一個瑞典人的網站, 2001年後就沒有更新, 有些很特別的資訊, 例如無線電或收音機裡面的中頻變壓器的歷史資料和使用方法. 雖然是舊有的資訊, 不過現在還是適用, 除非玩 SDR, 最近翻看, 結果它的網站搬家了, 舊的資訊找不到, 唯有試試看 WEBARCHIVE, 找回來這頁, 存起來看看, 除了錯別字以外, 所有資訊及版權歸原網頁或作者所有 (SM0VPO)
最重要的是它給出了中週為什麼要使用中心抽頭的原因和原理. 頻寬是圈數比的平方反比例, 意思就是, 窄頻和高選擇性, 只通過或接收 “單一的頻率"
Declaration : all those information copied here (blow) is for reference only, the original author (it is believed SM0VPO, the call sign and own those material and perhaps abandon a website only but not to neglect the copyright of those information). Those information presented here is slightly edited with errata and illustration re-rearrangement.
IF-CAN (Intermediate Frequency CAN)
This side presents some facts about IF-CAN (IF-transformer).
All contribution to this page are most welcome
These transformers are specially designed tuned circuit in RFI-tight groundable metal packages for narrow bandwidth IF application. They are called IF cans. As shown in the 1968 specification sheet of figure at right,
this unit includes a 125-pF capacitor, and the arrow between primary and secondary. An arrow indicates that the tuning is attained by tuning-tool (a non-metallic screwdriver) adjustment of the ferrite core (slug). The purpose of the primary winding tap is to increase the effective Quality (Qeff) of the collector circuit in the narrow band IF of the standard broadcast receiver.
Each IF transformers has self resonances with an impedance max at predefined frequency. The resonances frequency can be adjusted by turning the colored ferrite core. In an ordinary radio, you will most often find 4 types of IF-cans.
For the FM part the IF frequency is 10.7MHz. The color of the slug in this CAN is most often pink.
For the AM part the IF frequency is 455kHz.
RED – Oscillator. With 30pf – 300pf = 1MHz to 2MHz
YELLOW – First 455KHz IF filter transformer
White – Second 455KHz IF filter transformer (not always used)
Black – Last 455KHz IF filter transformer
How to connect the IF transformer?
The IF transformer uses a tuned primary winding of typically 110 – 160 turns of wire with a 180 pf – 200 pF fitted across the coil. This winding is usually tapped at about 20 – 25% and connected to a centre pin. Unless you have any data on the coil then it is debatable from which end of the coil the tapping is made.
The diagram above shows the impedance as function of frequency. The phase angle is also plotted. The ferrite core (slug) is yellow 455kHz. As you can see from the diagram the Impedance has a maximum at the resonance frequency. At the resonant frequency the phase is zero and the impedance is pure resistive.
Primary winding tap and Q-factor
The schematic above show the IF-transformer. RT is the resistance in the amplifier stage.
For instance, suppose the tap is not used. The equivalent circuit is (figure at left), of course,
Qeff = RT/XL and the bandwidth BW= fo/Qeff.
IF the power supply line (ac ground) is connected to tap point, the resulting equivalent circuit is that of figure right.
Here, L1 + L2 = L, so the circuit is resonant at the same frequency. However, since L is proportional to N^2, where N is the number of turns for the inductor
XL2 = n^2 XL where n is the turns ration defined by the tap point n = n1/(n1+n2).
Ignoring finite inductor Q, the effective tapped circuit Q is QT = RT / XL= RT / (n^2 XL) = Qeff / n^2.
Since n<1 ,QT> Qeff of the untapped transformer.
Determine the Q of the two circuits. The tap point is 1/3 of the inductor turns from the bottom.
Qeff = 2500 / 500 = 5
XL = n^2 * XL2 = (1/3)^2 * 500 = 55.5 ohm.
QT=2500 / 55.5 = 45
The Q has been increased by 1 / n^2 = 9 times.
The bandwidth is 1/9 of the untapped value.
RULE: By tapping the transformer the Q-value increase and the bandwidth decrease.
What is inside the CAN?
The two pictures below explain the inside of the CAN.
Declaration : all those information copied here (above) is for reference only, the original author (it is believed SM0VPO, the call sign and own those material and perhaps abandon a website only but not to neglect the copyright of those information). Those information presented here is slightly edited with errata and illustration re-rearrangement.