Receiver front-end filter #rcvr


Glen Leinweber
 

Completed QRP-Labs receiver module, meant for most any band.
Hans supplies a fixed-tuned bandpass filter module between antenna & input for some RF selectivity.
Modules for most any HF band are available from QRP-Labs.
Why not attempt a similar module with a big dual-gang variable capacitor that tunes over all HF?
Hans conveniently does allow direct inputs right on the PCB from such a filter.

The circuit arrangement is exactly the same as Hans' filter: two toroids (tuned by each gang
of the big variable), coupled together by a fixed, small coupling capacitor. A link winding couples
antenna signals into one toroid, while the other toroid couples another similar link winding for output.

I used T50-2 toroid cores, with a 5-turn link winding on each....30 turn main winding #24 AWG.
Those dual-gang variable capacitors are rare today. You want one from an AM radio with equal
capacitance, of the 365 pf variety. I pulled one from an old transistor desktop AM/FM radio. The
20pf capacitance sections normally used for FM are left unused.
The fixed 6.8 pf coupling capacitor (hidden in the photo) couples signals from one gang to the other.

How wide a tuning range can such a filter accommodate? Those two toroids must be matched
carefully, especially at the high-frequency end. That 6.8pf coupling capacitor is a bit small at the
low frequency end (3.8 MHz) where capacitor plates are fully meshed. Attenuation is 6.5dB there,
but it is a very effective filter.
Filter losses were quite small at higher frequency, but filter bandwidth grew to be quite wide.
Frequencies from 3.5 - 23MHz were accommodated.
With a Si5351 VFO module, this makes a pretty nice general-coverage receiver.

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