Receiver Rf Circuitry - Vhf Reciter; Front End; Mixer; If Circuitry - Tait TB8100 Service Manual

2.3

Receiver RF Circuitry - VHF Reciter

Figure 2.5 Reciter VHF receiver RF circuitry block diagram
RF In
Low Pass
Filter
Post-mixer
Buffer
2.3.1

Front End

2.3.2

Mixer

2.3.3

IF Circuitry

38 Reciter Circuit Description
Band Pass High Pass
Filter 1 Filter
Narrow Bandwidth
Crystal Filter
Attenuator
Pad
Switch
Wide Bandwidth
Crystal Filter
The incoming signal from the BNC connector is fed through a low pass
filter, then through a band pass "doublet" filter, and finally through a high
pass filter. These networks attenuate harmonics and spurious responses.
The signal is then amplified and passed through another band pass "doublet"
filter before being passed to the mixer.
The RF signal from the front end is converted down to the 16.9MHz IF by
a high level (+17dBm local oscillator) mixer. The voltage controlled
oscillator (VCO) generates a level of 20dBm which is fed to the mixer
through an attenuator pad. A diplexer terminates the IF port of the mixer
in 50Ω, thus ensuring a good match for all mixing products, as well as
enhancing the linearity. The post-mixer buffer amplifier provides gain and
isolation between the mixer and crystal filter. It also compensates for the
insertion loss of the crystal filter.
The signal from the mixer is fed to the IF amplifier through a 2-pole crystal
filter which provides protection from strong off-channel signals. Note that
there are two 2-pole crystal filters, one for narrow bandwidth and one for
RF Band Pass
Amplifier Filter 2
Amplifier
Switch
© Tait Electronics Limited September 2006
Local Oscillator
Diplexer
Mixer
IF Anti-alias
Filter
IF Output
TB8100 Service Manual