Thursday, October 14, 2010

New Project - 2304 MHz Transverter

Its been so long since I've posted in here, I almost forgot I had this blog site.  I haven't been very active since my last post, but I feel its time to document a new project that I just started today.  This is a major project for me.  What I'm doing is building up a transverter for 2304 MHz, partially from scratch, partly from an old MMDS receive converter.  I'm building this unit around the California Amplifier model 31732 MMDS downconverter.  These units were originally used to receive over the air cable television.  I have successfully retuned one for Amsat Oscar 51 (AO51) ham satellite S band downlink on 2401.200 MHz.  The converter is extremely sensitive and they appear to be stable enough to use as a basis for this project.  This is the first time I've attempted to build my own microwave rig from either converting surplus or scratchbuilt.

The design goal for this transverter is to build a portable 13 cm station.  Power output of a couple of watts, and later perhaps as much as 75 watts using converted surplus.  This band is easy to do this with, being there is a nice variety of surplus hardware that can be used on these frequencies.  The project will use the MMDS converter for the entire receive chain, including the LO.  A probe (first modification) is installed just above the LO filter striplines, and exits the chassis to an SMA connector to be used to drive the TX mixer.

The TX mixer, bandpass filter and low level transmit amplifier will be homebrew, external to the MMDS whitebox. A surplus double balanced diode ring mixer will be used as the transmit mixer.  From the RF port, a homebrew 3 resonator filter will pass only the LO + IF output of the mixer, then will be amplified to approximately +10 dBm, which will drive a commercial PA, which, depending on what I use, will run anywhere from 2 to 75 watts, depending on the model I'm able to acquire.

The first modification I have done was to install the TX LO probe to tap off some 2160 MHz energy to drive the TX mixer.  This involved milling down a lip  on the case of the converter and installing an SMA connector with a 30 mm probe inside the whitebox.

Today, I received the necessary crystal to change the frequency of the LO synthesizer to output the 2160 MHz.  The crystal frequency is calculated by taking the operating frequency (2304) minus the IF frequency (144), then dividing the result (2160) by 256 (8.4375).  This is the frequency of the crystal I ordered.  I ordered this crystal from International Crystal, and it came in today.  I installed the crystal, and checked the frequency, and it netted on frequency with very little effort.  The output from the probe seems to be enough to drive the TX mixer.

The next step will be in tuning the receiver printed circuit bandpass filter, lowering its frequency from 2500 MHz down to 2300 MHz.  My next post will describe the procedure, and will keep you informed as to my progress.  So until then, I'll have to wait until I get some RG59 and some connectors so I can hook up the IF rig to the unit and start adjusting the hairpins on the pc board.

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