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Re: Transystems Modifications



Ward WC0Y wrote:

The 122 to 222 MHz IF output band is below the LO frequency.  Its image input is in the 2400 to 2500 MHz band where we hams want it to operate.  The designers could not allow the 2400 to 2500 MHz span to add noise and interference, so they added the tuned stub after the LNA to filter it out.  That stub is what removes the image noise and keeps a good noise figure when using a 123 MHz IF.
So, one image noise span is suppressed for each IF frequency case; for the 123 MHz IF it is the stub, and for the 145 MHz IF it is the RF filter roll-off.

Wayne replies:

With a 2278 MHz L.O. and no filtering, both 2401 and 2155 MHz will mix down to the 123 MHz IF.  We only want to "hear" the 2401 MHz signal.  To accomplish that, the mixer input needs either a 2155 MHz notch filter or a 2401 MHz bandpass filter.

I don't understand Ward's statement that the 2400 MHz stub (notch) will reduce the undesired signal unless he is referring to the MMDS application where 2400 MHz is the undesired signal.  For use with AO40, the undesired signal is 2155 MHz with a 2278 MHz LO and 123 MHz IF.  Or 2111 MHz with a 2256 MHz LO and 145 MHz IF.

I understand how Pieter Ibeling's 2400 MHz ceramic bandpass filter would greatly reduce any noise mixing down from 2155 MHz.  Howie DeFelice reported that the 2155 MHz signal does indeed get through the mixer and stock filters (even stronger than the desired 2400 MHz signal because of the 2400 MHz stub).  It appears to me that the Murata ceramic filter would offer a significant improvement in the downconverter's performance under every circumstance that has been discussed (stub cut or not cut, 123 or 145 Mhz IF).  So would a longer stub cut to 2111 or 2156 MHz.

I think that anybody who uses the AIDC3733 with a helix or 60cm dish should add the Murata 2400 MHz ceramic filter.  Maybe it's not necessary with higher-gain antennas.  It may also be necessary if there are any obstructions such as trees.

Wayne Estes W9AE
Mundelein, IL, USA
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