Mixer; If Circuitry - Tait T880 II Series Adjustment Manual

M880-00
2.3

Mixer

(Refer to the front end circuit diagram (sheet 4) in Section 6.2 and
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 +20dBm (typical) and
this is fed to the mixer via a 3dB 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 6.2 and
Losses in the mixer are made up for in a tuned, common gate, post mixer amplifier
(Q300). 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 50 ohms on its
input and directly to the impedance of the next stage on its output. This stage is fol-
lowed by a cascode amplifier (Q302) whose output is matched into a 2-pole crystal filter
(&XF302). The signal is then mixed down to 455kHz with the second crystal local oscil-
lator (44.545MHz).
The 455kHz signal is filtered using a 6-pole ceramic filter (IC345) before being limited
and detected.
The second IF mixer, limiter and detector is in a 16-pin IC (IC300). This IC also provides
an RSSI signal on pin 13. Quadrature detection is employed, using L345, and the recov-
ered audio on pin 9 of IC300 is typically 1V p-p for 60% system deviation.
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T885 Circuit Operation
Figure 2.2.)
Figure 2.2.)
B2.5
10/07/00