CDM1250 + Kantronics TNC

[cleb]

Hello,

I am interested in interfacing my VHF or UHF CDM1250 to a Kantronics KPC-3+ TNC. I have not purchased the TNC yet, just looking at the feasibility of it. I would like to use the Mic jack on the CDM, so I can switch the TNC between the VHF and UHF CDM's easily. Has anyone interfaced a KPC-3+ to a CDM before through the Mic jack? Does the volume have to be up for the TNC to receive data? Is the audio still routed through the speaker when the audio out of the mic jack is used to go to the TNC? Also, I know the CDM has a bias on some of the input/outputs. Is this an issue with the KPC-3+'s input/outputs?

TIA,

c

[KE7JFF]

I have my Kantronics KAM+ with my Maxtrac using the Mic connector which of course uses the same mic connector. The connector sends handset audio at a fixed level so the volume knob doesn't change much; you do need to set levels on the TNC itself, but thats easy. As for the bias; I'm not 100% sure on the CDMs but if thats an issue, you can switch a jumper over on the TNC to bump up the TX audio volume.

[Jim202]

I have my Kantronics KAM+ with my Maxtrac using the Mic connector which of course uses the same mic connector. The connector sends handset audio at a fixed level so the volume knob doesn't change much; you do need to set levels on the TNC itself, but thats easy. As for the bias; I'm not 100% sure on the CDMs but if thats an issue, you can switch a jumper over on the TNC to bump up the TX audio volume.

There is a DC bias on most Motorola mic connections. Make sure the TNC has a coupling cap in series with the mic connection.

[Will]

Yes a coupling cap, 2.2uF plus to the Mic HI on the radio.

You will also need a RC filter to get balance between the low tone and the high tone modulation. Kantronics overlooks this.

[RFguy]

Mic connector is pre-emphasized audio, handset audio is de-emphasized. This is not ideal for FFSK data.

The rear accessory plug would be a better choice with flat Rx/TX audio being available (along with +12 volts to power the TNC).

[cleb]

Will: Can you recommend a RC filter to use for this application? I am not familiar with those.

RFguy: Although it will not be ideal, will it still be functional if hooked to the Mic connector? I would really like to avoid the accessory connector due to the radio location/mounting.

Thanks,

c

[Will]

Yes the Mic connector is good.

The R C filter would have to be set up using a service monitor to get the low and high tones somewhat equal deviation.

R C Filter: A resistor in series, a capacitor in parallel with the mic input. Also referred to as a low pass filter.

[RFguy]

RFguy: Although it will not be ideal, will it still be functional if hooked to the Mic connector? I would really like to avoid the accessory connector due to the radio location/mounting.

Yes,

i have seen many installs using the mic connector for packet. There are more issues in lower signal areas. It won't be idael, but will probably be satisfactory.

[Will]

[snip. There are more issues in lower signal areas. It won't be idael, but will probably be satisfactory.

There are lots of Motorola radios out here in one of the biggest RF sinkholes and never a problem with Kantronics interfaced into the Mic jack.
The thing is getting the modulation balanced. The RC filter is the key component, even on the hammy radios.