SA LPF

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This page describes the input/TG low-pass filter for the DIY Spectrum Analyzer.

Summary

Schematic for the input LPF.

The input must be low-pass filtered to allow only the desired passband of 30MHz-2.5GHz through to the rest of the system. Likewise, the tracking generator must have an output low-pass filter to prevent LO and IF feedthrough at 3.5-6GHz. The low-pass filter is a 7-pole Chebyshev lumped element design, chosen for its performance, size, and simplicity. The filter has a -3dB bandwidth of 2.57GHz, with 50dB of rejection at the 1st IF of 3.5GHz, and >50dB rejection through 8.5GHz.

Testing

  • 8.6.2013: The filter was designed and simulated in Qucs using S2P part definitions from Murata's SimSurfing utility. Results of the simulation are shown below.


Frequency (MHz) Power (dBm)
150 -0.06
200 -0.02
250 -0.03
300 -0.07
350 -0.06
400 0.03
450 0.04
500 0.02
550 -0.07
600 -0.07
650 0.05
700 0.09
750 -0.05
800 -0.03
850 0.02
900 0.00
950 -0.07
1000 -0.10
1500 -0.17
2000 0.21
2500 0.04
3000 0.62
3500 1.72
4000 1.47

Revisions

  • 1.0: Initial revision.

Bill of Materials (v1.0)

Part Quantity Unit Cost Total Cost Source
ADF4350 1 $12.02 $12.02 Digi-Key
FOX924B-10.000 1 $3.76 $3.76 Digi-Key
ADP124 1 $1.95 $1.95 Digi-Key
AP1115 1 $0.54 $0.54 Digi-Key
MSP430G2553 1 $2.58 $2.58 Digi-Key
0402 100nF Capacitor 10 $0.011 $0.11 Digi-Key
0402 10pF Capacitor 10 $0.0116 $0.12 Digi-Key
0603 Capacitor 9 $0.0096 $0.09 eBay - mib_instruments
0603 4.7uF Capacitor 3 $0.1484 $0.45 Digi-Key
0805 10uF Capacitor 1 $0.30 $0.30 Digi-Key
0603 Resistor 10 $0.00294 $0.03 eBay - mib_instruments
0603 Green LED 1 $0.10 $0.10 Digi-Key
0603 Red LED 1 $0.27 $0.27 Digi-Key
0603 Ferrite Bead (470 Ohm) 2 $0.0472 $0.09 Digi-Key
Male Header 8 $0.01 $0.03 eBay
SMA Female Connector 4 $3.37 $13.48 Digi-Key
PCB 1.75"x1.1"/3 $10.00 $6.42 Laen's PCB
Total Unit Cost $42.32

PCB/Construction Photos