Ramsey Electronics FX-440 Manual page 90

St_age 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 420 - 450 MHz range before they
reach the 1st mixer input of U1 (pin 20). 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, L12,23,24 and C72 form a LOW pass
filter, suppressing unwanted signals that are higher than the desired
receiving range.
The RF present at C27 now strongly favors signals that are lower than
460 MHz. This RF is amplified by 03, a fairly tiny device favored for its
high gain and low noise. At this point, the desired 420 - 460 MHz RF has
been both filtered and amplified. It could be applied now to U1 with
reasonable results. However, note that we've been using words like
"suppress" and "favor,• not absolutes like "reject.• Unwanted RF is still
lurking in the background and could result in unwanted "ghost• signals
throughout the tuning range of the receiver.
Components C20,26-28,30,31, L2-6 form a HIGHPASS filter, which sets
lower limits on the RF passing from 03 to 02, 17 for further amplification.
The RF from C78 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 •+BR" and "BT" 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 L 17, which prevents the antenna RF from
being shorted to ground.
During Transmit, D7 passes RF from the transmitter to the antenna, and
L 17 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.
Assm. 30
FX-440