儀表信號基礎(Part-1)<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

1.      射頻信號強度的單位意義:  常用的單位如: uV, mV, dB, dBuV, dBm, dBuV/M等等.

‘uV 及 ‘mV為基本信號電壓強度單位 1 V="1000mV"     1 mV="1000uV"

dB 為倍數值的概念. 0 dB="1"倍,  1 dB="1".1倍,  2 dB="1".25倍,  3 dB="1".4倍 

6 dB="2"倍,  10 dB="3"倍,  20 dB="10"倍,  30 dB="30"倍,  40 dB="100"倍.

例如: 36 dBuV= (20+10+6) dBuV= (10*3*2) uV= 60 uV

     86 dBuV= (20+20+20+20+6) dBuV= (10*10*10*10*2) uV= 20 mV

dBm 其意義為 dBmW.  0 dBm 即為相當於1 mW功率時之信號電壓強度.

     在射頻信號儀表上: V*V/50 ohm= 1 mW  因此0 dBm= 0.22 V= 107 dBuV

     例如: 60 dBuV= -47 dBm= 1 mV

在音頻信號儀表上: V*V/600 ohm= 1 mW  因此0 dBm= 0.775 V

     例如: -8 dBV= -6 dBm= 380 mV

dBuV/M其意義為某一特定距離所能接收信號源之射頻信號強度.舉例如下, FCC規定30-88 MHz 3Meters 距離之輻射強度需低於 40dBuV/M.

2.        集中信號元件概念: 常用者為Splitter/Tap/Booster/ Attenuator /Cable/Connector等信  號元件.

信號分配器 Splitter: 4 Ways. –7 dB out.  信號分歧器 Tap: 2 Taps.  –20 dB out.

信號放大器 Booster: With gain-adjust and channel balance.

信號衰減器 Attenuator:   Programmable Type           Fixed type

unbalanced Tee type         

unbalanced Pi type

K = power in / power out, or in decibels = 10 * log (K) dB

RF Coaxial Cable: 常用種類有 3C2V/5C2V/7C2V, RG-58U/59U/62U等.

                阻抗特性: RG-58 50 Ohm, RG-59/RG-6 75 Ohm, RG-62 93 Ohm.

RG-59U規格表:

RF Connector: 常用種類有F-3C/F-5C/F-7C, BNC, RCA, UHF(M type)/ N等.

    F type      BNC type     RCA type     M type        N type

儀表信號基礎(Part-2)

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2. 集中信號儀表:

A.     掃描信號產生器Sweep Generator: 全頻帶調整及檢查用.

此儀器產生三組信號: RF Sweep, Marker, Saw-tooth sweep.

RF Sweep: 全頻帶無調變之射頻信號. 一般連接待測機器之測試輸入端.

Marker: 顯示RF sweep射頻信號之特定頻率點之位置. 一般連接示波器之亮度調變輸入端 (Intensity), 或是與待測機器之輸出混合後, 連接示波器之垂直輸入端 (Pulse).

Saw-tooth sweep: 低頻率之鋸齒波信號.決定頻帶寬度及同步鎖定.一般連接示波器之外部水平輸入端.(頻率一般為25或30Hz)

B.     單點信號產生器Spot RF Generator: 與一般標準信號產生器相同特性.

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3. 相關注意事項:

A.     欲將多組信號混合輸送時可利用分配器裝置. 信號由分配器原有輸出端接入,然後由分配器原有輸入端將信號送出.

B.     一般分配器為射頻用途, 並不適用於Marker 及Saw-tooth sweep之低頻信號.可直接輸出並接使用或以電阻分配器取代.

C.     分配器輸出端即使在未連接使用時, 亦需裝上匹配用之負載電阻.

D.     多組射頻信號混合時需注意諧波及互調變干擾產生. 一般射頻信號至少會產生高於 -40dB之二次諧波.

BRIDGED T PAD ATTENUATOR DESIGN

The "BRIDGE T" attenuator is a very useful tool to use when ever you need to reduce the level of a signal without changing the impedance of the circuit.

First note that two of the four resistors are always equal to the input and output impedance. In the case of an attenuator for 75 Ohm cables, R3 and R4 are 75 Ohms! This is also the Characteristic Impedance of the circuit.

Select the desired attenuation voltage ratio. Subtract one (1) from the voltage ratio. That becomes the Factor to derive the resistance value of R1 and R2.

FORMULA FOR VOLTAGE RATIO VR1: VOLTAGE RATIO = ANTILOG DB/20

The value for R1 is VR1 divided into R3 the Characteristic Impedance value. The value for R2 is VR1 multiplied by R3 the Characteristic Impedance value. In practice, the closest 1% value is sufficiently accurate for all the resistors.

75 OHM IMPEDANCE ATTENUATORS    FORMULA:  (R3) / VR-1  VR-1 X R3     dB             VOLTAGE

REDUCTION             RATIO          VR-1        R1(Ohms)     R2(Ohms)

   1                    1.122            .122                  614.75      9.15

   2                    1.259            .259                  289.57      19.42

   3                    1.413            .413                  181.60      30.97

   4                    1.585            .585                  128.20      43.87

   5                    1.778            .778                   96.40      58.35

   6                    1.995          .995                   37.59      74.62

   7                    2.239        1.239            60.53       92.92

   8                    2.512        1.512        49.60      113.40

   9                    2.818          1.818            41.25      136.35

  10                    3.162         2.162           34.69      162.15

  20                10.000        9.000             8.33      675.00

  30                31.623       30.623             2.45     2296.72

  40               100.000       99.000             0.76     7425.00