Centre tapping a tuned RF transformer - UK Vintage Radio Repair and Restoration Discussion Forum

Alan_G3XAQ · 2nd Jun 2020, 12:54 pm

I've seen three different approaches to making a centre tapped tuned circuit and I wonder which if any are better than the others.

A physical centre tap on the inductor seems easiest. In my case in point I can make a bifilar winding to get tight coupling between the two halves of the winding. A single capacitor between the two "hot" ends of the inductor tunes the circuit to resonance. Yes, it needs an insulated screwdriver for adjusting the trimmer.

Second easiest is to dispense with the tapped inductor and use two capacitors in series with their junction acting as the centre tap. If most of the capacitance is provided by fixed capacitors with just a small trimmer end to end I guess the balance might be good enough, or make the fixed caps different values and add a trimmer across the smaller one?

The third option is to have both split capacitance and a physically centre tapped inductor. Does the added complexity buy you anything?

The "famous" G3PDM receiver on p10.106 of the old blue RSGB handbook has examples of all three options in the one design!

What do the cognoscenti on here recommend?

Alan

2nd Jun 2020, 12:54 pm	#1
Alan_G3XAQ Pentode Join Date: Apr 2018 Location: Canterbury, Kent, UK. Posts: 189	Centre tapping a tuned RF transformer I've seen three different approaches to making a centre tapped tuned circuit and I wonder which if any are better than the others. A physical centre tap on the inductor seems easiest. In my case in point I can make a bifilar winding to get tight coupling between the two halves of the winding. A single capacitor between the two "hot" ends of the inductor tunes the circuit to resonance. Yes, it needs an insulated screwdriver for adjusting the trimmer. Second easiest is to dispense with the tapped inductor and use two capacitors in series with their junction acting as the centre tap. If most of the capacitance is provided by fixed capacitors with just a small trimmer end to end I guess the balance might be good enough, or make the fixed caps different values and add a trimmer across the smaller one? The third option is to have both split capacitance and a physically centre tapped inductor. Does the added complexity buy you anything? The "famous" G3PDM receiver on p10.106 of the old blue RSGB handbook has examples of all three options in the one design! What do the cognoscenti on here recommend? Alan Last edited by Alan_G3XAQ; 2nd Jun 2020 at 1:18 pm.