SA LPF
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This page describes the input/TG low-pass filter for the DIY Spectrum Analyzer.
Summary
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
- Rev0VTx VCO PCB Bottom.jpg
Bottom of unpopulated PCB
- Rev0VTx VCO PCB Top.jpg
Top of unpopulated PCB
- Rev0VTx VCO Top.jpg
Populated PCB