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RE: Downconverter for AO40 ?

Doug Faunt wrote:

A low droput regulator would allow the unit to operate from 13.8V up the coax, and still provide some protection if a higher voltage, either continuously, or as spikes, were applied. It also provides some isolation if you want to feed multiple devices up the coax.

I have chopped the regulator out of mine, and have now observed some frequency instability (it may be unrelated, of course).

What voltage does the FT-847 put on the coax for the preamp?

Wayne replies:

There is at least 0.2V drop across the saturated transistor that switches the "preamp power" on and off.  Also, there is some voltage drop in the DC power cable and inside the radio when transmitting.  We could probably ignore the small voltage drop on the coax cable that goes to the downconverter.  I haven't measured the "preamp" voltage from my FT-847, but it's definitely less than the power supply voltage.

I suspect that the downconverter (with 12V regulator disabled) can handle the same reasonable voltage fluctuations as the transceiver.  The sensitive parts in the downconverter are powered by still another voltage regulator (5V, I think).  A regulated 13.8V power supply should give adequate protection from voltage fluctuation and spikes.

I have two TSI downconverters with the 12V regulator disabled.  One is used with my portable antenna setup.  It does drift noticeably when the sun goes in and out of clouds, but it's stable at night or on a cloudy day.  The other downconverter is on my home antenna setup.   When the temperature is above freezing it doesn't drift because a PTC thermistor stabilizes the crystal at 60C.  It does drift when it's really cold and windy because the PTC doesn't dissipate enough heat to stabilize the crystal at 60C.  I need to wrap the downconverter in pipe insulation, and/or put some insulating foam around the crystal and PTC.

Wayne Estes W9AE
Mundelein, IL, USA
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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