OK.? ?Updated results.
FIRST -- I had forgotten or missed (although I made comments about it!) Ashhar's prescription to add 470 ohms in parallel with the drive resistors for the "pin" diodes.
I did that today -- on both sides.? ? On R201/202 and on R202/203, one 470 ohm for each pair.? ? This should raise the bias current on each PIN diode to approx 75 mA
ERROR CORRECTION:? previously I stated that increasing the idle bias current made a substantial improvement in the 2nd harmonic of the 20m CW signal.? ?My notes show a SUBTRACTION error to get an "improved" -36dBc.? ?The correct subtraction is -26dBc? (-47dBm versus -21 dBm carrier) which is basically UNCHANGED from the original measurements.
TONIGHT -- I made new measurements using the SAME SETUP, after adding in the new resistors.? ?I also made certain to tighten the case screws "snug".
3.510? ?20W? 73%? -20.6dBm? ? ? ? ? 7MHz -66dBm? -45.4 dBc? ? ? ? ? ? ? ?10.5MHz -77 dBm? -56.4 dBc
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?slightly better? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?slightly better -- and both PASS
7.010? ?20w? ?72%? ?-21.5dBm? ? ? ? ?14MHz -60 dBM? ?-38.5dBc? ? ? ? ? ? 21MHz? ?-66dBm? ? -44.5dBc? ? ? ?little change
10.11? ?~20W 81%? ?-21 dBm? ? ? ? ? ? 20.2MHz -57.3dBm? -36.3dBc? ? ? ? 30.3MHz -62 dBm? -41 dBc
14.010 20W? ?78%? ? ?-21 dBm? ? ? ? ? 28MHz? -48 dBm? ?-27dBc? ? ? ? ? ? ? 42MHz? ?-52.4dBm? ?-30.4 dBc? ?little change
18.1 MHz 15W 88%? ?-22dBm? ? ? ? ? 36.2MHz? -65 dBm? -43 dBc? ? ? ? ? ? 54.3 MHz -82.4 dBm? ?-60dBc? ? ?PASS
1.? What is monotonous is the LACK of change in most of these numbers despite all the things I have tried.? ?It is eerie how much they repeat.
2.? I presume that the 20meter and 18.1 MHz use the same filter.? ?The 18.1 MHz PASSES while the 20 meter fails miserably.? ?Is the filter out of adjustment?? That the 18.1 MHz signal PASSES certainly shows that there is adequate signal islolation and that a workable filter can get as much as -60 dBc!? ?This is a much better result
3.? Do the 40 meter and 30 meter use the same filter?? ?I was expecting a similar improvement of the 30 meter signal -- but it might be even worse than the 40 meter!? ?(I had to interpolate my attenuator corrections so this might be off a dB)? ? ??
It doesn't seem that the PIN diodes can be blamed for all this.? ? The ICOM 718 has (with my measrueemnt setup) undetectable 2nd harmonics but these 2nd harmonics are as strong as -27 dBc (20 meters) and generally running in the -36 to -45 range on most bands.? ?While that is certainly weaker than the fundamental, it doesn't compare to the ICOM 718!? ?It makes you think that we may have 2nd harmonics being generator in the earlier stages, possibly the 4-transistor driver. ?
I don't know, folks.? ? Anyone else have actual DATA or suggestions on how to fix this?? ? From my measurements, 80 meters and 17 meters are good.? Is Serial # 141 just an outlier, and how am I so unlucky??
73
Gordon KX4Z
