Copyright TEL
31/01/96
2.3
Mixer
(Refer to Figure 2.2.)
IC301 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 +22dBm (typical) and
this is fed to the mixer via a 2dB attenuator pad. A diplexer terminates the IF port of the
mixer in a good 50 ohms, thus preventing unnecessary intermodulation distortion.
2.4
IF Circuitry
(Refer to Figure 2.2.)
Losses in the mixer are made up for in a tuned, common gate, post mixer amplifier
(Q303). Several stages of amplification and filtering are employed in the IF circuitry.
The first crystal filter is a 4-pole device (&XF301) which is matched into 50 ohms on both
its input and output ports. This stage is followed by a two-stage amplifier which is
designed as a 50 ohm block, after which the signal is mixed down to 455kHz with the
second local oscillator (20.945MHz).
The 455kHz signal is filtered using a six-pole ceramic filter (&XF303) before being lim-
ited and detected.
The second IF mixer, limiter, detector and RSSI is in a 16-pin IC (IC303). Quadrature
detection is employed, using L319, and the recovered audio on pin 7 of IC303 is typi-
cally 0.3V p-p for 60% system deviation.
Summary of Contents for T830 Series
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