DANIELS
ELECTRONICS LTD.
TM
3-4
IM10-VR3H040-AB VHF Enhanced Extra Wideband Receiver Instruction Manual
Operational amplifier U19A is AC coupled to the recovered audio line providing the standard
6 dB per octave de-emphasis response from 300 Hz to 3 kHz. The de-emphasis audio output can
be routed, via jumpers JU13, JU14 and JU15, through a voice band filter and/or connected
directly through the audio squelch gate U7.
Operational amplifier U8A provides adjustable (R66) flat audio gain. With a standard input
signal applied (-70 dBm on-channel signal modulated with a 1 kHz tone to 60% system
deviation) , the flat audio output of U8A can provide up to 7 Vpp into a 10 k
Ω
load connected to
pin B22 of the main 48 pin connector.
Operational amplifier U8B provides adjustable (R58) de-emphasis audio gain. With a standard
input signal applied, the de-emphasis audio output of U8B can provide up to 7 Vpp into a 10 k
Ω
load connected to pin B20 of the main 48 pin connector.
High level drive IC U9 is used to drive the 600
Ω
line transformer T1 through jumper JU22 to a
level of +3 dBm. With JU19 installed, U9 can also be used to drive an external 4 to 16
Ω
speaker connected to main connector pin B4 with up to 750 mW audio power. Jumpers JU16
and JU17 select the high level drive source from either the de-emphasis or flat audio signal path.
Potentiometer R64 adjusts the gain of the high level drive to a maximum undistorted level of
+3 dBm into a 600
Ω
load connected to main connector pins B26 and Z26. Jumper JU24, when
installed, unbalances the 600
Ω
line transformer for single ended audio routing applications. For
low power applications, a 4 mA reduction in receiver current is possible by removing jumper
JU18, thus disabling the high level amplifier U9. Outputs of operational amplifiers U8B (de-
emphasis) or U8A (flat audio) can then be used to drive the 600
Ω
line transformer by installing
jumper JU20 or JU23, provided that the 600
Ω
line transformer output is adjusted for an output
less than -8.0 dBm.
3.1.8
Post Discriminator Filtering
The 21.4 MHz IF / Audio Main Board circuitry includes two active filters; a 4 pole Butterworth
low pass subtone filter with a cutoff frequency of 250 Hz and a 6 pole voice band high pass
Butterworth filter with a cutoff frequency of 300 Hz. Both active filters are normally disabled to
conserve supply current unless required for a particular application. Operational amplifiers U4A
and U4B form a 4 pole subtone low pass filter that is used to remove modulation components
above 250 Hz. Enabled via installation of jumpers JU6 and JU7, the subtone filter provides an
output at main connector pin B10 that can be used to drive external sub-audible tone decoders.
Alternatively, with the installation of jumper JU38, the subtone filter output can be output on pin
B-14.