Philips EM6E Service Manual page 121

EN 160 9. EM6E AB
Multiplier and Power Amplifier
F113
4102
+8b
5116
2192
7105
47p
BFG520
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 (V_pil, "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 trim potmeter 3179
towards the varicap in order to achieve FM modulation. Trim
Circuit Descriptions, Abbreviation List, and IC Data Sheets
+8b
7104
BC857B
5118
8n2
5n6
2188 2189
2185 2186
22p 8p2
3p3 3p9
Figure 9-19 Multiplier and power amplifier
+8b
2126
3125
68K
47p
F114
2127 2128 5112
8n2
3p9 2p2
7106
5107
BFG520 4101
4n7
potmeter 3179 is aligned for a deviation of 50 kHz. There are
three ways to align the trim potmeter:
•
Use a FM modulation analyser meter such as FAM (Rohde
and Schwarz) 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 V_rms.
9.7.4
Receiver
Block Diagram
Downconverter
synthesiser TSA5060
I2C data I2C clock
10V supply
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.
1102
RT-01T 0002
AERIAL BRACKET
4104
2131 2206
2p2 3p9
6110
BAV99
5129
6111
8n2
BAV99
CL36532008_072.eps
LA1836M
SA572D
IF receiver
expander
Stereo decoder
Tuned Stereo
MUTE
pin 2
pin 6
pin 7
PIC12C508A
pin 4
pin 5
supply
powersave
4 position channel switch
130503
Powersafe supply
Left out
NJM4565M
Amplifier
Right out
Mute
Powersave
pin 3
CL36532008_078.eps
130503