Hmm, it looks interesting:
- to make an "appendix LPF" that would work for both 3.6 and 5.3 MHz does not seem to be possible
- however, adding an elliptical PI-network C-L-C, sort of an "extra stage", seems to work for 3.6 MHz:?
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first shunt capacitance: 727 pF (680+47), together with the 270 pF output capacitance in the PA it makes 997 pF
inductance: 1.77 uH (19 turns on T50-2), and 270 pF in parallel with it makes a parallel resonant circuit at 7.2 MHz
output capacitance: 727 pF (670+47)
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This combination should suppress 7.2 MHz by 50-60 dB (resonance is critical and there might be tolerances in capacitor values as well as winding). Optimum values are slighly different by 2 to 3 pF but impact is insignificant (order of 0.001 dB according to simulation).
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Now I have to check how will this appendix behave standalone in the "opposite direction", i.e. transceiver in RX, PA is bypassed and RX signal goes only through this LPF. But I think it should be fine. Tomorrow or on Tuesday I will try this. Unfortunately I think I have no BNC connectors in the drawer.
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73 Jindra
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On Sun, Feb 23, 2025 at 10:26 PM, ok4rm wrote:
I like this idea, Don. I built the PA for 7 MHz (for emcomm this would be the highest band).? For the other two bands, 60 meters and 80 meters, I could perhaps live fine with just one simple elliptical lowpass aimed at suppressing 7 MHz.? 10 and 14 MHz are out of the game in this case.
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Two ideas:
- in LTspice I was unable to design a lowpass that would work fine for both 80 and 60 meters (in OK we have the WARC 15-15 segment, i.e. 5351.5 to 5366.5 15W PEP) - 7 MHz is too close to 5.3 so 2nd harmonic suppression was not great, despite the assumption that push-pull amp creates relatively weak 2nd harmonic. 3rd harmonic of both 3.6 (central spot of all digital mods on 80m) and 2nd harmonic of 5.3 should be well suppressed by the original lowpass. So I need to resolve just the 2nd harmonic from 80 meters (7.2) and whatever else above that is attenuated is a bonus.
- I cannot just take some 5.3 MHz lowpass and put it as-is after the original 7 MHz lowpass because the input shunt capacitor of the appended LPF would add to the output capacitor of the original LPF and that would result in a mess. So the "appendix" LPF must be designed as "continuation" of the original built-in LPF. But that should be easy with LTSpice.
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I will try to simulate this, build it, and if it works I will post it here.
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73 Jindra
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On Fri, Jan 5, 2024 at 01:25 PM, Donald S Brant Jr wrote:
To keep it simple in an emergency situation you could just use outboard lowpass filters, swapping them manually as needed.? ?Put your highest-band filter in the box (always inline) and add lower-band ones when required.??
Instead of "...and a small SMPS giving ~ -300 volts to shut down 1N4007..."? ?you could just tap and rectify/voltage-double the RF output.? This would guarantee always having sufficient voltage to bias your diodes off.
73, Don N2VGU
