Direct frequency synthesizer for offset loop synthesizer

Details Direct frequency synthesizer for offset loop synthesizer Direct frequency synthesizer for offset loop synthesizer

2003-07-31

2004-10-19

Nguyen, Linh M. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Synthesizer

C327S356000

Reexamination Certificate

active

06806746

ABSTRACT:

BACKGROUND OF THE INVENTION
High-frequency spectrum analyzers typically include a series of frequency conversion stages to facilitate analysis of applied input signals. In
FIG. 1
, a block diagram of frequency conversion stages within a conventional high-frequency spectrum analyzer are shown. An input signal S
IN
at frequency f
IN
that is applied to the spectrum analyzer is converted to a first intermediate frequency signal IF
1
by mixing the input signal with a signal S
LO1
at frequency f
LO1
provided by a first local oscillator LO
1
. This first intermediate signal IF
1
is further converted to intermediate frequency signals IF
2
, IF
3
having successively lower frequencies f
IF2
, f
IF3
, respectively. Bandpass filters BPF
1
-BPF
3
eliminate image signals resulting from the mixing to provide an unambiguous representation of the applied input signal S
IN
at the third intermediate frequency f
IF3
.
The third intermediate frequency signal has a frequency f
IF3
=f
IF2
−f
LO3
=f
IF1
−f
LO2
−f
LO3
=f
LO1
−f
IN
−f
LO2
−f
LO3
, indicating that a detected response at the frequency f
IF3
is attributable to the input signal S
IN
having a frequency f
IN
=f
LO1
−f
LO2
−f
LO3
−f
IF3
. Because the frequency f
IF3
of the third intermediate frequency signal IF
3
and the frequencies f
LO1
, f
LO2
, f
LO3
of the local oscillators are known, the frequency f
IN
of the input signal S
IN
is readily established. The frequencies f
LO2
, f
LO3
, and f
IF3
are generally fixed, whereas the frequency f
LO1
of the first local oscillator LO
1
is generally tuned to accommodate input signals S
IN
over a wide range of frequencies f
IN
.
The measurement accuracy of a spectrum analyzer depends on the quality of the signals S
LO1
-S
LO3
provided by the local oscillators LO
1
-LO
3
in the frequency conversion stages. Ideally, the local oscillators provide stable, low-noise signals and don't contribute significant noise to the intermediate frequency signals provided by the frequency conversion stages. However, practical implementations of the local oscillators provide signals that have short-term frequency instabilities and fluctuations, commonly referred to as phase noise. Since the phase noise of a local oscillator generally increases with operating frequency, the first local oscillator LO
1
, being the highest frequency local oscillator in the spectrum analyzer, is typically a higher phase noise contributor than the other local oscillators in the spectrum analyzer. In addition, the frequency tuning feature of the first local oscillator LO
1
tends to further increase the phase noise the local oscillator. Because the phase noise of the local oscillators can degrade the measurement accuracy of the spectrum analyzer, there is motivation to improve the noise performance of the local oscillators in the frequency conversion stages of the spectrum analyzer, particularly by lowering the phase noise of the first local oscillator LO
1
.
A known way of lowering phase noise involves using an offset-loop synthesizer
2
to generate the first local oscillator signal S
LO1
, as shown in FIG.
2
. The offset-loop synthesizer includes a course-step synthesizer phase locked loop (PLL)
4
that provides an offset signal S
0
to a main (PLL)
6
of the offset loop synthesizer that provides the signal S
LO1
. The offset signal S
0
eliminates the need for frequency division in the feedback path of the main PLL, thereby reducing the phase noise of the signal S
LO1
. However, because the frequency f
0
of the offset signal S
0
is multiplied by a harmonic mixer
40
in the main PLL, the phase noise of the offset signal S
0
is also multiplied. Accordingly, it is advantageous for the offset signal S
0
provided by the coarse step synthesizer PLL to have especially low phase noise.
Low phase noise is typically achieved in the coarse step synthesizer PLL by eliminating frequency division in the feedback path in the M/N loop used to implement the coarse step synthesizer PLL. This is done by setting the value of M to unity and by varying N, the divide ratio of the programmable divider D, to set the frequency f
0
of the offset signal S
0
. The harmonic mixer in the main PLL uses the H-th harmonic of the signal S
0
to produce a signal with a frequency close to the resulting frequency f
LO1
of the signal S
LO1
, whereas an interpolation signal S
INT
provides fine frequency resolution for the signal S
LO1
. This results in the signal S
LO1
having a frequency f
LO1
=H*fo±f
INT
, where f
INT
is the frequency of the interpolation signal S
INT
. This frequency relationship illustrates that the phase noise of the signal S
LO1
is the harmonic number H times the phase noise of the coarse step synthesizer PLL, plus the phase noise of the interpolation signal. Thus, while the noise gain of the main PLL with respect to the interpolation signal S
INT
is unity, the noise gain with respect to the coarse step synthesizer PLL is H, the harmonic multiplier of the harmonic mixer in the main PLL of the offset loop synthesizer. This noise gain results in multiplication of the inherent phase noise of the VCO and other components of the coarse step synthesizer PLL, which can degrade the measurement accuracy of the spectrum analyzer within which this type of offset loop synthesizer is included.
SUMMARY OF THE INVENTION
Embodiments of the present invention are directed toward a direct frequency synthesizer suitable for replacing the coarse step synthesizer PLL in an offset loop synthesizer. The output signal from the direct frequency synthesizer is derived from a high frequency reference signal that is frequency divided and mixed to satisfy the coarse step synthesis requirements of an offset loop synthesizer. The absence of a VCO within the direct frequency synthesizer provides the direct frequency synthesizer with lower phase noise than a typical PLL-based coarse step synthesizer. Though applicable to a variety of types of synthesizers and signal generators, the direct frequency synthesizer can provide especially low phase noise when used to generate an offset signal for an offset loop synthesizer of the first local oscillator of a spectrum analyzer, where the second local oscillator of the spectrum analyzer provides the reference signal for the direct frequency synthesizer. Alternative embodiments of the present invention are directed toward a direct frequency synthesis method.


REFERENCES:
patent: 5596290 (1997-01-01), Watkins et al.
patent: 6295020 (2001-09-01), Koechlin

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