HMC856 Eval board Interfacing

Question

I am reviewing the HMC856 eval board schematic on page 12 of the HMC856 data sheet. I am trying to figure out how to interface the control (B0-B4) to LVTTL (1.8, 2.5 or 3.3V) control system. Is it possible?

salemdar · Accepted Answer

VT should be tied to GND, you are correct.)

salemdar · Answer

Control pins have internal 600ohms and there is a voltage division with externally connected resistors which are pulled down to -3.3V.

J5 has jumpers on it, pulling down B0+~B4+ (positive pins) to -3.3V with 2.7k or not. With the internal 600ohm pull-up, positive pins are either set to -600mV(jumper installed) or 0mV(jumper not installed) 
 As for the negative pins, 5 control pins are connected in parallel with an equivalent pull-up of 1 20ohms(600/5). With the external pull-down( 1 . 2k ), negative pins are set to -300mV. This jumper is not meant to be removed, it should always be on 
 Thus, negative pins set to -300mV, positive pins are set to 0mV or -600mV. The differential control voltage is +300mV or -300mV.

salemdar · Answer

Hi, 
 One way to do is connect positive ports B0-B4 to a fixed voltage with a resistance, say 1.2k to GND and connect transistor switches to negative ports(B0~B4) with 2.7kohms each. Transistor switches can either be open or short to Vee. This way (B0-B4) change state. LVTTLs can drive switch transistors. 
 If you supply shift the part between 3.3V and GND, and if LVTTLs can supply enough current, you wouldnt need interfacing transistors/level shifters. The RF signal though should be AC couple in that case. 
 Sinan

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HMC856 Eval board Interfacing

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