Ramsey Electronics FX-440 Manual page 84

ADDITIONAL RECEIVER IC FUNCTIONS AND FEATURES
In addition to audio output at pin 12, a separate high speed data output
(up to 35000 baud) is available at pin 14 and may be linked directly to a
packet TNC or other data controller using the jumper wire pad provided
on the PC board. The MC3335 is capable of detecting true FSK
(frequency shift keying) in addition to AFSK
The receiver has good "hysterisis" characteristics, the ability to hold the
squelch open once it has been broken by a marginal signal, even if the
signal becomes weaker. Hysterisis is established by R6. The IC also has
meter drive and carrier detect circuitry which is put to good use in the
transceiver design by providing very effective squelch action. The
squelch is activated by signal strength, not by noise. When no carrier is
detected, the voltage at pin 1 O is high, which causes 06 to mute the
audio amplifier U2 as explained in Stage 8.
When a carrier is detected, the voltage at pin 1 O drops low, turning off
06. Resistor R13 permits squelch adjustment. If actual measurement of
relative carrier strength is desired, suitable metering circuitry may be
connected to the RSSI output point marked on the PC board (Received
Signal Strength Indicator). 60 db. of RSSI range is available. Finally, the
carrier detect circuitry affords the same COR ("Carrier Operated Relay")
action as needed in any repeater, which is why the output of pin 1 O is
also available on the PC board, designated "COR. • It is up to creative
users to determine their own practical implementations for this feature.
Stage CR
ASSEMBLY PROCEDURE:
( ] [ Jl
CR1. Install the socket supplied for U1, the MC3335
20-pin DIP FM Receiver IC. Use the same care in soldering the
socket as if you were soldering the IC directly. Make sure all 20
pins are visible through the holes before soldering. Use the same
care in inserting U1 into the socket and be very sure that the
notched end is oriented as illustrated.
Install the following resistors:
7
( ] (
\~ 1 CR2. R11, 1 OK ,,lt;>rown-black-orange).
/
[ ] ( VJ
r
CR3. R5, 1 OK (brown-black-orange).
i ..-
/'
/
I I
r·
I
GR4. R4, 33K (<?range-orange-orange).
I
J [ \1/cRs. R24, 1 OK (qrown-black-orange).
[ ] 1'J
CR6. R9, 4.3 or 4. ?megohm (yellow-orange-green or
yellgw~viol~t-green).
I ]
k.,YtR?. R3, 1 K (brown-black-red).
Assm. 24
FX-440