MGL Avionics User Forum

Hi,

When I press the PTT, I initially get silence. If I speak immediately all is good. Clear transmission and no feedback.

However, if I say nothing, after about 1 second feedback starts and builds (just like when you get a microphone too close to a PA speaker). The only way to get it to stop is to talk over it, in which case the feedback gradually stops. After that any more than 1 sec silence while transmitting causes it to be begin again.

I'm getting a VSWR of between 1.2 and 1.3.

I'm wondering if it is some kind of weirdness between the V16 and A16 and/or something to do with VOX levels?

Ideas welcome ...

Gareth.

If you do not speak the V16 starts to increase the mic gain to some extent - it is part of the AGC system that aims to modulate your carrier to the fullest possible extent. The reason why you tend to get very good range out of this radio.
The possible gain increase is limited however (the increase is intended to cater for typical ranges of microphone signal variation as you would find in normal use). This AGC is NOT present for the normal intercom.

There are three possible feedback paths:
1) Pure audio from headset speaker directly to microphone. Reducing volume sorts that.
2) Coupling from headset output to mic input. This is inductive or capacitive coupling and usually caused by long audio cables where MIC signal is not properly shielded from headset output signal.
3) RF feedback. This is the nasty cousin. This is a feature of AM radios. Part of the TX signal makes it back into your microphone input (either intercom or radio). One of the ways I sometimes use to find this is get the system to "just" start feedback and clamp your hand tightly around the various wires. You may locate the one that is playing "receive antenna" this way. Then figure out why it is happening. Could be routing, not shielded properly etc.

Hi All,

Just in case anyone is searching with a similar problem, this is solved.

It was a bad case of failure to follow the instructions in the manual! In my defence when I originally installed it was with an early draft of the manual (A16 serial number 3) so I did not install the Bourns 5001 isolating transformer. In that version it was listed as optional and in the context of solving problems; later versions of the manual recommend it in all installations.

Anyway ... if you don't have the Bourns 5001 transformer installed between the V16 and A16 then you MUST turn down the mic gain on the V16 to -12db; the default is 0db which caused the problem.

Per the manual:
For a direct, non isolated connection:
Using the V16 setup menu – please adjust both Pilot and PAX MIC Gain to -12.00db (the lowest setting). On the A16 – adjust the COM output level to about the 2/3 setting and the COM input level to about 1/3. It is recommended to use the V16 VOX system at a fixed level of “1” (do not use the bypass setting). You will be using the VOX functions of the A16 – so leave the V16 at “1” which is just enough to switch the intercom in the V16 off when not it use.

Gareth.

Just an update on the above.

I am working on new firmware for the V16 and A16 which is starting to look quite promising.

The goal is to completely eliminate the audio and PTT wiring from A16 to one or two V16 radios, effectively using the unused RS232 port 2 on both V16 and A16 devices to transfer both audio (MIC audio and RX audio) as well as PTT signals.

This completely eliminates the possible interference issues that can easily be introduced in a setup like this due to the amount of sensitive audio wiring, accidental ground loops etc. At the same time very significantly simplifying wiring. As there is no conversion from digital audio to analog and back to digital there is also no loss of audio quality.

This firmware update will work on all units in the field.