Telecommunications – Transmitter – Angle modulation
Reexamination Certificate
1998-09-23
2001-06-12
Kuntz, Curtis (Department: 2643)
Telecommunications
Transmitter
Angle modulation
C455S112000, C455S118000
Reexamination Certificate
active
06246864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a UHF band carrier FM transmitter suitable for use in a wireless microphone such as a hand-held type which can be held and carried in the hand or a mounted type which can be mounted on the body or clothing, and more particularly relates to an improvement of a PLL system in the transmitter.
2. Description of the Related Art
Conventionally, as an acoustic wireless microphone transmitter, a crystal oscillation/ multiplication system used at a very high frequency (VHF band: 30 MHz to 300 MHz) has been known. Recently, due to revisions in the regulations of different countries relating to radio waves, operation of acoustic wireless microphone transmitters even at a ultra high frequency (UHF band: 300 MHz to 3000 MHz) has come to be recognized. The crystal oscillation/ multiplication system compatible with the UHF band comprises, for example, as shown in
FIG. 4
, a modulation/oscillation/3× multiplication circuit
1
for performing frequency modulation (FM modulation) by voltage controlled oscillation by changing a load capacitance of a crystal oscillator X
1
having a resonance frequency of 23 MHz by an input acoustic signal, i.e., a modulation signal AF having an audible frequency of 10 Hz to 20 KHz and for multiplying the frequency of the modulated wave by 3; a 3× multiplication circuit
2
for multiplying the frequency of an output wave thereof (center frequency: 69 MHz) by 3; a 4× multiplication circuit
3
for multiplying a multiplied output wave thereof (center frequency: 207 MHz) by 4; a radio frequency amplifier
4
for amplifying the electric power of the multiplied output wave (center frequency: 828 MHz) to create a transmission electric power (10 mW); a bandpass filter FL
1
such as a helical resonator having a sharp selection characteristic; and an in-housing antenna
5
.
When the transmission frequency changes from a VHF band to the UHF band, in a crystal oscillation/multiplication system, due to the point that a number of multiplication steps inevitably becomes large and the point that filters of sub-tuning circuits (tank circuits) for reducing spurious emission have to be inserted for every multiplication circuit, a great increase in the number of individual parts such as inductors and capacitors unsuitable for formation on an IC is induced. It is therefore difficult to use this system for an acoustic wireless microphone transmitter—for which a reduction of size and a lowering of cost are required. Particularly, since a circuit constant of the filter of each multiplication stage has been adjusted to make it compatible with the crystal oscillator X
1
, it is not possible to deal with changes in channels just by replacing or switching crystal oscillators X
1
having different resonance frequencies, so in principle the circuit configuration is unsuitable for an increase in channels.
Therefore, at present, even in acoustic wireless microphone transmitters, a PLL frequency synthesizer system compatible with an increase in channels has become the mainstream. A wireless microphone use UHF band carrier FM transmitter using a dual modulus prescaler system for this PLL frequency synthesizer, for example, as shown in
FIG. 5
, comprises a reference counter (variable reference frequency divider)
12
for obtaining a more stable reference frequency f
r
from the oscillation frequency f
osc
(4 MHz) of a reference oscillator
10
using crystal oscillation and making a channel interval (frequency step) variable by dividing the oscillation frequency f
osc
to 1/L by a frequency division ratio setting signal given from a microprocessor
11
; a dual modulus prescaler type PLL (phase locked loop) circuit
20
having a voltage controlled oscillator
21
for effecting FM modulation by the use of the input modulation signal AF that is an acoustic signal; a buffer amplifier (buffer)
22
which is loosely coupled with the voltage controlled oscillator
21
to prevent the effect of the next stage on from being fed back to the voltage controlled oscillator
21
; a radio frequency amplifier
23
for amplifying the electric power of the output wave thereof (carrier 800 MHz band) to create a transmission electric power (10 mW); a bandpass filter
24
compatible with the output wave; and an in-housing antenna
25
.
This PLL circuit
20
comprises a digital phase comparator
26
which compares phases of a reference signal V
r
input from the reference counter
12
and a comparison signal V
c
input from a variable comparison frequency divider
30
and issues a phase difference detection signal in accordance with the phase difference detection signal (phase up signal U, phase down signal D); a charge pump unit
27
for flowing, based on the phase difference detection signal (U, D), a current corresponding to the phase difference into or from a loop filter
40
; a loop filter (low bandpass filter) circuit
40
for integrating the current to obtain a DC voltage; a voltage controlled oscillator (VCO)
21
which generates an oscillating signal having a center frequency (carrier frequency) under the control of the DC voltage, effects an FM modulation of this oscillating signal by the input modulation signal AF, and outputs an FM modulated wave V
o
; and a variable comparison frequency divider
30
for dividing the output frequency f
o
of the FM modulated wave V
o
with a frequency division ratio of (MN+A).
This variable comparison frequency divider
30
is a dual modulus prescaler comprising a prescaler (fixed frequency divider)
31
having two types of frequency division ratios of 1/M and 1/(M+1) and a program counter
32
having a pulse swallow counter for switching the frequency division ratio of the prescaler
31
. The pulse swallow counter is a variable frequency divider comprising an A down counter, an N down counter, and an RS flip-flop and is given the frequency division ratio setting signal from the microprocessor
11
. By changing this frequency division ratio setting signal (A, N), the output frequency f
o
can be changed by the frequency step equal to the reference frequency f
r{l .
A PLL circuit by nature has a lockup time which is strongly influenced by the time constant of the loop filter. When the time constant of the loop filter is made small, the lockup time becomes short, however, since the amount of high frequency components in the output signal becomes large, the output of the loop filter changes at a high rate in comparison with the low frequency side of about 100 Hz of the input modulation signal. Because of this, when the time constant of the loop filter is small, the loop works to suppress the input of a modulation signal at the low band side and therefore low band FM modulation becomes difficult.
If the time constant of the loop filter is made large in order to prevent the difficulty in FM modulation, however, the lockup time of the loop filter becomes longer so that the frequency pull-in (drawing) process takes a long time to start up when turning on the power supply, etc.
Therefore, in the PLL circuit shown in
FIG. 5
, a loop filter switching system is adopted.
Namely, the loop filter circuit
40
has a first low bandpass filter
41
and a second bandpass filter
42
connected in series with the output of the charge pump unit
27
and an analog switch
43
for short-circuiting the first low bandpass filter
41
to the charge pump unit
27
in response to a detection signal of an unlock detection circuit
28
for detecting a lack of phase synchronization. In this loop filter switching system, only the second low bandpass filter
42
having a small time constant (i.e., wide band width because of an increased amount of high frequency components or, in other words, because the high band side cut off frequency is high) is used until the PLL circuit pulls in the frequency so as to ensure a high speed lockup at the time of turning on the power supply, etc. After the PLL circuit pulls in the frequency, the first low bandpass filter
41
and the second low bandp
Armstrong, Westerman Hattori, McLeland & Naughton
Circuit Design Inc.
Kuntz Curtis
Ramakrishnaiah Melur
LandOfFree
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