5.3.7
With no RF power, the quiescent output voltage can now be adjusted sing P5. Provided
the output is not negative, this adjustment can also be made later.
5.3.8
The unit is now ready to have a transfer curve taken. First run a transfer curve with no
linear extension (i.e. without P7). The curve will fall off at high power as shown in Fig. 5.3.8.
Measure the transfer slope, and adjust P8 until the desired slope is achieved. Replace P8 by R20,
R15, and S1 (as appropriately chosen, see sub-section 8.3), and set R17 equal to the value of P8.
This adjustment is only needed on the first unit, and thereafter these values can be pre-loaded.
Extrapolate the straight-line portion of the transfer function, e.g. -35 dBm to -20 dBm, to the
maximum desired power. With the RF set to this power, adjust P7 until the output voltage is at the
value given by the extrapolation (V
MAX
). Re-run the transfer curve, which should now be
acceptable. If the curve is too high at powers near the point at which the original curve began to
roll off, connect a pot from pin 10 to ground, and adjust it remove the bulge. Replace with R110.
Fig. 5.3.8
If you do not get a satisfactory curve, check the following:
a) A1, A2 and A3 outputs should be within 2 mV of ground with no input power;
b) The detector output as a function of input power (check this at the output of A1);
c) Check that A1 is not limiting before maximum power is reached;
d) If the problem is at low or intermediate powers, make sure the gain of A2 is 4
0.2, the
gain of A3 is 16
1, and that both A2 and A3 limit above 450 mV; and/or
e) Make sure the output amp is not limiting from current overload in the event you are not
using an external drive transistor.
-17-
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