Tait T860 Series II, Service Manual

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M860-00-Preliminary
T865 Circuit Operation B2.5
Copyright TEL 30/06/98
2.3 Mixer
(Refer to the front end circuit diagram (sheet 4) in Section 6 and Figure 2.2.)
IC420 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 the IF section circuit diagram (sheet 3) in Section 6 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 (&XF310 and &XF320) which is matched into 50
ohms on both its input and output ports. This stage is followed by a two-stage amplifier
(designed as a 50 ohm block) and second crystal filter (2 pole, &XF330), after which the
signal is mixed down to 455kHz with the second local oscillator (20.945MHz) by IC340.
The 455kHz signal is filtered using a six-pole ceramic filter (&XF340) before being lim-
ited and detected.
The second IF mixer, limiter, detector and RSSI is in a 16-pin IC (IC350). Quadrature
detection is employed, using L360, and the recovered audio on pin 7 of IC350 is typi-
cally 0.3V p-p for 60% system deviation.