This section will discuss some of the more detailed tests carried out on the final circuit which
was discussed in section 5. Graphs and pictures will be used to aid in the final analysis of the
Design.
The equipment used in analysing circuitry is vital in yielding the correct information about the
advantages and disadvantages of any design. During the course of final test the equipment used
were a spectrum analyser, a frequency meter, digital multi-meter, an analogue and digital FM
radio receiver was used.
A spectrum analyser is exactly what its name implies, it shows the frequency response over a
specified width in the frequency domain. The spectrum analyser that was used, was the MS610C
from Antristu. The Antristu has a dynamic range of 9Khz to 2 GHz. The spectrum analyser was
used to view the varying effects of the carrier when it was modulated by the baseband audio signal.
The signal strength was also measured using the analyser, using a conversion formula that will be
shown later.
The frequency meter used was the GX240 from ITT instruments. It has a maximum frequency of
120MHz. This was used to check the output frequency of the oscillator. To effectively use it, an
impedance of 20W was attached to the meter's probe and
placed at the output of the transmitter, this was used to properly match the 50W
transmission line of the probe with that of the transmitter
output.
An analogue (dial turn) and a digital (push-button) receiver was used in demodulating the modulated carrier
wave generated by the transmitter.
The unmodulated carrier spectrum at 100MHz
The modulated carrier spectrum at 100MHz
The y-axis of the spectrum analyser measures the power of the output wave in milli-decibels, the formulae for converting
either way from mill-watts to milli-decibels are given below
All measures were taken with a 20 W impedance onto the end of the probe to ensure the matching
of the 50W transmission line to the output impedance of the oscillator.
|
Supply Voltage(Volts) |
Supply Current(mA) |
Output Power(dBm) |
Output Power(mW) |
|
9 |
8.37 |
8.1 |
6.456542 |
|
8 |
7.31 |
7.1 |
5.128614 |
|
7 |
6.24 |
6 |
3.981072 |
|
6 |
5.23 |
3.5 |
2.238721 |
|
5 |
4.27 |
1.88 |
1.5417 |
|
4 |
3.33 |
-1.42 |
0.721107 |
The table above shows the supply current and output power as a result of the dc supply of the
PP3 battery decreased during continuous operation over 18 hours.
It can be easily seen from the 2 graphs above that the supply current decreases linearly as the supply
voltage supplied by the battery decreases, also the output power is decreases as voltage supply to the
transmitter decreases. The typical voltage input from the battery source will be 9.2V in the first 30 minutes
of operation decreasing to the 8.8volts to 7 volts band in the next 14 hours of continuous operation
and decreasing to the 7 to 4 volts band of operation in the following 6 hours. This last band of voltage
operation produced fairly low levels of transmission. The Middle band 7 to 8.8 volts emitted a very
strong transmission indeed, which lasted for 14 hours, this is the main feature that make the PP3 such
a popular battery for intensive dc supply to "walkie talkie's", because of this a stable transmission over
a long period of time can be sustained.
