DANIELS
ELECTRONICS LTD.
TM
VHF Enhanced Extra Wideband Receiver Instruction Manual IM10-VR3H040-AB
3-5
Operational amplifiers U5 and U6 form a 6 pole voice band high pass filter that is used to
remove modulation components below 300 Hz. Enabled by the installation of jumpers JU8 and
JU9, the voice band high pass filter provides an output that can be routed through either the flat
or de-emphasis audio paths via jumpers JU12 or JU15. The primary purpose of the voice band
high pass filter is for the effective removal of low frequency modulation components, such as
sub audible tones (CTCSS) from the receiver audio path. This may be required in certain
repeater applications where sub-audible tones are detected and regenerated independent of the
receiver.
3.1.9
Squelch Circuitry
The 21.4 MHz IF /Audio board has two squelch detection methods: one based on noise operated
squelch and the other based on carrier level (RSSI) operated squelch. Both methods provide the
COR function (Carrier Operated Relay) in a flexible and efficient manner. Noise operated
squelch is the preferred and standard method normally employed.
The demodulated signal at pin 9 of U2 contains noise, second local oscillator bleed through, and
the desired voice band demodulation products. Noise based squelch works on the simple
principle that receiver noise power varies with received signal strength (FM quieting effect).
The demodulated signal from pin 9 of U2 is filtered by a selective passive filter consisting of
C106, C107, C108, L7 and L8. This filter removes modulation components below 10 kHz,
leaving band limited noise from 10 kHz to 100 kHz. It is important to remove modulation
components below 10 kHz in order to prevent variations in squelch trigger point due to
modulation changes in the received signal.
Band limited noise is then presented to operational amplifier U13A, where it is amplified with
gain as a function of ambient temperature. This temperature compensation is necessary to
account for changes in receiver noise over the wide operating temperature range capability of the
Receiver family. The temperature compensated, band limited noise is connected to a "True
RMS" power detector U14; the output of which is directly proportional to RMS noise power.
Operational amplifier U16A is configured as a comparator monitoring the output of power
detector U14. The trigger point of this comparator is established by the DC level at the non
inverting input (pin 3). This level is set by the "Squelch Threshold Adjust" potentiometer R88
from the 2.5 Vdc reference U15. As the level of a received signal increases, the noise
power decreases, which results in a lower output from the RMS detector U14. This causes the
output of comparator U16A to go to the positive rail (approxi8.5 Vdc) turning on
transistor Q9 which further lowers the level at the inverting input of U16A by an amount
established by the "Squelch Hysteresis Adjust" potentiometer R115. The positive feedback set
up by Q9 requires the received signal to decrease in level (increasing noise power) to a point
higher than the original squelch trigger point before the receiver will return to a squelched
condition, indicated by U16A's output going to the negative supply rail (approximately -5.7
Vdc).
