Progressive Radio EDU-KIT TUNING CIRCUIT

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Madeleine Walsh
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Tuning Circuit Purpose and Function The primary of the antenna coil couples the radio wave into the secondary. This method of coupling is known as "transformer coupling".

1 Tuning Circuit Purpose and Function The primary of the antenna coil couples the radio wave into the secondary. This method of coupling is known as "transformer coupling". The secondary of the antenna coil and the variable condenser act as a "tuned circuit"; they select any of the radio waves that you desire. If you used a condenser of fixed value instead of the variable condenser, you wouldn't be able to change stations. 1 William R. Robinson Jr. AJ4MC p1of 19

2 Theory and Design. Tuning Frequency The variable capacitor and the antenna form a parallel LC circuit. Changing the variable capacitance changes the resonate frequency. 1 o Frequency of resonance = Fr LC Tuning Bandwidth The tuning bandwidth is dependent on both the coupling and the load. o I do not know how to account for coupling o BW = Q f 3 o Q = R Xl 4 o Substituting the nd into the first gives F * R BW = Xl o Expanding Xl = πfl yields o Substituting the nd equation into the first yields F * R BW = FL o The F s cancel out leaving us with R BW = L Input Series Resonance Unfortunately C1 and the primary of the antenna coil form a series LC circuit with resonance also. In some designs an antenna tuner can be used to move this resonance to the frequency of interest, but in our case it is fixed as we change frequencies, possibly causing unwanted signals to be received. 1 o Frequency of resonance = Fr LC Additionally most wire antenna s are short for the broadcast band making the antenna capacitive about pf. 5 This capacitance is in series with C1 so this must also be taken into account. C1* Canttenna o Ceff C 1 Cantenna Input Series Bandwidth The input series bandwidth is dependent on both the coupling and the effective load. o I do not know how to account for coupling William R. Robinson Jr. AJ4MC pof 19

3 o Similar to Tuning Bandwidth F * L BW = R Loose Coupling At 1st glance it appears that the nd purpose of the Antenna coil is to match the relatively low impedance of the random wire antenna (10-00 ohms 6 ) to the higher impedance of the grid circuit. Matching the antenna ground impedance to the high impedance input of the 1st stage tube s grid allows more power to be transferred from the antenna to the 1st stage. The impedance as seen by the antenna ground path can be calculated from the formula below. But further examination shows that the transformer is a step down transformer so it actually would create more mis-match! (See Real Circuit for measurements of the transformer.) Np 7 o Rin Rload * Ns With a measured input inductance of 3.4 mh, the inductive impedance is high (around 1K) so that most of the antenna to ground circuit voltage appears across the primary. This is perhaps a high impedance loosely coupled front end. In an ideal front end both the input and the tuning side would be resonate at the frequency of interest. Loose coupling provides a higher Q (selectivity) at the 8, 9, 10 expense of signal strength (sensitivity). William R. Robinson Jr. AJ4MC p3of 19

4 Calculated Tuning Frequency The variable capacitor is adjustable from about 40 to 400 pf The secondary side of the coil has been measured at 174 uh with a resistance of 10 ohms o 1 Fr LC o F _ lower o Fr_lower = 603 KHz 1 174uH * 400 pf o 1 Fr _ upper 174uH * 40 pf o Fr_upper = 1.9 MHz This is a little high for the commercial broadcast band 540 to1600 khz 1 but the inductance of the antenna coil is adjustable. Tuning Bandwidth R BW = L 10 o Fr_lower_BW = 174uH Fr_lower_BW = 9.15K o As frequency does not play a part in the equation (without coupling affects) Fr_upper_BW = 9.15K Input Series Resonance C1 and the primary of the antenna coil form a series LC circuit at resonance; however a typical (too short wire) has a capacitance of about 100 to 500 pf. 5 The 3-4 side of the antenna coil measures 3.4 mh and 41.1 ohms o C1* Canttenna Ceff C 1 Cantenna 0.01uF *500 pf Ceff 0.01uF 500 pf Ceff = 476 pf William R. Robinson Jr. AJ4MC p4of 19

5 o Fr _ series 1 LC Fr _ series Fr_series = 15 KHz 1 3.4mH * 476 pf Input Series Bandwidth I do not know how to calculate the input tuning. At a minimum it is a function of the coupling which is unknown. R o Fr_Series_BW = L 41.1 Fr_Series_BW = *3.4mH Fr_Series_BW = 1.9K Loose Coupling This calculation shows that the input impedance of the antenna coil is high. The transformer turns ratio is about 1/0.135 (see Real Circuit transformer turns ration below.) Np 7 Rin Rload * Ns 1 o Rin 10K * o Rin = 548K The documentation does not specify the coupling. I do not know how to calculate the changes caused by loose coupling to resonance or gain. William R. Robinson Jr. AJ4MC p5of 19

Simulation The coupling capacitor C1 provides low impedance but grid resistor R and the Grid provide very high input impedance so no Rload was used in simulation.

6 Simulation The coupling capacitor C1 provides low impedance but grid resistor R and the Grid provide very high input impedance so no Rload was used in simulation. The tricky part here was of course the transformer. Modeling is usually problematic, see ref 1. o The impedance ratio is the measured gain squared. This value is then inverted because of the way the model defines it as primary to secondary! Np Rin Rload * Ns Rload Rin 1 Impedance ratio = o The coupling factor affects both the gain between the resonant points of about -5.7 Db, and the frequency response. I finally settled on a compromise model with a coupling of See Real Circuit for measurements used to create the model. William R. Robinson Jr. AJ4MC p6of 19

7 Tuning Frequency This simulation looks at the response without resonance on the antenna side of the antenna transformer. Cvariable = 400 pf Fr_lower = 69 KHz Cvariable = 40 pf William R. Robinson Jr. AJ4MC p7of 19

8 vdb(out) Tuning Cuircuit-Small Signal AC-4-Graph k k 1.000M Frequency Fr_upper = MHz Tuning Bandwidth Fr_lower_BW = 0.8 khz Fr_upper_BW = < 1 khz William R. Robinson Jr. AJ4MC p8of 19

9 Input Series Resonance This simulation looks at the response without resonance on the receiver side of the antenna transformer. Fr_series = 15 khz Input Series Bandwidth Fr_series_BW = 4 khz William R. Robinson Jr. AJ4MC p9of 19

10 The next two simulations below show the response with both resonant circuits at the same time. Cvariable = 400 pf William R. Robinson Jr. AJ4MC p10of 19

Cvariable = 40 pf Progressive Radio EDU-KIT Fr_upper is slightly lower at 1.981 MHz from 1.

11 Cvariable = 40 pf Progressive Radio EDU-KIT Fr_upper is slightly lower at MHz from Mhz Fr_Series is slightly higher at 18 khz from 15 khz William R. Robinson Jr. AJ4MC p11of 19

12 Loose Coupling The simulation below has tighter coupling wit K =.9999 verses above Tuning Cuircuit-Small Signal AC-6-Graph vdb(out) k k 1.000M Frequency Fr_upper is significantly higher at.613 MHz from 1981 Mhz Fr_Series is slightly higher at 134 khz from 18 khz The gain is higher with tighter coupling 8, 9, 10 o This is in agreement with sources William R. Robinson Jr. AJ4MC p1of 19

13 Real Circuit Transformer Note the pin outs for the purchased antenna coil are not the same as the one originally provided with the kit. o Connected with pins and 3 to ground (as shown in the data sheet) seemed to work better other combinations that I tried. I measured the following on the P-C70-A o Pins 1- (receiver side as small coil this has proper resonance with pf) 174 uh 10 o Pins 3-4 (antenna side large coil) 3.4 mh 41.1 ohm o Therefore a step down transformer and impedance matching is backwards The turn s ratio is not provided in the documentation. This was found by measuring the voltage gain across a range of frequencies. I attribute the peak near 3 MHz to self resonance (about 1 pf of capacitance). At frequencies away from self resonance the gain is about (0 log(0.135 = db) so this was used as the turns ratio. William R. Robinson Jr. AJ4MC p13of 19

14 P-C70-A Antenna Coil Unloaded Voltage Gain Voltage Gain = N1/N Measured Simulation -0 Frequency Khz The nd issue was determining the coupling coeffiecent. Trial and error resulted in my using William R. Robinson Jr. AJ4MC p14of 19

15 Tuning Frequency Progressive Radio EDU-KIT Tuning Cuircuit Frequency Response Cvar = 470 pf Gain (db) Measured Simulation Frequency (khz) o Fr_lower = 580 KHz William R. Robinson Jr. AJ4MC p15of 19

16 Tuning Cuircuit Frequency Response Cvar = 47pF Gain (db) Measured Simulation Frequency (khz) o Fr_upper = 1.47 MHz This is a little low for the commercial broadcast band 540 to1600 khz 1 o But the inductance of the antenna coil is adjustable. o The scope probe capacitance is probably decreasing Fr_upper. Tuning Bandwidth Fr_lower_BW = 30 khz Fr_upper_BW = 40 khz o These are a little wide for the commercial broadcast band 540 to1600 khz 1 Input Series Resonance Fr_series = 16 KHz Input Series Bandwidth Fr-Series_BW = 7-13 khz Loose Coupling William R. Robinson Jr. AJ4MC p16of 19

17 There was no way to adjust the coupling in the real circuit so now experimental results are available. William R. Robinson Jr. AJ4MC p17of 19

18 Comparison The low value of the measured Fr_upper is caused by the scope loading. This was confirmed in the #1 Triode Grid leak detector tests when the output Fr was noted to change with /without the scope on the Grid circuit. This may also explain the increased bandwidths of the measured results. The narrow bandwidths of the simulation are what is desired in this cuircuit but perhaps the coupling for simulation is too loose o This is supported byt the lower gain of simulation o But tighter coupling really messes up tuning resonate frequencies. The table below compares the results Real-Measured Simulation Calculated Tuning Frequency Fr_lower khz Fr_upper MHz Tuning Bandwidth Fr_lower_BW khz K Fr_upper_BW khz 40 <<1 9.1K Input Series Resonance khz Input Series Bandwidth khz William R. Robinson Jr. AJ4MC p18of 19

19 References 1. UNKNOWN, The Progressive Radio EDU-KIT Instruction Book, (Progressive EDU-Kits INC. 1959), p.40. UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 010) p.54, (Eq. 116) 3. UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 01) P.56, (Eq. 1) 4. UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 010) P.60, (Eq. 130) 5. Anderson, Phil W0X1, A Great Teacher: The Crystal Set, (QEX ). online, accessed UNKNOWN, Crystal Radio, online, accessed UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 010) P.63, (Eq. 137) 8. UNKNOWN, File:Inductively coupled crystal radio circuit.svg, g, online, accessed UNKNOWN, RadioCoils and Circuit Applications-(High Impedance Primary, (Meissner), online, accessed Sousa, Joe (forum response), Optimized amplifier-detector and regeneration methods, e_receiver.html, online, accessed UNKNOWN, AM and FM Radio Frequencies, online, accessed Robinson, William AJ4MC, Transformer (another of these short papers), online, accessed 01. William R. Robinson Jr. AJ4MC p19of 19

AM/FM-108TK FM_RF_AMP

AM/FM-108TK FM_RF_AMP V1 is 50 mv at 88Mhz V2 is 7.73 Volts dc o Real circuit has supply voltage of 7.73 due to Ir drop across 220 ohm R25 and 100 ohm R9 Ir25 = (8.85-7.75V)/220 ohm = 5 ma Ir9 = (7.75-7.37V)/100 ohm = 3.8 ma