No matter what your IMD setup is, you need to run the measurement bypassing the DUT first, to see the capability of the setup.
My IMD bench is:
?- Si5351a generating F1=3.58 and F2=4.914MHz,? 4mA drive strength (important)
- 5-resonator xtal narrow bandpass filters, Cohn topology. I can slightly tweak the input frequencies to hit the maximum, it moves with temperature.
- hybrid power combiner
- step attenuator
- dummy aerial if the DUT is a loop preamp (in my case it is a 1:1:1 transformer and two 25/0.5 Ohm dividers, no reactive elements, flat response, 50dB CMRR up to 30MHz)
- here is the preamp under test
- 5-inductor bandstop filter, 2.5 .. 5.5MHz. F1/F2 Attenuation is about 35..40dB. All products of interest are outside of the stop band, but I measure their attenuation anyway to correct for it during the measurement. It is about 1 .. 1.5 dB.
- 20 or 40dB fixed attenuator to bring the signals within the range of the receiver
- Airspy HF+ Discovery
- HDSDR software
?
The floor (IMD detected without the DUT) is
+110dBm OIP2 at 2.24MHz (F2-F1)
+109dBm OIP2 at 8.5MHz (F2+F1)
+47dBm OIP3 at 2.24MHz (2*F1 - F2)
+39dBm OIP3 at 6.25MHz (2*F2 - F1) - this is the one which limits my measurements, I don't see it changing with LZ1AQ or other good preamp in the path. Probably limited by the Si5351a outputs cross-modulating.
?
73, Mike AF7KR
