Problems with PTT

[TF1987]

Hi, have several Problems with the PTT. The DRA818V seems to send all the time, also when PTT is LOW. Can anybody help?

[handiko]

Was the shield mounted on the Arduino or standalone?

[TF1987]

Hi Handiko, shield was mounted on the arduino. I wondered because the DRA818V need a voltage of 3,3 V. For your design it becomes the supply voltage of the Arduino (5V).

Also the level of the PTT pin on the side of the transistor is not 5V. What type is T1? It is not written in your sketch.

BR Thomas

[handiko]

First of all, try your Dorji Shield as a standalone board to eliminate all external causes. Put +5V and GND ONLY via the side connector. You can use +5V from an Arduino to do so, but it can only supply enough current to operate at low power on TX.
The T1 is a generic NPN switching transistor. You can use MMBT3904, MMBT2222, or any SMD small signal NPN transistor (as long as the pinout is compatible). Without the PTT pin of the shield being connected to anything, the base of T1 should be 0V AND the collector of T1 (pin 5 of the Dorji module) is more than 3V.

If the Dorji module is always TX, then it means, the pin 5 of Dorji is somehow pulled down to the ground. Maybe by the failed transistor T1, or there is some stray voltage at the base of T1 turning the transistor ON.

[handiko]

Ps: what I mean of test the Dorji board as a standalone module, is to remove it from the Arduino board. Don't attach it.
Use jumper cables to connect the +5V and GND from a power supply or an Arduino.

handiko

[handiko]

Another way to troubleshoot (must be checked in sequence):

1. remove R3 (5k6). If the problem goes away, then it's your Arduino that somehow outputs logic HI on D7. Check it and correct it. If the problem still persists, then,
2. remove T1. If the problem goes away, then it's the T1 which not compatible or failed. Replace T1. At idle, the collector of T1 should be high and at TX it should be low. If the problem still persists, then,
3. something is wrong with the PCB routing, soldering job, or the DRA818 module itself. The PTT pin of the DRA818 (pin 5) might be pulled to GND.

[TF1987]

Thanks. I will try.

Greetings from Germany

[TF1987]

Hi Handiko,

I tried all your suggestions. The PTT pin of the DRA818V is always low, when i only power the shield with 5V and GND, nothing else. When i put the shield onto the Arduino, the PTT pin of the DRA818V toggles between 0V and not more that 2.9 - 3.0 V. It seems that the DRA818V don't recognize the HIGH Level because it's too low?

Also i measured a resistance of 116 kOhm between collector of T1 and GND. It seems that this resistance comes from the DRA818V.

Maybe the DRA818V is damaged? Can I do anything else?

[handiko]

Hi Thomas,

Unfortunately, I don't have any module DRA818V at hand anymore. So I can't really compare your measurements with mine. But I'm sure my previous troubleshooting sequences are based on my past experiences. It could be DRA818V modules nowadays are a little bit different compared with past models. Your voltage measurements at the PTT pin of DRA818V are good, I think.

Maybe you could "force" the collector of T1 high by a pull-up resistor (a resistor from the collector of T1 to +5V) of maybe 4k7 to 47k. That way, at idle, the collector of T1 should be always high. When the T1 is turned on by a high signal on D7, the T1 collector should be pulled low without a problem.

If the problem still exists, I have to say that the module is problematic.

[TF1987]

Hi Handiko,

Thanks for your reply. I guess I found the problem. I tested the shield without an antenna and it works. In the moment I'll mount any kind of antenna there is much HF-noise on the PTT-pin of the DRA818V. So the pin will not go up to 3,2V anymore. I think I make this problem by my own, because I create a new shield-PCB routing. Maybe the antenna strip on the PCB is routed the wrong way. Maybe it's needed to create a new design...thats not good.

BR Thomas, DO4FT

[handiko]

Without seeing the PCB layout, it is almost impossible for me to tell whether the layout is "RF compatible" or not. It is a good practice to separate the ANT trace as far away as possible from the PTT trace. Since the RF transmitted by the antenna could couple back to the PTT trace, which could result in PTT hang. We could mitigate this by putting a couple of RF bypass capacitors at the PTT pin of the DRA818V module. These capacitors consist of 22pF + 100pF + 1nF paralleled. Preferably all use SMD capacitors.

All in all, RF PCBs are not as simple as we might think. And the best way to learn is by building our own design. There are plenty of resources out there which would teach us to properly design an RF PCB. Also, we could discuss our discoveries and obstacles here. Happy experimenting!!

[radio-satellites]

@handiko what is the purpose of the transistor (T1) on the PTT line? And is R4 (on the audio input line) a variable resistor?