Probably the first thing you notice is that this not a bandpass filter like the one analyzed above. This a low-pass filter, but still acceptable for use here because all of the harmonics we need to suppress are above the fundamental square wave frequency (14.0956 MHz)
This filter has very low insertion loss and the 3d harmonic is well suppressed (-74 dB).
Sensitivity Analysis
Universal Dual HF Band Filter (BPF)
The BPF provides adjustable capacitors that can be used tune the filter. The parts list does not provide a part number for the capacitor, but the vendor only sells two adjustable capacitors, so we can guess that it is the Sprague GKG40015 with a range of 7-40 pF.
Figure 5: BPF Capacitance Sweep Schematic
The key difference between this schematic and the one used for the nominal analysis, above, is the Parameter sweep simulation. Parameter sweep repeats another simulation, SP1 in this case, using a different value each time SP1 is run.
Another important difference in the schematic is that equations using the xvalue() and yvalue() functions have been eliminated. Those functions will not work with the data produced a Parameter sweep simulation, so we will have to read the values we are interested in from a graph, or use an external program to process the data. For this exercise I am just going to use the graph.
The values for C4 and C5 (the variable capacitors in the design) are replaced with a variable. The variable value is simple and it's sweep can be specified directly using settings in the Parameter sweep simulation block.
Figure 6: BPF Capacitance Sweep Results
Figure 6 shows us that adjusting the variable capacitors is going to be quite important. The pass band can be moved almost 3 MHz and, if miss adjusted, emission of the fundamental frequency could be greatly attenuated.
Another thing we can learn from Figure 6 is that GKG40015 does not provide enough capacitance to cover the whole
20m ham band (14.000 MHz to 14.350 MHz).
Seven Element Elliptical Filter
There are no adjustable parts on this filter, but variation in the toroids is likely either due to variations in the material (+/- 5%) or due to winding. I don't know how characterize the variation due to winding, so ignore it for now and look at just the material variation. The material variation is specified as a percentage of Al
, inductance per turns-squared.
Figure 7: Elliptical Filter, Toroid Sweep Setup
Equation Eqn2 in Figure 7 calculates the inductance for L2, L4 and L6 based on the Al value settings in the Parameter sweep block. My assumption is that all three toroid cores will be from the same batch and the Al values will therefore match.
Figure 8: Elliptical Filter, Toroid Sweep Results
The variation in Al (inductance per turns-squared) yields an inductance variation of approximately 0.01 uH. That shifts the corner frequency of the filter, but the WSPR frequency is well within the pass band and the 3d harmonic is still well suppressed.
Conclusions
We could learn a lot more about these filters by using sweep analysis on capacitance values and identifying worst case combinations of values. Still, the information from these first analysis is adequate to make a choice for a WSPR transmit filter.
The elliptical low-pass filter is better choice for this application because:
- It does not need tuning.
- The insertion loss is lower.
- It may not be possible to tune the BPF so that the 20m WSPR frequency is in the pass band.