Polyphase based Wideband Digital SSB Converter of CDAS. L. Chen; X.Z. Zhang; Y. J. Wu; R.J.Zhu

Transcription

1 Polyphase based Wideband Digital SSB Converter of CDAS L. Chen; X.Z. Zhang; Y. J. Wu; R.J.Zhu

2 Abstract CDAS (Chinese VLBI Data Acquisition System) has been developed in Shanghai astronomical Observatory since the beginning of year During the last few years, we have developed wideband Digital SSB Converter based on poly-phase filter-banks. The digital SSB Converter segments each IF into 2 n to USB and LSB channels. adjacent frequencies The experiments was done on the CDAS of 1U board s size.

3 Introduction THE ARCHITECTURES AND ARITHMETIC OF A WIDE BAND DIGITAL ADJACENT SSB CONVERTER (1) Channelization (a) ωk = 2π k /K for even arrangement (b) ωk = 2π k/k+π /K for odd arrangement

4 (2) Low-pass filter banks (a)the frequency characteristics of an ideal low-pass filter (b)the frequency characteristics of an practical lowpass filter The practical low-pass filter has transition bandwidth. The band-width of the filter output signal in each channel is 2π/D, so it is possible to perform down sampling by factor D on the output.

5 (3) The polyphase architecture for complex signal with even arrangement and noncritical sampling 2π kp 2π j j km K 1 ' y (m) = [ s( md p) * h ( m) ] e K }e F k p= 0 2π j km ' = IDFT[ s( md p) * h ( m) ] e F p p Where: D: decimator p K ' h( l ) p 0 1,, K DF l h( ), p F 1 Digital base-band converter architecture when,k=fd

6 If F = 2, e j 2π km F = j πkm e Digital base-band converter architecture: e jπkm = 1 for k even m ( 1) for k odd Thus, after channelization by the poly-phase filters and IFFT, the data rate of signal yk (m) has been reduced to 1/D of that of the input signal s(n).the prototype of a low-pass filter h p (m) has the bandwidth B = 1/(2D). K is the number of output channels, the output of 0-th channel and K/2-th channel are real, the others are complex. For real input signals, half of the channels are redundant.

7 (4) Digital adjacent SSB base-band converter based on the decimation structure of a real bandpass signal A real signal is symmetrical in the frequency domain. We denote its spectrum with X, X+ for positive frequencies and X- for negative frequencies. Time domain signal:

8 Architecture of the transformation from a real band pass signal to a real upper side band signal for F = 2, the data rata of yi (m) is Fs/D, where Fs stands for the data rate of the signal s(n). Its bandwidth is B = 1/2

9 SIMULATION AND VERIFICATION (1)Impulse response of prototype low-pass filter fpass : 0.9*32MHz fstop : 32MHz sample clock : 1024MHz Pass band ripple : 0.2dB stop-band attenuation :60dB Tap:1599

10 K=1024/32=32, decimation factor is D=16, a 32 point FFT, s( n ) = f f f f sin ( 2n ) + cos( 2nπ ) + sin ( 2nπ ) + cos( 2nπ ) + noi se F F F F s s s s Where: f0=20mhz, f1=60mhz, f2 =98Mhz, f3 =186Mhz, SNR=5dB,Fs=1024Mhz

11 (2)The input time series s(n) and its normalized spectrum and the relations with the polyphase filters:

12 Output signals generated by the new approach for the example of 0, 1st,2nd, 3rd,6th,7thUSB channels Magnitude (db) The output spectrum of the 0 channel Frequency (M Hz) The output spectrum of the 1st channel The output signal of each channel is real, with a bandwidth of 32MHz. The middle frequency of the k-th (k=1,2, 15) channel represents 16, 48, 80, 112, 144, 176, 208, 240, 272, 304, 336, 368, 400, 432, 464 MHz. The signal frequencies f0 to f3 are located in channels 1, 2, 3, 6. They appear there as 4, 12, 18, 10MHz base-band signals the corresponding base-band channel. Magnitude (db) Magnitude (db) Magnitude (db) Magnitude (db) Magnitude (db) Frequency (M Hz) The output spectrum of the 2nd channel Frequency (M Hz) The output spectrum of the 3rd channel Frequency (M Hz) The output spectrum of the 6th channel The output spectrum of the 7th channel Frequency (M Hz)

13 (3)The input signal with a 1024MHz clock is converted to an IF from 16 to 496MHz, into 15 real USB channels. Channel 0 is not useful due to aliasing. Since this channel is situated at the band-pass edge, it would be unusable anyway due to the shape of the analog band-pass filter ahead of ADC.

14 THE HARDWARE OF THE WIDEBAND DIGITAL SSB CONVERTER BASED ON POLY PHASE FILTERS The CDAS of 1U board 1U size signal process board

15 Control computer Xilinx FX60 IF Input 512MHz A/D Xilinx LX160-1 Xilinx LX160-2 Xilinx LX160-3 Xilinx LX160-4 MK5B/ VDIF Format 1024Mx8bit 16channel 32M 2bit VSI

16 A. The full spectrum of noise test:

17 B:Zero baseline test 50 X auto-corr AMP 60 XY co-corr AMP Amplitude(dB) Amplitude(dB) Amplitude(dB) freq(mhz) Y auto-corr AMP freq(mhz) pha(degree) freq(mhz) XY co-corr PHS freq(mhz) X: The correlation spectrum of one CDAS of 1U board's data Y: The correlation spectrum of another CDAS of 1U board's data XY: The cross correlation spectrum We could find that the phase spectrum is linear in each channel.

18 Thanks!