TDA5250D2 Datasheet, PDF (44/97 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

confidential
TDA 5250 D2
preliminary
Applications
An approximation of the losses of the input matching network can be made with
the formula:
Loss
=
−20 ∗
log_{}1 −
Q_L
Q_U

=
−20 ∗ log1 −

7.2
36


=
2dB
[4 – 4]
The noise figure of the LNA-input-matching network is equal to its losses. The
input matching network is always a compromise of sensitivity and selectivity.
The loaded Q should not get too high because of 2 reasons:
more losses in the matching network and hence less sensitivity
tolerances of components affect matching too much. This will cause problems
in a tuning-free mass production of the application. A good CAE-tool will help to
see the effects of component tolerances on the input matching more accurate
by tweaking each value.
A very high selectivity can be reached by using SAW-filters at the expense of
higher cost and lower sensitivity which will be reduced by the losses of the
SAW-Filter of approx. 4dB.
Image-suppression:
Due to the quite high 1^st-IF of the frontend, the image frequency is quite far
away. The image frequency of the receiver is at:
f _IMAGE = f _SIGNAL + 2 ∗ f _IF = 868.3MHz + 2 * 289.4 = 1447.2MHz ^{[4 – 5]}
The image suppression on the evalboard is about 16dB.
LO-leakage:
The LO of the 1^st Mixer is at:
f _LO
=
f _RECEIVE
*
4
3
=
868.3MHz
∗
4
3
= 1157.73MHz
[4 – 6]
The LO-leakage of the evalboard on the RF-input is about –98dBm. This is far
below the ETSI-radio-regulation-limit for LO-leakage.
Wireless Components
4-4
Specification, July 2002