DANIELS
ELECTRONICS LTD.
TM
3-2
IM10-VR3H040-AB VHF Enhanced Extra Wideband Receiver Instruction Manual
3.1.4
First IF Amplifier
MOSFET Q1 provides linear 21.4 MHz IF amplification, while crystal filters XF1 through XF4
provide the optimum bandpass characteristics for good selectivity and low distortion.
Potentiometer R13 is used to bias Q1 to an operating point drain current of 7.5 mA. The 21.4
MHz IF amplifier provides an overall gain of approximately 14 dB, including crystal filter
losses.
3.1.5
Second IF / Demodulation
The 21.4 MHz output of the IF amplifier is fed into the second mixer of U2, a low power, FM IF
IC. This IC consists of an oscillator, mixer, limiting IF amplifier, quadrature discriminator, and
received signal strength indicator (RSSI). A second local oscillator source of 21.855 MHz,
derived from a fundamental mode on-chip oscillator (crystal X1), mixes with the 21.4 MHz first
IF to produce a 455 kHz second IF. Second IF filtering is achieved through the use of CF1, a
multi-element 455 kHz ceramic filter. Inductor L5, capacitor C137 and resistor R16 form the
passive quadrature discriminator tuned circuit. Recovered audio is output on pin 9 of IC U2.
3.1.6
Received Signal Strength Indicator (RSSI)
A received signal strength indicator (RSSI) output is provided at pin 13 of IC U2. This pin
sources up to 60 µA over a linear 60 dB of input signal range. Operational amplifier U3A
translates the RSSI current to a +1 to +5 Vdc output range that is made available at B24. This
signal can be used for accurate signal strength measurements making the RSSI function useful
for path diagnostics or receiver voting systems. Operational amplifier U3B inverts, scales and
level shifts the RSSI output signal to achieve a signal range suitable for triggering the receiver
squelch on the basis of received signal carrier strength.
It should be noted that noise triggered squelch is normally employed in the receivers due to less
variation in squelch threshold caused by changes in received signal modulation. RSSI based
squelch does however offer squelch hysteresis ranges of up to 60 dB which may find limited
application in specialized radio systems.
Jumpers JU30 and JU31 provide selection of the desired squelch source signal option. RSSI
based squelch also improves the unsquelch (turn on) time for narrow band receivers driven by an
over-deviated signal.