Jim, IMD? (3rd order intercept is calculated)? is easily measured on any RX.? On the Drake tube gear, like R4B / R4C, it was never outstanding to begin with.? The problem is the 2nd mixer, it doesn't have any dynamic range to begin with.? Rob Sherwood's 'fix' for that was the various roofing xtal filters....which work superb.? The roofing filters? will eliminate or partially eliminate? one of the 2 x offending? signals.....and bye bye IMD resulting from them.?
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?On my yaesu FT-1000D? ?and also my FT-1000MP-MKV, after doing the W8JI noise blanker mod ( blanker is not totally shut off when switched off),? I can't find any IMD.? ?I need 2 x signals, both 50 db > S9, and 2khz apart,? to produce an IMD product equal to the noise floor....and that's on the bench.? ?That's just not happening with any ant connected.? My noise floor is S2? to S8, depending on the band.? ?Ok, now any IMD is well below my noise floor...so it's a non issue, since I can't hear it.??
IF you reduce the RF gain a bunch..... the agc threshold INCREASES ........ and any IMD from 2 x loud, closely spaced signals will be BELOW the agc threshold.? ?So yeah, if u had zero band noise, and 2 x super loud, closely spaced signals on RX,? and could actually? hear IMD, it will VANISH? if the RF gain is turned down.? ? It will also vanish if you insert some attenuation in the RX path.? ?The IMD will drop off at triple the value of the inserted attenuation.??
Jim? VE7RF
