Driving SBL-1 IF port

Andrew Holme 20 years ago

I'm building an experimental direct sequence spread spectrum modulator along the lines of that described in this article:

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The PRBS sequence XOR'ed with the data is fed to the IF port from a logic gate.

I think the mixer IF port is incorrectly terminated and the IF drive level seems excessive - even for a high-level mixer. Assuming the impedance looking into the IF port is 50 ohms, surely the drive is +/- 35mA not 25mA as claimed? Mini-Circuits don't recommend a drive level for the TFM1H IF port; they only specify the absolute maximum as 40mA.

I want to use an unsymmetrical resistive pad to both set the drive level and present the IF port with a true 50-ohm termination; and I only have an SBL-1 (Level 7) mixer to hand at the moment. What IF port drive level should I use for this?

TIA

PS Another interesting SS link:

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tim gorman 20 years ago

A mini circuits SBL-1 is listed as a +7dbm IF, +1dbm RF type of mixer.

25ma to a 50ohm load would give a voltage of 1.25v. That corresponds to a level of +15dbm - too hot for the SBL-1. A 50ohm, 6dbm pad between the mixer IF port and your IF system would provide a very good impedance match to the mixer.

tim ab0wr

Andrew Holme wrote:

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Mark 20 years ago

The IF port I beleive has DC coupled connections to the diode ring. Yout apply current to the IF port in one direction and you turn on one set of diodes, you apply current in the other direction and you turn on the other set of diodes and the RF signal gets inverted in polatirty (180 deg). So the drive to the IF port must be enough voltage to turn the diodes on (there may be a few in series) and current limited to limit the current flow through the diodes. The IF current should be much hight compared to the signal current.

A few volts in series with 50 Ohms works well but the key point is you want it to be bipolar assumming you are trying to create 180 deg phase changes in the RF signal passing through the mixer.

Mark .

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Andrew Holme 20 years ago

For the purpose of calculating drive current, the IF port does not look like

50-ohms. It looks like a 0.3V diode drop. I tried a 1uF polyester layer DC blocking capacitor in series with a 56-ohm resistor between the logic gate output and the mixer IF port - and it worked fine. Looking at the waveforms at either end of the resistor, the drive current was (2-0.3)/56 = 30mA.

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