Re: OPTIMIZING RSP RECEIVER OPERATION WITH ANTENNA NOISE
Posted: Sun Oct 07, 2018 8:59 pm
A complete 3 way receiver feed system for one antenna with controlled attenuation and SDRplay switching is shown below. A 400 Ohm antenna is perfectly matched by a parallel connection of three attenuators, each with an input impedance of 1200 Ohm. Loss for the parallel connection amounts to 10 dB. The attenuators introduce additional loss, but isolate each of the 3 receiver inputs. Tabled values for 1200:1000 Ohm attenuators are shown below.
Example: for a 22 dB total loss (well constructed transformers have negligible loss) we need a 12 dB attenuator, so component values will be as follows:
R1 = 2200 Ohm
R2 = 1500 Ohm
R3 = R4 = 1000 Ohm
1:1 transformer: 5t primary & 5t secondary on F43 ferrite 10 mm dia. toroid core.
20:1 transformers turns ratio 4: 16t primary & 4t secondary on F43 ferrite 10 mm dia. toroid core.
For other antenna impedances an input transformer with the required turns ratio will be used, as shown in the schematic. The balanced nature of these multiplexing systems guarantees minimum possible noise feedthru.
Example: for a 22 dB total loss (well constructed transformers have negligible loss) we need a 12 dB attenuator, so component values will be as follows:
R1 = 2200 Ohm
R2 = 1500 Ohm
R3 = R4 = 1000 Ohm
1:1 transformer: 5t primary & 5t secondary on F43 ferrite 10 mm dia. toroid core.
20:1 transformers turns ratio 4: 16t primary & 4t secondary on F43 ferrite 10 mm dia. toroid core.
For other antenna impedances an input transformer with the required turns ratio will be used, as shown in the schematic. The balanced nature of these multiplexing systems guarantees minimum possible noise feedthru.