E.F. Johnson Company 5100 ES, Service Manual, Page: 49 / 180

January 2007 5100 ES Series Portable Radio Service Manual 5-5
Circuit Description
5.2.1 Receiver
5.2.1.1 Front End Bandpass Filter
A harmonic filter is followed by a PIN diode transmit/receive switch. Following the
switch, two fixed tuned bandpass filters are used in the front-end of the receiver.
Depending on the desired receive band, the appropriate filter is selected using RF switches
(U37/U41). This filter provides first-image rejection with minimal loss in order to provide
the desired level of receiver sensitivity. A variable attenuator, which follows the filter,
increases the dynamic range of the receiver when receiving high-level signals.
5.2.1.2 Front End LNA and Bypass Switching
The Low Noise Amplifier (LNA) is critical in determining the overall noise figure of the
receiver chain. An MGA-71543 amplifier (U39) provides optimum noise figure, gain,
intercept point, and power consumption.
5.2.1.3 Post-LNA Bandpass Filters
Additional bandpass filters are positioned after the LNA. These filters are identical to the
front-end filters previously described. RF switches U42 and U40 are used to select
between the two bands. These filters provide additional image rejection.
5.2.1.4 Mixer and LO Filter
A double-balanced, low-level ADEX-10L mixer (MX1) with a LO drive level of +4 dBm
is used for the first conversion. This mixer provides good dynamic range with a 3 dB
lower LO drive than the more tradi7 dBm drive mixers. This reduces power
consumption and also the conducted and radiated local oscillator leakage from the
receiver.
For the low band UHF version, a high-side mix is used for the 380-444 MHz receive band
and a low-side mix is used for the 444-470 MHz receive band. For the high band UHF
version, a high-side mix is used for the 450-455 MHz receive band and a low-side mix is
used for the 455-520 MHz receive band. This band plan reduces the tuning range
requirements for the VCOs.
A LO filter is used prior to LO port of the mixer to reduce the impact of wideband noise
from the LO synthesizer on the receiver sensitivity.
5.2.1.5 IF Filter and Amplifier
A two-pole 64.455 MHz crystal filter (U2) is used to provide the desired level of adjacent
channel rejection while providing minimal amplitude and phase distortion within a 25