Circuit Descriptions, Abbreviation List, and IC Data Sheets
EN 143EM6E 9.
Multiplier and Power Amplifier
Figure 9-19 Multiplier and power amplifier
The LO signal is multiplied by two in a separate stage (7105) in
order to get a high isolation between the local oscillator and the
antenna. There is quite some filtering necessary at the output,
necessary to prevent the oscillator frequency and its harmonics
from reaching the power amplifier (PA) stage.
There is also a 2-stage band stop filter followed by an elliptic
low pass filter. This has to do with the legal requirements for
spurious radiation.
The PA stage with 7106 increases the power level to about +15
dBm at the output of the filter. The transistor is polarised into
class A for minimum harmonic content and furthermore there is
output filtering available to further reduce the harmonics. The
configuration differs with the frequency version.
The 864 MHz version uses an integrated antenna on the board.
The printed dipole is driven symmetrically by a balun for
optimum power transfer and symmetry. There are protection
diodes (6110 & 6111) foreseen but not stuffed for ESD when
applying an external antenna (the 433 MHz version uses an
external antenna).
Power Supply and Power Down
The board is having a low drop voltage regulator 7117, which
has an output voltage of 8.3 Vdc. The 5 V supply is extracted
via transistor 7119.
The "power down detection" is done via 6105 and 7118: when
the voltage drops, transistor 7118 comes out of saturation
rapidly, thereby putting the stereo coder in Hi-Z position (idle).
No AF output will be there from the stereo coder.
Suitable timing signals for the sub carrier and pilot frequency
are extracted out of a 12 MHz crystal by division with 256 and
512 respectively. The whole chip 7110 can be enabled /
disabled via the synthesiser port P2 (Vpil, "low"= enable). The
outputs are square wave 5 V compatible.
The multiplex output from the stereo coder is fed into a limiter
amplifier 7116B, which sets a limit to the output voltage. In case
the audio input signals increase towards 1.3 V, the signal will
be flat-topped and limited symmetrically. This of course
introduces distortion, but is necessary in order to observe the
bandwidth limitations as set by legal standards. The base-
emitter junctions of 7120 and 7121 form the limits.
This output signal is then summed with the pilot signal, which
is injected just as a plain square wave. The amplifier 7116A
forms a second order low pass filter that cuts at about 90 kHz.
This reduces the harmonics present in the composite output
signal. This signal is actually fed via trimpotmeter 3179 towards
the varicap in order to achieve FM modulation. Trimpotmeter
3179 is aligned for a deviation of 50 kHz. There are three ways
to align the trimpotmeter:
• Use a FM modulation analyser meter such as FAM
(Rohde und Schwartz) or similar.
• Put the RF output on a spectrum analyser and connect
both audio inputs together at 0.41 V/400Hz at the inputs of
the TX. There are two peaks visible on the screen. Align
until the difference between the -3dB peaks is 2 * 50 kHz
or 100 kHz.
• Use an accompanying receiver and while modulating both
inputs of the TX, align until the measured audio output of
the RX is 1.3 Vrms.
Receiver
Block Diagram
Figure 9-20 Block diagram receiver
Antenna Input
The antenna input is tuned for a 17 cm telescopic antenna. The
input is ESD protected by diode 6602. The SAW filter 1602
protects the receiver for out of band interference.
8n2
2127
3p9
5109
BFG520
7106
2121
220p
47p
2124
10K
3118
2185
22p
4101
5105
12n
2128
2p2
3125
68K
5116
8n2
2190
3p9
7105
BFG520
AERIAL BRACKET
0002
100R
3123
2205
47p
47p
2123
2p2
2131
2p2
2210
5n6
5118
8p2
2186
4104
6111
BAV99
BC857B
7104
6103
BZX384-C4V7
2188
3p3
2187
5p6
6n8
5104
F113
4102
5117
39n
2192
47p
5106
68n
4p7
2200
330R
5107
4n7
2207
3p9
0p68
2208
5129
8n2
8n2
5128
2206
3p9
2p7
2125
6p8
2130
3117
47K
3124
150R
2p7
2132
27n
F114
2193
47p
5120
F115
6K8
3120
2191
5p6
2189
3p9
6110
BAV99
2126
47p
33n
5121
1102
RT-01T
3122
2K2
8n2
5112
2p7
+8b
+8b
2129
Vosc
+8b
CL36532008_072.eps
130503
Downconverter
Right out
10V supply
synthesiser TSA5060
PIC12C508A
supply
powersave
I2C data
I2C clock
LA1836M
IF receiver
Stereo decoder
SA572D
expander
NJM4565M
Amplifier
Mute
Left out
4 position channel switch
pin 6
pin 7
pin 4
pin 5
Tuned
pin 2
MUTE
Powersave
pin 3
Stereo
Powersafe supply
CL36532008_078.eps
130503
To Next Page
Loading...
