M8SL2-00-002-812 T855 Circuit Operation B2.5
© Copyright Tait Electronics Limited August 2004. All rights reserved.
2.3 Mixer
(Refer to the front end circuit diagram (sheet 4) in Section 6.3 and Figure 2.2.)
IC410 is a high level mixer requiring a local oscillator (LO) drive level of +17dBm (nom-
inal). The voltage controlled oscillator (VCO) generates a level of +21dBm (typical) and
this is fed to the mixer via a 5dB attenuator pad. A diplexer terminates the IF port of the
mixer in a good 50Ω, thus preventing unnecessary intermodulation distortion.
2.4 IF Circuitry
(Refer to the IF section circuit diagram (sheet 3) in Section 6.3 and Figure 2.2.)
Losses in the mixer are made up for in a tuned, common gate, post mixer amplifier
(Q310). Several stages of amplification and filtering are employed in the IF circuitry. The
first crystal filter is a 4-pole device (&XF300) which is matched into 50Ω on both its
input and output ports. This stage is followed by a common base amplifier (Q320)
whose output is matched into a 2-pole crystal filter (&XF301). The signal is then ampli-
fied using a high gain MOSFET amplifier (Q350), after which the signal is mixed down
to 455kHz with the second crystal local oscillator (44.5455MHz).
The 455kHz signal is filtered using a 6-pole ceramic filter (&XF302) before being limited
and detected. Q340 provides a buffered 455kHz output for use with the optional RF
level detector (RSSI)
The second IF mixer, limiter and detector is in a 16-pin IC (IC310). Quadrature detection
is employed, using L390, and the recovered audio on pin 9 of IC310 is typically 1V
pp
for
60% system deviation.
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