SoftRock RXTX Ensemble Antenna Low Pass Filter

Transcription

1 V1 is 1.0 Vp-p at 7 Mhz Purpose and Function This Low pass filter is here mostly for FCC requirements for transmit, however it is used bi-directionally for receive, reducing any unwanted high frequency signals. Because the filter is symmetric and the source and load impedances are all 50 ohm the performance is the same in both directions The transceiver has a common antenna terminal and RF path which is switched between the RX Band pass filters (default) and the TX low-pass filters, via circuitry in the RF I/O and Switching Stage. The switching is performed in response to the /PTT signal from the microcontroller, as selected by the SDR program on the PC. 1 Need additional filtering for 40m Depending upon the option you have chosen to build and how you wish to operate within that built option, you may need to provide additional low-pass filtering between the transceiver and the antenna. The FCC requires that the mean power of any spurious emission from a station transmitter or external RF amplifier transmitting on a frequency below 30 MHz must be at least 43 db below the mean power of the fundamental emission. In the case of an 80m (3.5 MHz) transmitter, we must ensure that the mean power at the second harmonic (40m MHz) must be at least 43db below that of the fundamental 80m emission. William R. Robinson Jr. AJ4MC p1of 18

2 This rule applies to the RXTX Ensemble. With the current design, the following options require low-pass filtering in order to reduce the second (and higher order) harmonics for the following options to at least 43 db below that of the fundamental (first-mentioned) band (assuming you want to operate in that band): 1 Band Option Use LPF When Operating At To Attenuate 2nd Harmonic At 80/40m (80m) 3.5 MHz 7.0 MHz 40/30/20m (40m) 7.0 MHZ 14 MHz 30/20/17m (30m)10.1 MHz 20.2 MHz William R. Robinson Jr. AJ4MC p2of 18

3 Theory and Design T36 converts the unbalanced RF on the card to a balanced transmission line o It is interesting to note that this is then connected to BNC connector which is normally used for unbalanced systems. o It is wound as a 1:1 transformer. o The core material BN is good for use in a wideband Transformers from MHz 2 C24, C25, C26, L2 and L3 form a 5 pole low pass filter. The filter is being used as a compromise for the three bands 40M, 30M and 20M. o An additional filter is to be used for the 40M band 1 o Therefore design must provide for -43 db at the 1 st harmonic at the bottom the 30 M band 3 o The lowest frequency for the 30M band is 10.1 MHz 4 o Therefore we need at least -43 db at 20.2 Mhz At the same time we would like to not over-attenuate to top of the 20M band at MHz 4 Simulation of the circuit suggests the filter is a Butterworth with the cutoff frequency Fc at about 15.4 MHz. o At 20.2 Mhz the attenuation is only about 22.3 dbv 6 dbv for the pass band = 16.3 dbv Using table 3-9 in reference 5(p-40), with an F/Fc ration of 20.2 MHz/ 15.4 MHz and n =5 will provide something in the neighborhood of -12 db of attenuation This is much less attenuation than what is required IF the (harmonic) spurious emission had the same energy as the primary frequency. Using table 3-1 for Butterworth Equal Termination 5(p-41) with the following conditions o Fc = 15.4 Mhz o Rl = Rs = 50 ohms o And the equations Cn 5(p-41 eq3-12) C 2 * Fc * Rl Ln * Rl 5(p-41 eq3-13) L 2Fc Yields the following: (C in pf, L in uh) Device C1 L2 C3 L4 C5 Coefficient Value pf/uh Actual values The actual values are listed above also William R. Robinson Jr. AJ4MC p3of 18

4 o The inductors have good correlation o The capacitors do not show good correlation William R. Robinson Jr. AJ4MC p4of 18

5 Calculated Cutoff Frequency Fc = 15.4 MHz (design input) Ripple Ideal Butterworth has no ripple Ripple = 0 db Attenuation Slope Using Fig3-9 in reference 5(p40) for 5 elements o Slope = db@4/octave o Slope = 89-60/octave o Slope = 29 db/octave Insertion Loss An Ideal filter has no insertion loss as ideal indictors and capacitors only store energy. However there is of course the loss caused by the source and load resistances. This is simply that of a voltage divider, 20 log(rl/rs+rl) o Rsource = Rload = 50 ohms o Insertion loss Rx = 20 log(50/50+50) Insertion Loss = -6 dbv Inductance of L2, L3 (for 40, 30, 20 M bands) 16T #26 on T37-6 L = (AL*Turns 2 )/1000 uh 6 AL=3 +/- 5 % L = (3*16 2 )/1000 uh 6 L2, L3 = 0.77 uh Inductance of T6 (for 40, 30, 20 M bands) 4T Bifilar #26 on BN L = (AL*Turns 2 )/1000 uh 2 AL= 250 +/- 20 % L uh=(250*4 2 )/1000 uh 2 T6 = 4.0 uh William R. Robinson Jr. AJ4MC p5of 18

6 Simulation Rload is Rbandpass-filter (L4,C39,T5) + R54 +Rmixerswitch + Rinput to inverting opamp low pass filter o It would appear that there are two paths in parallel terminating in R58 and R55 but a study of the mixer will show that only one is on at a time o The band pass filter is made up of only inductors and capacitors which ideally dissipate no energy so Rbandpass filter does not contribute to the Rload o The Ron impedance of the FST3253 is about 4 ohms 7 But two inputs are used in parallel to reduce this to 2 ohms o The input of an opamp circuit is the impedance of the inverting input as this the op amp keeps this at (virtual) ground 8 In this case R58//XC41 1 Xc 9 2fC 1 Xc42 2f * 0.047uF Assuming the highest f = Mhz to get the lowest Xc we get to a very small number and the analysis appears to fall apart How ever if we notice that after the mixer we are only interested in frequencies below 96 Khz 1 then 1 Xc42 2 *96Khz *0.047uF XC41 = 35 ohms Rload = //4 + 10//35 Rload = 19.7 ohms But this goes through a 1:1/2 transformer 2 Np 10, 11 o Rin Rload * Ns 2 18 o Rin 19.7 * 9 o Rload = 78 Ohms Which is a little high of the desired 50 ohms but not horribly so The circuit is fairly sensitive to changes in Rload o The plot below shows the frequency response with Rload form 20 to 100 ohms in 20 ohm steeps William R. Robinson Jr. AJ4MC p6of 18

7 VDB(out)_0[0] VDB(out)_1[1] VDB(out)_2[2] VDB(out)_3[3] VDB(out)_4[4] Antenna_Low_Pass_Filter-Small Signal AC-6-Sweep-Graph Frequency M M Cutoff Frequency The insertion loss is -6dBV, so the cutoff frequency is at -9 dbv Fc =15.4 MHz Ripple Ripple = 0.92dBV (at 4.7MHz) Attenuation Slope Using points at 200Mhz and 400 Mhz o Slope = o Slope = /octave o Slope = 30 dbv/octave Insertion Loss o Insertion loss = -6.0 dbv William R. Robinson Jr. AJ4MC p7of 18

8 Real Circuit Results below are with 14 turns on L2 and L3 (design calls for 16 turns), when the entire the receiver section was completed. Below is the input and output of the o Local oscillator is at MHz o Antenna is at MHz Antenna Input amplitude was adjusted for near clipping at audio output Time Domain o Channel 1 is the input at the antenna J6 o Channel 2 is the output at C24/L4 Note the decrease in the high frequency noise from input to output I have no explanation for the output amplitude being higher than the input amplitude. When the two scope probes were put on the same signal they tracked each other very well. Frequency Domain o FFT of the output of the There is the expected peak at 7.0 Mhz There is a peak at 12 MHz which is -26 dbv from the expected signal Perhaps this is from the USB section There is a peak at 16 MHz which is -26 dbv from the expected signal I have no explanation for this signal There is a peak at 21 MHz which is -22 dbv from the expected signal This is probably the third harmonic of the square wave LO leaking back into the antenna as it disappears with removal of the power. When Power is removed the 7Mhz peak William R. Robinson Jr. AJ4MC p8of 18

9 looses about 7 dbv suggesting that the 1 st harmonic of the LO is also leaking out the antenna. Results below are with 14 turns on L2 and L3 (design calls for 16 turns), with a 50 ohm load unless otherwise noted. Cutoff Frequency Fc = 15.5 MHz Ripple Ripple = 0.44dbV Attenuation Slope My signal generator did not allow me to measure this but good correlation with simulation in the measurable range Insertion Loss Insertion loss = -6.0 dbv Inductance of L2, L3 (for 40, 30, 20 M bands) The inductance measured significantly higher than the calculated value, and as a result the circuit cutoff frequency was also too high. As a result I found I needed to reduce the number of turns to 12 to get near the design value of 0.8 uh This proved to have to high of a cutoff frequency and finally I chose 14 turns for the two inductors 16 turns = 1.2 uh 14 turns = 0.97 uh 13 turns =0.92 uh 12 turns =0.84 uh William R. Robinson Jr. AJ4MC p9of 18

10 Inductance of T6 (for 40, 30, 20 M bands) This inductor measured significantly lower than calculated 2.8uH Scope The screen shots below are mais inductor measured significantly lower than calculated William R. Robinson Jr. AJ4MC p10of 18

11 Comparison The difference in ripple between calculated and simulation is probably due to the usage of available real values The difference between ripple between simulation and the real circuit is most likely due to the points picked to measure the attenuation (simulation local minima was fairly sharp compared with the data points) The differences of the calculated inductor values seem to be the core material as one measures high and another measures low, other measurements by the same instrument have had good correlation and the circuit behaviors(s) are consistent with the measured values RXTX Ensemble Frequency Mhz Measured 16 Turns Measured 12 Turns Simulated l2=l3=0.8 Measured 13 Turns Measured 14 Turns -30 gain dbv The table below compares the results Real-Measured Simulation Calculated Cutoff Frequency Mhz Ripple dbv Attenuation Slope dbv/octave Insertion Loss dbv The chart below shows the response with the real load o The Frequency response curve with the real load is significantly different than calculated and simulation results William R. Robinson Jr. AJ4MC p11of 18

12 Measured 14 Turns 50 ohm load Measured 14 Turns real load o I was unable to reproduce the trend of the real behavior by in simulation by adding the when the RX Band Pass filter to the design (before the load) the trends are clearly visible except the peak at about 3 MHz. See the peak caused at about 6 MHZ by the real T5 in the RX Band Pass Filter (perhaps it is shifted when everything is put together Perhaps the filter requirements should have been as one filter (if that is possible). William R. Robinson Jr. AJ4MC p12of 18

13 A 50 ohm resistor (R1 below) in series between the two filters gives a response closer to what is desired but at the cost of another 4 db of insertion loss. William R. Robinson Jr. AJ4MC p13of 18

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15 References 1. Robson, Richard R. Sr., WB5RVZ, Ensemble RXTX Project, online, accessed kitsandparts.com, Specs for BN multi-aperture core, online, accessed FCC, Emission standards, online, accessed ARRL, US Amateur Radio Bands, online, accessed Bowick, Chris, RF Circuit Design 2 nd Edition (Elsevier Inc 2008), Butterworth Equal Termination Low-Pass Prototype Element Values, p kitsandparts.com, Specs for T37-6 RF Toriods, online, accessed Fairchild, FST3253Dual 4:1 Multiplexer/Demultiplexer Bus Switch, p2, online, accessed ERT, Inverting Operational Amplifier Circuit, online, accessed UNKNOWN, The ARRL Handbook For Radio Communications, (ARRL 2010) p2.27, (Eq. 54) 10. UNKNOWN The ARRL Handbook For Radio Communications, (ARRL 2010) P2.63, (Eq. 137) 11. Robinson, William AJ4MC, Transformer (another of these short papers). online, accessed William R. Robinson Jr. AJ4MC p15of 18

16 What I learned In spice the units must not have a space between number and the dimension suffix. Voltage divider below shows Vout = ½ Vin as expected when Rsource = Rload = 50 mohms Voltage divider below shows Vout = Vin when Rsource = 50 m and Rload = 50(space)m o This suggests Rload is much bigger than Rs (perhaps 50 ohms) William R. Robinson Jr. AJ4MC p16of 18

17 But voltage divider below shows Vout = Vin when Rsource = 50 and Rload = 50(space)m o This suggests that Rload is bigger than 50 ohms when marked 50(space)m o Same result when Rsource = 50 mega-ohms William R. Robinson Jr. AJ4MC p17of 18

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