Duplex MMDVM board not working correctly?

[K1SEH]

Part 2: this configuration does not work.

2020-06-05_16-15-00.png (103.04 KiB) Viewed 6081 times

2020-06-05_16-15-17.png (14.71 KiB) Viewed 6081 times

In this instance, both TS1 and TS2 are showing as enabled on the dashboard, but the hotspot and handheld are not communicating with each other.

Part 3 will have dashboard screenshots.

[K1SEH]

Part 3: dashboard screenshots.

This does work with my handheld:

2020-06-05_16-10-19.png (76.51 KiB) Viewed 6081 times

This does not work with my handheld:

2020-06-05_16-14-41.png (63.93 KiB) Viewed 6081 times

Please disregard the RF activity in the second screenshot, that does not represent reality (it's left over from the previous, working, configuration).

[K1SEH]

The possibility exists that I'm not adequately explaining my issue.

Since I'm using two different frequencies, I am, in fact, using duplex. My understanding is that a dual-hat modem is supposed to enable both timeslots, as well as allow me transmit on one timeslot, while the receiving another (or something along those lines). That is what's not happening.

[G8SEZ]

How do you have the antennas arranged? These dual Hats will have problems if you use the antennas connected to the SMAs on the PCB.

I also decided to buy a dual Hat via Amazon, I have a spare Pi sitting there not doing anything so adding a Hat to it enables further experimentation.

[K1SEH]

Umm...connected to the SMAs on the PCBs.

But...

Why would they work when I'm using the non-dual-hat config, but don't work when I am using the dual-hat config?

[G8SEZ]

The possibility exists that I'm not adequately explaining my issue.

Since I'm using two different frequencies, I am, in fact, using duplex. My understanding is that a dual-hat modem is supposed to enable both timeslots, as well as allow me transmit on one timeslot, while the receiving another (or something along those lines). That is what's not happening.

Yes, the idea is that the transmit side of the hotspot sends relevant data in the mid-slot bit-fields of the timeslot the mobile is using for receive so you get the immediate confirmation from the mobile that it is accessing the repeater/hotspot. You can then have two mobiles on opposite timeslots. Each individual mobile is operating as half-duplex.

[K1SEH]

So, I haven't lost my mind (today, anyway).

That's what I thought was supposed to happen; it's just that it's not.

[G8SEZ]

Umm...connected to the SMAs on the PCBs.

But...

Why would they work when I'm using the non-dual-hat config, but don't work when I am using the dual-hat config?

Right, well the answer is that the low isolation between the Tx antenna and Rx antenna means that the Tx signal into the receiver is pretty high and in addition the wideband Tx noise at the Rx frequency goes straight into the receiver and desenses it. You have a signal level from the Tx which is about 123dB above the receiver sensitivity, it needs a lot of isolation to reduce that to a level that the receiver can cope with.

You could try putting a dummy load on one of the antenna connectors or using some cables to separate them by a good few metres. Running the Tx below the maximum +13dBm output will also help.

[K1SEH]

My power level is at 50. I can borrow a dummy load, but I'll have to see if I have an SMA to PL-259 adapter.

[G8SEZ]

Each Hat will be different depending on exactly what components it was built with, some TCXOs might be closer to the correct frequency and/or have lower phase noise leading to less noise close to the RF carrier. Different decoupling capacitors might affect the wideband noise generated by the Tx ADF7021 chip. The same is true of the regulators and their decoupling.

It may be digital radio, but in the analogue domain it's still subject to these effects.