DANIELS
ELECTRONICS LTD.
TM
VHF Enhanced Extra Wideband Receiver Instruction Manual IM10-VR3H040-AB
4-7
4.7.4
Distortion and Sensitivity
•
Adjust L2, C17, C27, and L5 to get lowest distortion. Experiment with L2 and C17 to
get the best trade-off resulting in lowest distortion. Ensure distortion is < 3.0%.
Expect ~ 1.5% or less for most units.
•
To measure the sensitivity, decrease the generated RF level until the SINAD meter
measures –12dB SINAD. This RF level is known as the ‘sensitivity.’ Slightly tune the
five variable inductors if sensitivity is not within spec. Ensure you observe the shape
of the filter to make sure it is still acceptable after you do this adjustment.
4.7.5
Second IF / Demodulation
Adjustment of the second local oscillator frequency (21.855 MHz) is most easily performed by
employing the communications monitor as a sensitive receiver with the 21.855 MHz signal taken
from a small loop antenna placed near crystal X1. A short length of wire connected to a coaxial
cable works well. Adjust variable capacitor C130 until an oscillator frequency of 21.855 MHz is
achieved.
If the squelch circuitry has been incorrectly adjusted, it may be necessary to depress the squelch
override switch on the receiver front panel continuously (or ground pin Z14) for the following
receiver adjustments.
Terminate the balanced audio output lines B26 and Z26 with a 600
Ω
1/4 watt resistor. Connect
the communications monitor AF high impedance input across this resistor. Adjust potentiometer
R64 (High level drive adjust) to approximately mid-range position. Maximum AF signal and
minimum harmonic distortion should occur simultaneously. All distortion measurements should
be made through a 300 Hz to 3.0 kHz band pass filter to remove low frequency noise originating
from the RF communications monitor signal source. This filter is usually selectable on most
communications monitors. Adjust the high level drive potentiometer (R64) to obtain -8 dBm
(308 mVrms) across the external 600
Ω
termination resistor. A level of -8 dBm is the standard
factory setting for the balanced output.