Ramsey Electronics FR1 Assembly And Instruction Manual - page 5
FR-1 • 5
CIRCUIT DESCRIPTION
FM broadcast signals captured by the whip antenna or from the input jack are
applied to the RF amplifier Q2, a high gain, low noise microwave style
transistor. Signals are amplified about 20 dB, that's 100 times! After being
boosted, these signals are routed to the SA602 mixer-oscillator chip for
conversion down to the 10.7 Mhz Intermediate Frequency (IF). The SA602 is a
very popular chip used in a great variety of receivers from cellular telephones
to satellite receivers, in fact, here at Ramsey, we use it in every receiver kit we
sell! The reason for its popularity is that it contains most of a receivers 'front
end' circuitry on a single, easy to use chip.
Internal to the SA602 is an oscillator tuned by an external tank circuit; C11,12,
L1 and varactor diode D1. A varactor diode acts like a voltage variable
capacitor and the voltage applied across it comes from Tuning pot R12. This IF
signal is an exact reproduction of the desired signal, but at 10.7 Mhz. We then
band-pass filter it with ceramic filter FL-1. The ceramic filter allows only the
desired 10.7 Mhz signal to pass, rejecting any adjacent channel signals.
Transistor Q1 amplifies the IF signal before it is applied to the detector chip
U1.
One of the primary advantages of FM is that the program information
(modulation) is contained in carrier frequency variations and not in amplitude
changes like AM, which would be susceptible to noise and interference. This
means that we can limit or clip the signal, thus removing any noise and not
affecting the desired information. The first section of U1 performs this limiting
function, providing high gain and clipping any amplitude (noise) variations.
FM is detected in the second section using a process known as quadrature
detection. We'll not get into this too deeply because it requires a knowledge of
trigonometry identities - let's just say it extracts the frequency modulated
program information. Capacitor C1 and coil assembly L2 comprise a phase
shift network that is required by the quadrature detector. Demodulated signals
are amplified and sent out through pin 1 to the; volume control, AFC circuitry
and the SCA buffer amp. The AFC circuitry is used to properly keep the
receiver locked on to the tuned in radio station. Resistor R9 and capacitor C17
filter out any audio voltage variations so that only major frequency changes will
shift the tuning voltage to lock the frequency.
Audio signals from the volume control are amplified to speaker levels by U2,
an LM-386 audio amplifier chip. Transistor Q3 provides a buffer stage to
external SCA devices you may connect up to the FR1. SCA units decode
(demodulate, actually) 'hidden' audio and data services transmitted over
standard FM broadcast channels. These services include stock quotes, sports
scores, talk shows and even background music - commercial free!