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Optional Mods for the California Systems Downconverter
05/13/10 - Andy Flowers, K0SM

Optional Mods for the California Systems Downconverter

Andy Flowers, K0SM

Bias Tee

If you would like to get yourself a bias tee to power the 2.4 GHz downconverter we did the other night, there is a pretty good deal on Ebay at the moment: Item number 170018555520. This is essentially the same device we were using on the test bench.

Connections would be as follows:

"SAT": DC power (+15-30 V on center pin). I'll get out modification instructions for 13.8V power.

"IN/OUT": Downconverter

"ANT": Your 123MHz receiver

As is marked on the case, the "ANT" connection is isolated by a capacitor which will keep the DC out of your radio's antenna port.

2304MHz with 26MHz IF

As a side note, if anyone wants to convert these to 2304 MHz using a 26 MHz IF, you may very well have to build your own DC block with a higher-value capacitor, as it's possible that these things don't pass RF well at HF. Remember--they're designed for VHF/UHF TV. Our 123MHz IF for 2401 will pass through it just fine.


As for antennas, you probably could use a DSS dish, but they are starting to be less effective at S-band because they just aren't very many wavelengths across at 13cm. For OSCAR-51, you also want something you can aim easily. I recommend a horn antenna (like my tin-can in the presentation) or a 10- to 15-turn helix. I'm sure some directional antennas for 2.4 GHz WiFi use would work too. If you build it yourself, you probably want something with a male N on it so that you can screw it directly to the converter; no feedline, no loss. Directions for the tin-can approach: I improved upon this by putting an aluminum foil horn clamped to the end of the waveguide--that got me another 3dB or so. I hope that helps get folks started.

Isolated DC/RF

You can eliminate the need for a bias tee by moving the RF port to the other F-connector. The unused port, labeled “TEST”, is a -20dB RF sample port that is isolated from the other F-connector by a resistor and a capacitor.

First you need to cut the trace that runs along the edge of the board that links the two F-connectors. This trace is underneath the green silk screen on the 1995 boards, and is probably most easily cut right next to the “OUT” connector.

Next, solder a short jumper to that trace to bypass the capacitor and resistor on the “test” port. Sometimes soldering the jumper is easier if you remove these two components first (labeled “C” and “R” below), but it doesn’t really matter since you’ll be bypassing them anyway. Most importantly, be very careful that you don’t accidentally damage the inductors of the VHF output network by careless soldering.

The 1996 boards look slightly different in this area, but you should be able to find a convenient place to solder a jumper. Double check to make sure there is no conductivity between the two F-connectors—you don’t want to send +V down the coax to your radio.

DC input is now on the port labeled “OUT” while RF is on “TEST.”

13.8V Operation

The stock converter is meant to run on 15V-25V of input voltage. Below this voltage the regulator in the converter starts to lose regulation and drops below 12V. Y ou can replace the 7812 regulator on the board with a low-dropout regulator that will provide 12V output down to about 12.5 V of input voltage. This is useful if you plan on using your converter with 13.8V.

Mouser # 511-LF120CDT does the trick, and at the time of this writing it is only $0.88/ea. To remove the regulator you will probably want to desolder the pins first and then attack the solder tab with a hefty iron—I can assure you that the ground plane and nearby screw make a very effective heat sink! You can solder the new regulator in place of the old—don’t forget to solder the case to the board.

Some of the TO-252 regulators have been glued down such that it is nearly impossible to remove it with an iron. With a pair of needle-nose pliers you can remove the plastic body from the top of the regulator by gently wiggling back and forth. This exposes the silicon regulator (about 1 mm in size) sitting on the metal tab. You can then scrape off the silicon with a hobby knife leaving a polished surface to which you can solder the tab of the new regulator. You may need to bend the leads of the new regulator down slightly to reach the solder pads. When done, reapply solder to the both the old and new tabs to ensure a good ground connection.

Frequency Adjustment

You can use a calibrated frequency counter to check the frequency accuracy of your converter. I found that I can read the LO on my HP5350B counter (which, admittedly, has a very low input threshold) by plugging it into the antenna port. It actually reads twice the LO frequency (4556 MHz). I would let the converter warm up for a while before taking a reading. If you feel the need to adjust the crystal frequency, there is a trimmer right next to the crystal that can be tweaked to adjust the LO frequency. It is very sensitive, so this might take some trial and error. When finished, make sure the oscillator starts and stops properly.