Ramsey Electronics FZ-146 Specifications download pdf (Page 89)

FX-146 • 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.

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Ramsey Electronics FZ-146 Specifications Page 89