Driving SBL-1 IF port

[Andrew Holme]

I'm building an experimental direct sequence spread spectrum modulator along
the lines of that described in this article:
http://www.tapr.org/ss_g1pvz.html

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:
http://adamkar.republika.pl/spread_spec_en.htm

 

[tim gorman]

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

 

[Mark]

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
..

 

[Andrew Holme]

I'm building an experimental direct sequence spread spectrum modulator
along the lines of that described in this article:
http://www.tapr.org/ss_g1pvz.html

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?

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.