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Steve -- surprise, surprise, surprise!
Now we may be getting somewhere. I reconnected D30 and L201 to bring the switching systems back to where they are supposed tobe. Then using two 9V batteries in series, I applied +18V to the point marked "HV" on C201. That should back-bias the low pass filter diodes by 4-6 volts or so.... And surprise!? ?The isolation only improved by a small single digits dB.? ?Like maybe 2 or 3 dB; the entire curve with all of its series resonances (I'm looking from 2MHz all the way to 82 MHz now, with tracking generator and spectrum analyzer). I was hoping it would have become FLAT at -60db or -70dB.? ?It did not. (A) So I think we do have some evidence that signals can LEAK? ?*around* the excellent low pass filters. (B) But if you look at the photos that I shot of the filter performance WITH all the switching diodes in place (see:? ?/g/BITX20/message/105633?) you don't see much evidence of signals getting "around" the low pass filters. I don't understand how both (a) and (b) can be true.? ? ?I'm still missing something. Copied from that previous message, here are the measured (IN CIRCUIT) rejections of the various filters: BAND? ? ? -1db point? ? ? ?2nd H. Freq? ? -24 dB point? ? ?3rd H. freq 80 M? ? ? ? ? 5.4 MHz? ? ? ? ? -22dB? ? ? ? ? ? ~ 7.06MHz? ? ?~-60 dB 40M? ? ? ? ? 11.4 MHz? ? ? ? ? -21 dB? ? ? ? ? ~ 14.6 MHz? ? ?-51 dB 20M? ? ? ? ? ?16.4 MHz? ? ? ? -44dB? ? ? ? ? ? ?21.6 MHz? ? ? ? -57 dB 15M? ? ? ? ? ~3dB@...? ? ?-21 dB? ? ? ? ? ? ?43 MHz? ? ? ? ? -57 dB? ?(I had to change the Siglent to go to much higher frequencies to get this one measured) How can the 20meter 2nd harmonic rejection be -44dB and my measruements of the 20 meter 2nd harmonic be -30dBc??? ?That suggests the output diode is mixing to creating more of the 2nd harmonic on the spot.? ? In post?/g/BITX20/message/105656? ?I showed that a modest attempt at shorting out the "output" diode on the filter used for 15 meters resulted in a 6dB reduction in the 3rd harmonic (to almost legal levels) while the 2nd harmonic was not much changed.? ? ?I did NOT test shorting out the "input" diode -- indeed it could be making harmonics and on some bands the filter has only 20dB reduction on 2nd harmonic, so if the switching diodes are making harmonics....that could be again the source of the problem. I have not yet done the drastic step of disconnecting all of this switching circuitry and just going directly into a filter and out to the antenna, which I discussed in post?/g/BITX20/message/105666? ? ? Since I still can't really explain what is going on, I may? have to go ahead with THAT experiment....If that succeeds (who knows, it might!)? then the solution would likely be to go back to the DE circuity with relays to switch the LPF's and diodes to do the T/R switching.? ? But there is no proof at this point that this is the solution.? Gordon KX4Z But it looks like? |
