PTT Problems

[Nicholas Evans]

I have been running a HL2 with a Xiegu XPA125B Linear Amp for sometime without issue. Of late, the PTT output from the HL2 is no longer triggering the Xiegu into transmit mode. On further investigation, measuring the Dc voltage outputs on the HL2 gave the following figures. In Non-transmit mode --> 2.15V, in transmit mode -->3.15V. The transmit mode figure should be very close to 0V. What gives?  It seems I have some sort of failure on hand. Any suggestions would be welcomed on how to correct this problem.

Thanks 

[Jerry Kaidor]

What are the limitations of the PTT output?  How much voltage can it tolerate?  How much current can it switch?

                - Jerry, KF6VB

[Magnus Persson]

I wouldn’t PTT without optocoupler. I use an optocoupler and a relay. Then you’re safe, no matter what.

Magnus SA7EMP

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[keyboa...@gmail.com]

From the Wiki:

EXTTR
This is an "open collector" style output to turn on an external power amplifier for transmit. The center pin is pulled low by the HL2 during transmit. The ring is ground. An external PA should pullup the center pin with a resistor. The pin may be pulled up to 28V. Power should not exceed 1W, and should most often be much less than this. For example, don't exceed 80mA at 12V, so minimum pullup resistor of 150 Ohms. Typical pullup resistors will be in the 1K to 10K range.

At the least I'd recommend adding switching transistor or mosfet circuit possibly activating a relay. I use one with multiple isolated poles to switch a driver amp in addition to a larger one.

[Nicholas Evans]

I have done a replacement of the driver FET chip (Q5) NUD3124 which sits between the FPGA and External PTT. Things have improved to being able to transmit again. The problem I now an facing is that the PTT output activates the Xeigu into transmit mode whether the Hermes lite is transmitting or NOT. To that end, I am seeing 1.8V at testpoint 4 (input to gate of the NUD3124) when not transmitting, and 3.2V when transmitting. My question is, are these the expected voltages on the GATE to initiate triggering, or should there be something else coming out from the FPGA. Currently, the FET chip is switching on both 1.8V and 3.2V and activating the transmit mode on the Xiegu.

Thanks  

[Steve Haynal]

You may want to check if a pull down resistor is properly installed for the FPGA pin. I believe one is installed or at least a footprint for one exists. Perhaps it was damaged. Also, it is possible to damage the FPGA bank such that the pin no longer works.

73,

Steve

kf7o

[Nicholas Evans]

So I have a solution at hand! On further investigation, not only was the fet chip (NUD3124) blown and replaced, but obviously the internal circuitry to the FPGA chip was gone. On NO transmit, I was seeing 1.9V where it should be 0V and on transmit I was seeing 3.3V which is correct. Unfortunately, the 1.9V was activating the NUD fet into transmit mode. On studying the HL2 circuit diagram, the internal PTT is an identical curcuit which is used to drive a relay that controls the 5w output from the Hermes. I removed the 4.7K resistor from the EXttr circuit, sliced the tract between the FPGA and EXTTR fet, and hardwired a connection between the TP4 and TP1.

 In essence the Internal PTT drives both its own circuit and EXTPTT based on the 0V and 3.3V being sent from the FPGA. Does it work, yes and seemily without issue!

[Steve Haynal]

Hi,

I am glad you fixed your problem! Just to clarify, INTTR and EXTTR are different. When operating full duplex mode (low power output only) INTTR will not switch when transmit is enabled but EXTTR will. This is primarily for transverters which you may not need support for. Also, the FPGA IO is organized as banks. Usually if there is one damaged pin in the bank it can often affect other pins in the bank. I am glad that does not seem to be the case for you.

73,

Steve

kf7o