertion loss

37、from generator port to output port in dB and D is directivity in dB. The transformer T1 is the crucial component in the return loss bridges. This conventional transformer acts as a wideband filter, providing the best return loss bridge performance in the central region of the “filter” passband, wher

38、e loss is lowest and transformation accuracy is best. Summary At low frequencies, below 100 MHz, low cost test accessories can be constructed from readily-available components, using simple point-to- point assembly methods. Coaxial test pieces can be built directly on back-to- back connectors, with

39、short lead lengths that provide acceptable per- formance for many test requirements. Audio and IF return bridges can be constructed readily as engineering models. Through-lead components and point to point wiring, without printed circuit boards, yields useful performance. Above 100 MHz, design consi

40、dera- tions require much greater sophisti- cation. Expensive test equipment can become necessary. Parasitic circuit elements can entail use of surface mount components with critical attention to layout and mechanical design. At these higher frequencies, usable coaxial components and return loss brid

41、ges are available from commercial vendors. References 1.R.M.Kurzrok,“Coax Connectors Make Low-Cost Test Pieces,” EDN, pp. 168, 170, June 22, 2000. 2. J. Sevick, Transmission Line Transformers, 2nd ed., American Radio Relay League, Newington, CT, 1990; current (4th) edition: Noble Publishing Corp., N

42、orcross, GA, 2001. Acknowledgement The author would like to acknowl- edge the invaluable assistance and technical contributions of Gary Breed. About the Author Richard M. Kurzrok, MEE, is an independent consultant specializing in filters, equalizers, and other pas- sive circuits. He can be reached a

43、t RMK Consultants via e-mail at: rmk- Freq.50 ohm RL Bridge 75 ohm RL Bridge 600 ohm RL Bridge (kHz)L (dB)D (dB)L (dB)D (dB)L (dB)D (dB) 0.05 1640174016.840 0.114.6 4015.24015.240 0.2144014.44014.340 0.513.74013.84013.740 113.54013.54013.440 513.54013.44013.340 1013.54013.44013.040 1513.64013.54012.

44、939.8 20144013.54012.937.5 3014.54013.94012.833.9 50164014.740 10020401735.5 Freq.50 ohm RL Bridge 75 ohm RL Bridge (MHz)L (dB)D (dB)L (dB)D (dB) 0.115.540 0.513.14014.4 40 113.2401440 313.240 513.24013.240 1013.24013.340 2013.24013.040 4013.23813.235 5013.236 8013.83313.328 10013.83013.330 1502613.224.4 20015.7 2313.020.6 Table 5 Audio return loss bridge insertion loss (L) and directivity (D) per- formance. Table 6 RF/IF return loss bridge performance.