IF Transformer. V2 is 2Vp-p sinusoidal

Transcription

1 V2 is 2Vp-p sinusoidal Purpose and Function These transformers are specially designed tuned circuit in RFI-tight groundable metal packages for narrow bandwith IF application. 1 Theory and Design C30 and L5 represent internal parts of an IF can (yellow) 2 o Note: The capacitor is often omitted from schematics C30 = 180 pf is a typical value 455 Khz is the am intermediate frequency 1 3 o Fr 2 LC L5 = 1/(2*pi)^2 * Fr^2 * C1 L5 = 1/(2*pi)^2 * 455 Khz^2 * 180pf L5 = 680 uh The center tap is used to increase the Q without changing the resonate frequency. 1 William R. Robinson Jr. p1of 9

2 Simulation The model does not allow for changing the location of the tap (center only) o Fixed at 1:1 for a center tap so q is not enhanced as below The model does not do well in the frequency domain o See references 4,5,6 Rm is just a matching resistor o If Vout = ½ Vin at resonance then the reactance of L5 = Rm Rl is the load resistance Rl Rm infinite 500K 500K 200K 50K 27K 5K 3K The Frequency response is shown below vm(vout) vm(vin) vm(vnstage) IF_transformer-Small Signal AC-0-Graph m m m m k k k Frequency k k k k k k William R. Robinson Jr. p2of 9

3 The simulation below shows the lower q when the same L and C are used but not using the transformers center tap to increase the Q. William R. Robinson Jr. p3of 9

4 Real Circuit With the yellow can adjusted for 455 Khz the half power points were o 448 Khz o 463 Khz William R. Robinson Jr. p4of 9

5 Comparison The comparison is reasonable for calculated and measured Simulation however leaves a lot to be desired Simulation does not do well with either the center frequency or the band width See references 4,5,6 Measured Simulated Calculated Book Resonate Adjustable Frequency Khz Band Width Khz ( )= 15 ( ) = 84 NA Q = Fr/BW NA 80+/- 20% The wider Simulated Bandwidth (lower q) is to be expected as the simulation turns ratio is much lower with the fixed center tap William R. Robinson Jr. p5of 9

6 References 1. UNKNOWN, IF-CAN, online, accessed IF-CAN (Intermediate Frequency CAN) This side presents some facts about IF-CAN (IF-transformer). All contribution to this page are most welcome Background These transformers are specially designed tuned circuit in RFI-tight groundable metal packages for narrow bandwith 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 indicates that the tuning is attained by tuning-tool (a non methallic screwdriver) adjustment of the ferrite core (slug). The purpose of the primary winding tap is to increase the effective Q of the collector circuit in the narrowband IF of the standard broadcast receiver. Each IF transformer has self resonance with an impedance max at predefined frequency. The resonant 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. o o o o 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 use a tuned primary winding of typically turns of wire with a 180pf - 200pF fitted across the coil. This winding us 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. Impedance The diagram at the right 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 0 and the impedance is pure resistive. Primary winding tap and Q-factor William R. Robinson Jr. p6of 9

7 The schematic above show the IF-transformer. R T is the resistance in the amplifier stage. For instance, suppose the tap is not used. The equivalent circuit is (figure at left), of course, Q eff = R T /X L and the bandwith BW=f o /Q eff. IF the power supply line (ac ground) is connected to tap point, the resulting equivalent circuit is that of figure right. Here, L1 + L 2 = L, so the circuit is resonant at the same frequency. However, since L ~ N 2, where N is the number of turns for the inductor X L2 =n 2 X L where n is the turns ration defined by the tap point n = n 1 /(n 1 +n 2 ). Ignoring finite inductor Q, the effective tapped circuit Q is Q T = R T / X L = R T / (n 2 X L ) = Q eff / n 2. Since n<1,q T > Q eff of the untapped transformer. EXAMPLE R T =2500ohm X L =500 ohm Determine the Q of the two circuits. The tap point is 1/3 of the inductor turns from the bottom. Solution Q eff =2500 / 500 = 5 X L = n 2 X L2 = (1/3) 2 *500 = 55.5 ohm. Q T =2500 / 55.5 = 45. The Q has been increased by 1 / n 2 = 9 times. The bandwith is 1 / 9 of the untapped value. RULE: By tapping the transformer the Q-value increase and the bandwith decrease. What is inside the CAN? The two pictures below explain the inside of the CAN. William R. Robinson Jr. p7of 9

8 2. XICON, IF Transformers (2007), pdf, online, accessed William R. Robinson Jr. p8of 9

9 3. UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 2008) p4.47, (Eq. 110) 4. Robinson, William AJ4MC, Transformer (another of these short papers). online, accessed Robinson, William AJ4MC, Center Tapped Transformer (another of these short papers). online, accessed Robinson, William AJ4MC, Non-Center Tapped Transformer (another of these short papers). online, accessed William R. Robinson Jr. p9of 9