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Old 11th Dec 2017, 10:35 pm   #10
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Hatfield modulator: info.?

It looks to me like a classic (double balanced) diode ring mixer but they have added baluns at the LO and RF port to optimise it for modulator use.
Ideally, you would want high isolation between ports A and C if you want a modulator like this to be able to act as a decent On Off Keying (OOK) modulator that has a decent 'off' performance. It looks like the two input transformers are twisted wire TLTs. This will help provide good balance up through the VHF region. Are the inner transformers trifilar wound?

I'd advise against moving the windings in your attempts to reverse engineer the circuit. Someone at the factory probably nudged/tweaked the windings to get the best isolation between ports A and C across the full RF range. So best left alone.

As Jon says, the LO drive level is probably +7dBm. You can use this as a classic DSB SC modulator or for AM if you allow a small DC offset current to be bled into port B along with the modulation. This will spoil the balance and allow a carrier to leak to the RF port and this will create AM. You can also use it as an attenuator by feeding in a DC current to port B.
Or you can switch in positive and negative current (-5mA/+5mA as a square wave) to port B to make a BPSK modulator. Or use it for OOK if you just switch in 0mA / 5mA at port B to turn the RF on and off between ports A and C.

I'm not sure what the diodes are rated for but I'd be very careful not to overdrive port B. Probably best kept to a few mA modulator drive here? The circuit is probably something like the one below but I am just guessing.
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Last edited by G0HZU_JMR; 11th Dec 2017 at 10:48 pm.
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