WJ-8611 DIGITAL VHF/UHF RECEIVER
CIRCUIT DESCRIPTIONS
5-5
Table 5-1. Preselector Band Selection
Tuned
Frequency
Preselector
Band
TP1
TP5
TP8
TP6
TP7
TP3
500
-1000
MHz
A
-5V +5V +5V +5V +5V +5V
250 - 500 MHz
B
+5V
-5V
+5V
+5V +5V
+5V
125 - 250 MHz
C
+5V
+5V
-5V
+5V +5V
+5V
60 - 125 MHz
D
+5V
+5V
+5V
-5V +5V
+5V
20 - 60 MHz
E
+5V
+5V
+5V
+5V -5V
+5V
*Bypass
F
+5V +5V +5V +5V +5V -5V
* The Bypass may only be selected when the receiver is tuned below 100 MHz.
In addition to the selection of the tuning band, the preselector tuning
voltage (PRE TV, P1, pin 9) is provided by the Digital Control PC
Assembly (A2) to center the preselector response about the tuned
frequency of the receiver. This analog voltage enters the 1st LO/1st
Converter Assembly as an analog voltage ranging from 0 to +5V. It is
then passed through a scaling circuit that increases the voltage range to 0
to +24 V. Test Point TP2 provides a point to verify the preselector tuning
voltage and the scaling circuit. The actual voltage that is provided is
determined by tuning data stored in non-volatile memory in the Digital
Control PC Assembly (A2). This data is factory selected to produce the
best overall response by the preselector.
From the preselector, the RF spectrum is amplified and filtered prior to
conversion. The lowpass filter is an elliptical filter that provides a flat
response from 0 to 1000 MHz and rapidly rolls off above the 1000 MHz
point. It provides a minimum of 30 dB of attenuation at the receiver’s
1350 MHz IF frequency. The filtering provides improved IF rejection and
also provides reverse isolation to reduce 1st LO radiation at the RF Input.
The 1st Converter mixes the RF input spectrum with the 1st LO signal to
produce the 1st IF signal. The 1st LO signal ranges from 1370 MHz
(when the receiver is tuned to 20.0000 MHz) to 2350.0 MHz (when the
receiver is tuned to 1000.0000 MHz). Tuning is performed in 2.5 MHz
increments. The resultant output is an up-converted IF with the frequency
sense of the spectrum inverted. For example, for RF tuned frequencies
ranging from 20.851 to 23.350 MHz, the 1st LO frequency is fixed at
1372.5 MHz. After the conversion, a 20.851 MHz RF signal will appear at
1351.649 MHz in the 1st IF, and a 23.350 MHz RF signal will appear at
1349.150 MHz. The IF spectrum, with the signals of interest positioned
within the 1349.150 to 1352.649 MHz range, is then bandpass filtered to
select the correct mixing product from the mixer and to limit the overall
bandwidth of the IF to approximately 14 MHz.
Courtesy of http://BlackRadios.terryo.org
Summary of Contents for WJ-8611
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Page 119: ...5 i SECTION 5 CIRCUIT DESCRIPTIONS Courtesy of http BlackRadios terryo org...
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