Ramsey Electronics FX-146, User Manual

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FX 146
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58
[ ] [ ] R43 10K G26
[ ] [ ] R44 22K G20
[ ] [ ] R45 82 ohm 1/2W TX23
[ ] [ ] R46 10K trimpot M19
[ ] [ ] R47 10K G31
[ ] [ ] R48 10K G33
[ ] [ ] R49 10K M9b
[ ] [ ] R50 10K M9a
[ ] [ ] R51 47K M10
[ ] [ ] R52 10K G27
[ ] [ ] R53 22K G21
[ ] [ ] R54 200 or 220 (1/2W) DR3
[ ] [ ] R55 10K G19
[ ] [ ] R56 47K M11
[ ] [ ] R57 2.2K M6
[ ] [ ] R58 270 ohm M8
[ ] [ ] R59 100K M7
[ ] [ ] R60 10K M23
[ ] [ ] R61 2.2K M12
[ ] [ ] R62 10K M24
[ ] [ ] R63 470 ohm M27
[ ] [ ] R64 470 ohm M28
[ ] [ ] R65 1K H4
[ ] [ ] R66 47K E8
[ ] [ ] R67 10K H5
[ ] [ ] R68 1K H1
[ ] [ ] R69 1K H2
[ ] [ ] R70 1K M41
FX-146
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89
Stage DR: Antenna Input and RF Preamplifier
The operation of the FM receiver IC was discussed in Stage CR. The
purpose of the following circuitry is to minimize the strength of unwanted
signals and to boost signals in the 140 - 160 MHz range before they reach
the 1st mixer input of U1 (pin 22). In addition, the circuit includes PIN diodes
for proper T-R (transmit-receive) switching.
Let's follow the signal path briefly from the antenna jack, remembering that
the antenna is "picking up" thousands of signals from all over the radio
spectrum. Capacitor C71,19,72 and L12,22 form a low pass filter,
suppressing unwanted signals that are higher than the desired receiving
range. The filtered signals are coupled through C47 to Q3, a low noise
preamp stage and then on to a band pass filter network consisting of
C30,31,28,104 and L2,5,6. For use in very high RF environments, an
optional helical filter may be installed in place of this band pass filter.
Helical filters provide excellent filtering characteristics, although at a cost -
typically $20 to $25. We leave this option open to you!
The RF present now strongly favors signals that are in the desired frequency
range of interest. This RF is amplified again by Q2 and then applied to U1,
the main receiver IC chip. The RF from the antenna input to U1 now can be
said to be restricted to the desired tuning range of the transceiver. Unwanted
signals have been doubly rejected, and the desirable signals have been
doubly amplified.
Take a look at PIN diodes D2, D6 and D7. These tiny diodes are amazing
devices which have made relays and so forth all but obsolete for T-R
switching purposes. To put it simply, a PIN diode can pass RF energy either
way when it is turned on by DC voltage and also block RF from the other
direction when it is not powered by DC. Just picture RF passing with the
anode "arrow" and picture the cathode band as a barrier.
The symbols "+8R" and "8T" mean that voltage is present at such a point
during Receive or Transmit only, not both at the same time.
During Receive, D6 is "on" and permits RF to flow from the antenna through
C47 to the filter and amplifier stages just discussed. Because any DC device
needs a ground connection as well as +DC, D6 is grounded through RF
choke L17, which prevents the antenna RF from being shorted to ground.
During Transmit, D7 passes RF from the transmitter to the antenna, and L17
again prevents loss of RF to ground.
During transmit, D6 is blocking transmitter RF from the receiver circuit.
For maximum protection of the more delicate receiver circuitry, D2 is
turned on during transmit to short any stray RF directly to ground.