Pulsar TCF-10B, System Manual, Page: 146 / 249

The first mixer (I
3
) is a high level type that can
withstand large input levels without creating inter-
modulation products. The oscillator level, injected
at pins 7 and 8 of the mixer, is +17 dBm (1.6
Vrms). The 5.02 MHz mixer output, on pins 1 and
2, is fed through a diplexer to the first IF
Amplifier (Q1/Q2 and associated components).
The diplexer provides 50 ohm termination to all
frequencies, but only passes frequencies below 6
MHz. The first IF Amplifier is a high level, low
distortion amplifier capable of delivering 100
milliwatts maximum into R24. The amplifier has
approximately 14 dB of power gain. A matching
attenuator network (R69, R70 and R71) estab-
lishes the proper driving impedance for filter FL1.
This network has 6 dB loss.
Filter FL1 determines the bandwidth of the
receiver. Extra Wide Band (approximately 1.2
kHz), Wide Band (approximately 600 Hz) and
Narrow Band (approximately 300 Hz) are
available. The stop band attenuation of the filter is
greater than 60 dB. FL1 has about 3.5 dB loss. The
output of the filter is fed to the 2nd IF amplifier
(see Schematic sheet 2, Figure 14-4). A second
path from the 1st IF Amplifier is fed to Q3 and Q4
and associated components, which form a buffer
amplifier to provide an auxiliary output for the
optional voice adapter. This output is brought to
connector pins A/C-24. R67 is an adjustment for
setting the voice output level.
The combination of I
4
, I
5
, T2 and associated
components form the 5.02 MHz IF amplifier
(sheet 2). Amplifier I
5
is resonated by L15 and
C42; R68 adjusts the overall gain of the I
4
-I
5
combination to present the proper level to the
second mixer (I
6
). I
4
and I
5
have a combined
voltage gain of approximately 54 to 84 dB,
depending on the setting of R68. Transformer (T2)
has a 16 to 1 impedance ratio and matches the
50 ohm mixer input (pins 1 and 7) to the 800 ohm
output of I
5
and R40. There is a voltage loss of
18 dB from I5 pins 7 and 8 to the input of the
mixer.
The second mixer (I
6
) has a 5.02 MHz input from
the IF amplifier and a 5 MHz input from the
crystal oscillator. The output from the second
mixer, 20 kHz, is fed through a bandpass filter to
the output on pin A-28. This mixer has a 5 MHz
injection of +7 dBm (.5 Vrms) applied to I
6
pins 2
and 8. The 5.0 MHz reference crystal oscillator is
adjusted to 5 MHz by C68, and consists of QN1
(A, B, C and D). Because the input is 5.02 MHz
and the injection frequency is 5.00 MHz, the
mixer output is 20.0 kHz plus other mixer
products. An active bandpass filter is configured
from the combination of I
7
and associated compo-
nents. It is used to drive the receiver output, at
connector pin A-28. This filter is tuned to a center
frequency of 20 kHz with approximately 4 kHz
bandwidth. The bandpass filter has 32 dB voltage
gain.
On-board voltage regulation and reverse voltage
protection is provided by D3, D4, D5, D6 and
associated components. All functional circuit
blocks operate from +18.6 and +3.6 or -18.6 and -
3.6 to p15 Vdc for operation. The
synthesizer uses both plus and minus 18.6 and 3.6.
The synthesizer plugs onto the PC Board with J1,
J2 and J3. You may remove it by unscrewing the
three hold down screws and unplugging it.
RF filtering is provided by L6, L7, L12, L13, L18
and L19 (Sheet 1) and by R28, R30, R31 and R34
(Sheet 2) to prevent stray coupling from circuit to
circuit. All RF bypassing is to common.
14.3 Receiver Troubleshooting
With the PC Board plugged into the chassis, you
can check the following functions:
14.3.1 Input Signal
You can use the following three test points on the
control panel to indicate if a signal is getting to the
module:
TJ1 Line Input
TJ2 Mixer Input
TJ3 Common
April 1996 Page 14–3
Chapter 14. Receiver Module & Synthesizer Module
14