38 Reciter Circuit Description TB8100 Service Manual© Tait Electronics Limited September 20062.3 Receiver RF Circuitry - VHF Reciter2.3.1 Front EndThe incoming signal from the BNC connector is fed through a low passfilter, then through a band pass “doublet” filter, and finally through a highpass 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.2.3.2 MixerThe RF signal from the front end is converted down to the 16.9MHz IF bya high level (+17dBm local oscillator) mixer. The voltage controlledoscillator (VCO) generates a level of 20dBm which is fed to the mixerthrough an attenuator pad. A diplexer terminates the IF port of the mixerin 50Ω, thus ensuring a good match for all mixing products, as well asenhancing the linearity. The post-mixer buffer amplifier provides gain andisolation between the mixer and crystal filter. It also compensates for theinsertion loss of the crystal filter.2.3.3 IF CircuitryThe signal from the mixer is fed to the IF amplifier through a 2-pole crystalfilter which provides protection from strong off-channel signals. Note thatthere are two 2-pole crystal filters, one for narrow bandwidth and one forFigure 2.5 Reciter VHF receiver RF circuitry block diagramBand PassFilter 1Band PassFilter 2RFAmplifierLow PassFilterHigh PassFilterAttenuatorPadMixerLocal OscillatorDiplexerPost-mixerBufferNarrow BandwidthCrystal FilterWide BandwidthCrystal FilterIFAmplifierAnti-aliasFilterIF OutputRF InSwitch Switch
