Tait T850 II Series User Manual

M850-00 T855 Circuit Operation B2.5
Copyright TEL 31/09/98
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 50 ohms, 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 50 ohms 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 (Q330), 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 p-p
for 60% system deviation.

Tait T850 II Series Specifications

General

Brand	Tait
Model	T850 II Series
Category	Accessories
Language	English