FM radio receiver

Details FM radio receiver FM radio receiver

1998-11-19

2001-01-23

Vo, Nguyen (Department: 2745)

Telecommunications

Receiver or analog modulated signal frequency converter

Signal selection based on frequency

C455S197100, C455S182200, C455S286000, C455S154100

Reexamination Certificate

active

06178315

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to an FM radio receiver containing a radio frequency tuning circuit for selecting a signal from an antenna and a local oscillator tuning circuit tunable to an oscillating signal for converting the frequency of a receiving signal to an intermediate frequency.
In such a kind of super-heterodyne FM radio receiver, a desired broadcast signal received at an antenna is converted to a frequency called an intermediate frequency (IF), which is then detected and demodulated. For FM reception, 10.7 MHz is normally used as this intermediate frequency. A local oscillator circuit is oscillated at a frequency always apart from the frequency of a desired signal (that is, receiving frequency) by 10.7 MHz. A 10.7 MHz IF signal is generated as a beat component as a result of mixing the oscillating signal of this local oscillator circuit and the received signal so as to convert the received signal to the intermediate frequency. At this conversion, if the local oscillator circuit is oscillated at a frequency higher by 10.7 MHz than the receiving frequency, it is called the upper heterodyne method. On the other hand, if the local oscillator circuit is oscillated at a frequency lower by 10.7 MHz than the receiving frequency, it is called the lower heterodyne method.
Whether an FM radio receiver is designed in the upper heterodyne method or lower heterodyne method is determined depending on the frequency allocation for FM radio broadcasting in each country and the presence of interference signals at image frequencies. In a super-heterodyne FM radio receiver, if an interference signal is located at the image frequency which is 10.7 MHz apart from a local oscillating frequency in the opposite direction to a receiving frequency, (i.e., 21.4 MHz apart from the receiving frequency,) image signals are mixed into the intermediate frequency signal resulting in interference. In Japan, the frequency allocation for FM radio broadcasting is 76-90 MHz, and channel 2 of TV broadcasting is allocated at frequencies 21.4 MHz higher than the FM band. In the USA, the frequency allocation for FM radio broadcasting is 88-108 MHz, and channels 4 to 6 of TV broadcasting are allocated at frequencies 21.4 MHz lower than the FM band. Accordingly, to avoid image frequency interference by strong TV broadcasting signals with a high electric field strength, the lower heterodyne method is used in Japan and the upper heterodyne method is used in the US (and also in Europe).
A conventional FM radio receiver will now be described referring to a car radio receiver with a front-end circuit as shown in FIG.
6
.
The front-end circuit of the conventional FM radio receiver comprises a radio frequency tuning circuit containing an antenna tuning circuit
61
and an RF tuning circuit
63
, an RF amplifier circuit
62
, a local oscillator circuit containing a local oscillator tuning circuit
64
, and a mixer circuit
65
. The antenna tuning circuit
61
comprises a tuning coil L
9
, a pair of variable capacitance diodes D
13
, D
14
which cathodes are connected to each other, and a capacitor C
11
. The RF tuning circuit
63
comprises a tuning coil L
10
, a pair of variable capacitance diodes D
15
, D
16
which cathodes are connected to each other, and a capacitor C
12
. The local oscillator tuning circuit
64
comprises an oscillator coil L
11
, a pair of variable capacitance diodes D
17
, D
18
which cathodes are connected to each other, and a capacitor C
13
. The variable capacitance diodes D
13
, D
14
, D
15
, D
16
, D
17
, D
18
have the same voltage versus capacitance characteristic. A common tuning voltage from a source
66
is applied to each pair of the variable capacitance diodes of the antenna tuning circuit
61
, RF tuning circuit
63
, and local oscillator tuning circuit
64
. Generally, this tuning voltage is set in a range of from 1 to 8 volts which can be supplied stably from a vehicle battery.
For an FM radio receiver which is intended for use in Japan, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit
61
and RF tuning circuit
63
tune to FM radio broadcasting signals of 76-90 MHz and the local oscillator tuning circuit
64
tunes to oscillating signals (i.e., 65.3-79.3 MHz) which are lower by 10.7 MHz than the tuning frequencies of the antenna tuning circuit
61
and RF tuning circuit
63
.
For an FM radio receiver which is intended for use in the US, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit
61
and RF tuning circuit
63
tune to FM radio broadcasting signals of 88-108 MHz and the local oscillator tuning circuit
64
tunes to oscillating signals (i.e., 98.7-118.7 MHz) which are higher by 10.7 MHz than the tuning frequencies of the antenna tuning circuit
61
and RF tuning circuit
63
.
For the FM radio receivers described above, the receiving bands are different between receivers for Japan and the US, and, moreover, the oscillating frequencies of the local oscillator circuits are entirely different because of a difference between upper heterodyne and lower heterodyne methods. Consequently, as shown in Table 1, the values of the coil elements and capacitance elements in the radio frequency tuning circuit and the local oscillator tuning circuit are significantly different for manufacturing the separate receivers. It is noted that, for both cases in Table 1, variable capacitance diodes used in each tuning circuit are those whose capacitance values change from 12 to 22.5 pF when the tuning voltage is changed from 1 to 8 V.
TABLE 1
radio frequency tuning
local oscillator tuning
circuit
circuit
Capacitance
Oscillator
Capacitance
tuning coil
element
coil
element
L9, L10
C11, C12
L11
C13
for Japan
120.476 nH
14.03 pF
182.127 nH
10.117 pF
for US
104.258 nH
 8.77 pF
 76.42 nH
11.525 pF
Therefore, conventional FM radio receivers need to be designed and manufactured separately for Japan and the US. Consequently, more design work is involved and the handling of components used in manufacturing the FM radio receivers becomes complicated since more components are required, thereby spoiling productivity.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an FM radio receiver wherein both a circuit necessary for receiving FM radio broadcasting signals in Japan and a circuit necessary for receiving FM radio broadcasting signals in US can be incorporated by means of a common front-end circuit without increasing the number of components, thereby preventing an increase of design work and a decline of productivity.
To achieve the above object, the present invention provides an FM radio receiver comprising a radio frequency tuning circuit for selecting a frequency modulated signal from an antenna and a local oscillator tuning circuit for tuning to an oscillating signal so as to convert the frequency of a receiving signal to an intermediate frequency. The radio frequency tuning circuit and the local oscillator tuning circuit each have variable capacitance diode elements. The variable capacitance diode elements of the radio frequency tuning circuit and variable capacitance diode element of the local oscillator tuning circuit are connected to a tuning control voltage source. The radio frequency tuning circuit has a capacitance variable ratio set so as to be capable of receiving continuously a first receiving band and a second receiving band. The local oscillator tuning circuit is so constructed that its inductance value and capacitance variable ratio can be switched according to the receiving band, so that the local oscillator circuit is set to lower heterodyne when the first receiving band is selected and to upper heterodyne when the second receiving band is selected.


REFERENCES:
patent: 3828257 (1974-08-01), Puskas
patent: 4237556 (1980-12-01), Naito
patent: 427

Affiliated with

Also associated with

Rating

FM radio receiver does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with FM radio receiver, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and FM radio receiver will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2452311