[DSP-10] PA Brick Modules

[Scott]

There was some discussion about PA modules that could be used in the 
DSP-10 yesterday.  I suggested some modules from RF Parts and suggested 
a couple parts that should be suitable (low power of approx. 7 Watts 
out) such as the M57732L.  In the case of the M57732L, you can make the 
output power adjustable by varyi the DC volatge to VCC1 between about 
2-14V.  The chart on the datasheet has a graph of output power vs. VCC1 
voltage.  The Datasheet, at the top of the first page says it is for 
135-160 MHz, 12.5V, 7W, FM Portable Radio.  This would make one think 
that it is non-linear.  The chart at RF Parts says it is Class AB, which 
would indicate it is linear (good for SSB, AM, etc.).  I believe that 
the module would indeed work in linear fashion because in addition to 
VCC pins, there is a VBB pin (Base Bias Voltage).  The base bias is 
specified as 6 VDC max.  By making this voltage adjustable (from 0-6 V), 
one should be able to use it in class AB.  The usual amount of bias is 
used so that there is ICC (DC current) flowing at the rate of 4 to 5 mA 
per Watt of output power with no RF applied.  So, as a starting point, 
you would put a DC mA meter in line with the VCC2 pin and adjust the VBB 
voltage (starting at zero and going up) until you read 25-35 mA of 
current with no RF appiled to the module.  Then you could fine tune VBB 
with on the air signal reports.  If the far-end station reports 
distorted audio from you, increase VBB a bit and try again until you get 
a "good audio" report from the other station.  To save on some heat 
generation, I would only switch VBB on during transmit.  This can be 
done by routing the adjustable voltage through a set of relay contacts 
that are open when the relay is NOT energized.  Energize the relay with 
a PTT signal to turn on the bias supply.  Any module with a VBB pin (and 
listed as class AB at the RF Parts webpage) should be able to be used in 
linear operation.  This particular module is only about $30, so it 
wouldn't break the bank for most folks to play with it a bit...

http://www.rfparts.com/module.html


-- 
Scott N0EDV
http://corbenflyer.tripod.com/
Flying Corben Junior Ace - Building RV-4
Gotta Fly or Gonna Die