Re: VK4PP uBitx LPF BANK add-on Board
Gordon Gibby <ggibby@...>
Warren, you are right on, but you might not have seen all the posts where I did exactly what you are saying. Both BEFORE the daughterboard and AFTER the daughterboard so i had proof of its effectiveness. I also did some calibrations of the S-meter using
BNC attenuators (which is all I have). I then did the suggested measurements of today, measuring bleed-through (unmeasureable, whereas I was clearly able to measure the harmonics and able to measure their reduction by moving the input side of the relay chain
OFF the board.)
Here, I'll see if I can find the posts (I'm not too used to that):
(1)Very first "before" measurements: https://groups.io/g/BITX20/message/57840
(2)Complete baseline data: https://groups.io/g/BITX20/message/57974
(3) Calibration of the S-meter and proof that the measurement setup itself didn't have bleed thru: https://groups.io/g/BITX20/message/58070
[This distinctly showed that 1 S unit in the range of 7-9 is 12 db onthat receiver on that frequency)
(4)Photos of the measurement setup: https://groups.io/g/BITX20/message/58070
(5) DAta AFTER daughterboard: https://groups.io/g/BITX20/message/58070
What I CANNOT prove is that the receiver maintains identifical response at all frequencies (hence, is "flat") however it is reasonable that the differnces aren't mroe than a few dB based on specifications for rigs of this price range etc.
Poor man's spectrum analyzer. What is problematic is the 24MHz harmonic range --- here I achieved only 6 dB reduction so there is something else going on there...maybe a series resonance in a filter or somethign I haven't thought of. Every other range
was dramatically reduced by the daughterboard system. So for my purposes --- a low pass filter above 20 MHz and I have every digital band I need for what I need out of the radio and I don't need to use the bands where the "spurs" are a thorn.
Hope that makes more sense.
From: BITX20@groups.io <BITX20@groups.io> on behalf of Warren Allgyer <allgyer@...>
Sent: Friday, August 31, 2018 2:24 PM
Subject: Re: [BITX20] VK4PP uBitx LPF BANK add-on Board
Your reasoning makes perfect sense. But I think you can measure at the actual harmonic frequencies which is where the issue would be . Your IC-718 can tune the third, fifth and 7th harmonics of 80 meters. So you can know exactly (within the limits of the S meter calibration and linearity) what the residual level is at the harmonic frequencies themselves. You have checked your S-meter and you know what represents 45-50 dB below S9+30 (normally one S unit = 6 dB but in real life S meters rarely replicate that). So just tune the harmonics directly and measure them. Or am I missing something?