Spectrum analyzer and method of controlling the same
A spectrum analyzer having a memory function to adopt a digital-data-based frequency sweep scheme while achieving performance comparable to performance of a high-speed FFT spectrum analyzer, and a method of controlling the spectrum analyzer, in which the spectrum analyzer includes: an ADC for converting a BWP signal, which is at least one analog unit frequency band signal, into a digital data sample at a predetermined sample rate according to a span set by a user; a digital sweep part for sweeping the data sample passed through the ADC while digitally decimating the data sample through a decimation processing block having a two-stage cascaded structure, and processing the swept data sample to increase a frequency sweep speed; and a control unit for controlling the digital sweep part according to various items input, set, and selected by the user to perform spectrum analysis and output a spectrum analysis result.
1. A method of controlling a spectrum analyzer, the method comprising:
(a) fixing a trace point (TP) determined in relation to a multiplication (TP 1 *TP 2 ) of primary and secondary digital frequency sweep counts (TP 1 and TP 2 ) to a preset value when a user selected span, which is a measurement target frequency bandwidth, a center frequency (f c ), a resolution bandwidth (RBW), and a video bandwidth (VBW) are set;
(b) calculating a number (N bwp ) of bandwidth parts (BWP), which is a unit frequency bandwidth at every RF frequency sweep in an RF processing part, by N BWP =ceil(span/40 MHz);
(c) calculating N d1 and N d2 , which are first and second decimation rates, for each of the BWPs, and configuring an RBW filter and a VBW filter;
(d) capturing output samples, N S , of an ADC in a first input buffer at a synchronization time interval;
(e) performing primary digital sweep on the samples stored in the first input buffer by an amount corresponding to a first frequency step, and decimating the samples through first cascaded integrator comb (CIC) filtering using the first decimation rate (N d1 );
(f) capturing the samples decimated through the first CIC filtering in a second input buffer, performing secondary digital sweep on the samples stored in the second input buffer by an amount corresponding to a second frequency step, and decimating the samples through second CIC filtering using the second decimation rate (N d2 ), sequentially passing the output samples of the second CIC filtering through the RBW filter, an envelope detector, a third CIC filter, the VBW filter, and extracting output values from the VBW filter, and repeating step (f) until processing for all TP 2 is finished; and
(g) repeatedly performing the primary and secondary digital frequency sweeps and decimation processing until processing for all the BWPs is finished.
2. The method of claim 1 , wherein TP≤TP 1 *TP 2 .
3. The method of claim 2 , wherein the first frequency step is set to bin*TP 2 , where bin=span/TP, and
the second frequency step is set to bin.
4. The method of claim 3 , wherein the capturing is performed until a predetermined number (N s ) of samples are filled in the first input buffer, and
when a time interval ends during the capturing, the capturing is held and continuously performed at a next time interval.
5. The method of claim 1 , wherein h[n]′, which is a coefficient of the RBW filter, is determined by h[n]′=h[n]*w[n]′,
the RBW filter is configured based on a window function of
w
[
n
]
=
a
0
-
a
1
cos
(
2
π
n
N
)
+
a
2
cos
(
4
π
n
N
)
-
a
3
cos
(
6
π
n
N
)
,
where a 0 =0.3635819, a 1 =0.4891775, a 2 =0.1365995, and a 3 =0.0106411,
a final window function is determined by w[n]′=w[n] α , where α is a constant for obtaining a bandwidth of the RBW filter,
H[n], which is a frequency domain value of the RBW filter, and a length of the filter are determined by a RBW sampling rate and the RBW, and
h[n], which is a time domain value, is obtained through inverse fast Fourier transform (IFFT).