Medium Wave Radio - Band Pass Filter
The medium wave radio band runs from 531–1,602 kHz with 4.5kHz bandwidth in Europe, and 530–1,700 kHz with 5kHz bandwidth elsewhere. A bandpass filter should only pass 500kHz – 2MHz approximately for good front-end filtering.
A filter was calculated on rf-tools.com using standard component values and the following parameters:
- 5th Order Chebyshev Bandpass
- Conventional, Series First
- Lower Cutoff Freq. = 500 KHz
- Upper Cutoff Freq. = 2 MHz
- Passband Ripple = 0.10 dB
The filter schematic diagram
Filter parts list
2 x 4.7nF 3 x 2.2 nF 2 x 5.6 µH 2 x 8.2 µH 1 x 10 µH
The filter was simulated in QUCS with these parameters and equations:
- Start : 25kHz
- Stop : 40 MHz
- Points : 1001
- dBS21 = dB(S[2,1])
- dBS11 = dB(S[1,1])
- Group Delay = -diff(unwrap(angle(S[2,1])),2*pi*frequency)
S21 Plot of the filter
Plotting S21 vs frequency gives this very nice filter shape. I suspect the actual filter I build will be nowhere near as good as this, and definitely not reach >100dB of attenuation, but it’ll be interesting to measure.
Part selection for the filter isn’t critical, as the frequencies involved are low, and the wanted bandwidth is fairly wide. I built a prototype on copper-clad board using polyester film capacitors and some miniature axial inductors. The inductors were a bit of an unknown, but they seem to work well in this application.
The filter prototype was measured using a nanoVNA and the data captured with the vna-saver software. Unfortunately, the vna-saver only saves in Touchstone files, whereas the QUCS simulation is magnitude vs frequency CSV. I haven’t found a way to overlay the two yet, to directly compare simulation and realisation.
The prototype on the copper board was too fragile for actual use, so it was re-built on some protoboard. It re-measures on the VNA as essentially identical to the copper board version - there’s not too much effect from all the distributed capacitance of the protoboard.
The filter has degraded a bit by being built on the protoboard, but it still more than good enough for the purpose