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:

(2)Complete baseline data:

(3) Calibration of the S-meter and proof that the measurement setup itself didn't have bleed thru:

  [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:

(5) DAta AFTER daughterboard:

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: <> 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?


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