SDR FM Pre-selector filter and direct digital SDR
In one of my earlier posts I’ve already described the direct digital SDR architecture and the possibility to receive in higher Nyquist bands.
If we are to use this possibility, however, we have to carefully avoid spectrum overlaps, for which we need an analog SDR FM pre-selector filter. Let me show you how I’ve designed and implemented one.
SDR FM pre-selector filter design
SDR FM Filter design
FM boadcast is in the 88-108 MHz band in most countries, which is in the 80-120 MHz 3rd Nyquist zone of our 40 MHz converter. The relative bandwidth is ~20% (20/98=0.24), which suggests to use an High-Pass Filter (HPF) – Low-Pass Filter (LPF) structure.
We can approximate the filter using the Dyonusos filter design software. I’ve manually fine tuned the autogenerated results, so that the band edge insertion loss falls in the 1-3 dB range while the inductor values are close to standard values. The capacitors can be implemented with parallel connected standard values. I’ve employed the same method for both the HPF and LPF filters, and decided to use 82 nH and one 100 nH inductor.
After the design, I’ve collected the components, and utilized a previous PCB design to solder the components together.
Before assembling the complete band-pass SDR FM filter, I’ve measured the high-pass and low-pass parts separately.
Finally, I’ve connected them, thereby forming a Band-Pass Filter (BPF), and I’ve tested the close-in pass-band performance and the higher frequency response as well. The latter is important for rejection of miscellaneous signals, like DVB.
As expected, the amplitude response of the assembled structure is not so good, and we see more insertion loss, and also other degradation due to the inaccurate component values.
I’ve introduced a simple method to design and implement an SDR FM pre-selector filter. It is designed with a filter design software with some manual fine tuning and adjustments to achieve the right performance in the corner frequencies and for amplitude response. During this process, one has to take into account the standard chip inductor values, e.g., from the E6/12 component line. Non-standard capacitor values can be constructed with parallel connected smaller values. Due to the limited quality factor of the inductors, the insertion loss in only moderate. Due to the high tolerance of the components, the pass-band response is high. With tunable high-Q inductors we could reach better values in terms of insertion loss and response.