My SDR – Nice band pass filter

Braahh… I made a mistake in my last order and I got a 12.1 ohms resistor instead of a 12.1k ohms. By misfortune, the value of this resistor is critical and I can’t replace it by a 10k or something close. It’s used by the internal voltage regulator of the Ethernet chipset (LAN8720A). So, I made an extra order, payed my 11 cents for the component and the rest for the shipping cost :/

Beside, I assembled and tested the first filter. The result is excellent and I can’t expect have a better response. This band pass filter (4 poles, Butterworth) was originally designed to be used on my contest transceiver, with some close antennas in the same area. For this board, I scale it down with a maximum permissible power is 10 Watts (air cooled) on a side band (ex: parasitic TX on the 14 MHz, while the RX is centered on 7MHz).

I use a miniVNA for my measurements and I cannot read below -40dB, but the expected results looks good. The attenuation on 14 MHz should be around 70/75 db. The calculated insertion loss is around 0.5dB, and I measured in practice 0.66 dB, simply great! Some screenshots follows :


The LPF, using Amidon toroids (T37-2), and SMC capacitors


Schematic of the filter, and response (in red, a Monte-Carlo analysis)


Zoom on the top flat, around 0.5dB of insertion loss


S11 response, 1-30MHz (measurements here)


S11, close view on the 40m band (measurements here)


S21 response, 1-30MHz (miniVNA can’t read below -30dB with the directional coupler in place. Measurements here)


S21, close view, -0.66 dB on the flat top (measurements here)