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3.3 Performance Tests
3.2.3 Adjusting the freq adjust
3.3 Performance Tests
SIM911 BJT Preamp
The clocking of the microprocessor is done by an RC oscillator, which
must be adjusted to 1% of 2.5 MHz in order to insure the serial baud
rate is 9600 baud. This is done with the "Freq Adjust" trimcap
(C402). Locate C402, the "Freq Adjust" trimcap, on the inside of
the SIM911. Locate the Frequency Test Point, TP401, and connect
a probe to monitor the clock signal. To override the clock-stopping
circuitry, depress and hold down one of the front-panel buttons.
Adjust the "Freq Adjust" trimcap to make the frequency at the test
point 2.5 MHz.
The following curves are typical noise density vs. frequency for the
SIM911.
50
20
10
5
2
1
0
1
10
10
Figure 3.1: The SIM911 noise density versus frequency.
These are noise plots with the input grounded internally.
If you are amplifying a small signal far from the SIM900 Mainframe
it could be preferable to run the SIM911 on an external supply, i.e.
the one which is powering your other electronics. To do this, simply
supply 15 V and a signal ground to power the signal electronics
and 5V and Power ground to power the microprocessor. Use the
shield floated option on the module to use the external ground for
G 1
G 10
G 100
2
3
4
10
10
10
Frequency (Hz)
3 – 3
5
10
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