30-06-2020, 11:24 AM
(This post was last modified: 30-06-2020, 11:43 AM by Mike Watterson.)
This doesn't actually need twisted together wire, just winding the two coil in parallel (at the same time) is good enough, though obviously actual twisted bifilar is better balance (less LO on the ferrite rod or radiated on the aerial. The actual twisted pair enamel is easier to use on cores or binocular cores, but using regular wire is easy on a former on a rod.
A wideband RF transformer with a centre tapped primary, secondary or both, on a suitable binocular core of course needs the actual twisted bifilar wire to do a centre tap as otherwise the balance is poorer and it's hard to wind. A typical example that might use a small core or toriod is the pair of centre tapped transformers to feed a ring diode mixer. The RF in on one transformer. The ring of diodes is between the transformers and the Local oscillator feeds the centre taps. One centre tap can be antiphase LO or 0V. The second transformer has the mixer products.
So a 1j42a or 1j37b simply replaces the two transformers and four matched diodes (in a ring, not bridge) with one centre tapped transformer on the input and any sort of suitable transformer (no centre tap needed) on the anode. A ring diode mixer is very good but needs well matched diodes, two accurately made bifilar transformers for centre taps and quite high level Local Oscillator. The unique split grid Rod Pentodes with a g1a and g1b are doing the same thing.
The Heptode/Octode and transistor combined Mixer/Oscillators are particularly dreadful for radiating LO on the RF in. They also suffer more from intermodulation due to a strong signal causing pulling (FM) of the local oscillator so that the strong unwanted signal has FM on the IF, which then will convert to AM via the IF filter slope.
The Hexode and Pendode mixers take the loc oscillator on a later grid than RF, so less leakage. Obviously a Heptode or Octode with RF on g1 and the LO from a separate oscillator fed to a later grid leak less LO.
A wideband RF transformer with a centre tapped primary, secondary or both, on a suitable binocular core of course needs the actual twisted bifilar wire to do a centre tap as otherwise the balance is poorer and it's hard to wind. A typical example that might use a small core or toriod is the pair of centre tapped transformers to feed a ring diode mixer. The RF in on one transformer. The ring of diodes is between the transformers and the Local oscillator feeds the centre taps. One centre tap can be antiphase LO or 0V. The second transformer has the mixer products.
So a 1j42a or 1j37b simply replaces the two transformers and four matched diodes (in a ring, not bridge) with one centre tapped transformer on the input and any sort of suitable transformer (no centre tap needed) on the anode. A ring diode mixer is very good but needs well matched diodes, two accurately made bifilar transformers for centre taps and quite high level Local Oscillator. The unique split grid Rod Pentodes with a g1a and g1b are doing the same thing.
The Heptode/Octode and transistor combined Mixer/Oscillators are particularly dreadful for radiating LO on the RF in. They also suffer more from intermodulation due to a strong signal causing pulling (FM) of the local oscillator so that the strong unwanted signal has FM on the IF, which then will convert to AM via the IF filter slope.
The Hexode and Pendode mixers take the loc oscillator on a later grid than RF, so less leakage. Obviously a Heptode or Octode with RF on g1 and the LO from a separate oscillator fed to a later grid leak less LO.
