Category: design and layout

lpfl_v1

HF preselector filter

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Direct digital HF receiver

Earlier we’ve introduced the concept of direct digital receivers in this blog.

http://spectrafold.com/quadrus/digitizer-hardware/sdr-pre-selector-filter-direct-digital/

Furthermore, VHF/UHF direct digital filters discussed:

http://spectrafold.com/quadrus/radio-software/direct-digital-sdr-receiver/

http://spectrafold.com/quadrus/digitizer-hardware/direct-digital-uhf-sdr-radio-receiver-dru-244/

In this operation mode the ADC input has to be kept clean from various spectrum components in other Nyquist bands. The input preselector filter helps in selecting the right part of the given Nyquist band.

The HF preselector filter

In case of high performance reception of HF signals, the main challenge is to prevent interference from higher frequency VHF/UHF signals. This may be achieved with a Low-Pass Filter (LPF) cutting the components above say 30-32 MHz, but that depends on the reception requirements. We have to carefully construct the filter such that it provides sufficient attenuation in the 88-108 MHz FM band, where the highest level signals may be found usually. The input bandwidth of the DRU-244A hardware is up to 440 MHz, thus the input filter should attenuation in the upper VHF/UHF band as well. This can be ensured with careful design and implementation using small, surface-mounted components. If we are concentrating on real HF signals, and are not interested in lower band mid- or long-wave reception, the input should be clean from high level AM broadcast signals as well, usually present in the spectrum below 1.6 MHz. For this purpose, a High-Pass Filter (HPF) should be inserted in the signal path with a 1.6 MHz corner frequency.

HF preselector filter design and realization

To satisfy the requirements of the HF input preselector filter described above, we’ve designed an input filter block with a cascaded HPF and LPF using a professional filter design software. The approximation is elliptic, ensuring a cutoff at 40 MHz. The implementation is a small PCB containing the HPF and LPF sections with SMA connectors at the ends and a place for an optional choke for remote powering capabilities of an active receiver antenna.

The photo of the finished HF preselector filters:
lpfa
The measurement results up to 500MHz:

lpfb

The full design package of the HF preselector filter, including the schematic and layout design in Protel (Altium Designer) file format, are available for download.

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