ALC Overrun


Algenon M0JIO
 

Hi Microwave Group 

I’m new to the forum and looking for some advice on what I believe is called ALC Overrun. I am looking at purchasing an SG Labs 1296 transverter to get started on 23cms. All the advice is to use an IF radio with a max output power of 5W. I would like to use my Icom-7100 with the power turned down to 5W as the IF radio but I understand that some radios can momentarily output more power than set  thus damaging the transverter. Does the group have experience of this combo and/or is there a way for me to detect if the radio does output momentarily in excess of the set power. I have basic test equipment: dummy load, power meter, 50Mhz scope which I suspect will not detect any blip. 


Andy G4JNT
 

Use a 50 ohm resistor as a load and connect a diode detector across it.  A 1N914 etc will suffice  .   Transmit from your rig into that and use the scope to monitor the diode voltage.   That should show any ALC overshoot.



On Thu, 8 Apr 2021 at 09:48, Algenon <alg_edwards@...> wrote:
Hi Microwave Group 

I’m new to the forum and looking for some advice on what I believe is called ALC Overrun. I am looking at purchasing an SG Labs 1296 transverter to get started on 23cms. All the advice is to use an IF radio with a max output power of 5W. I would like to use my Icom-7100 with the power turned down to 5W as the IF radio but I understand that some radios can momentarily output more power than set  thus damaging the transverter. Does the group have experience of this combo and/or is there a way for me to detect if the radio does output momentarily in excess of the set power. I have basic test equipment: dummy load, power meter, 50Mhz scope which I suspect will not detect any blip. 


Greg - ZL3IX
 

Damage to the transverter isn't the only problem with using a rig with the ALC working very hard. You need to get someone to check your signal for splatter, or better still use a spectrum analyser if you have one. Making a fast-acting ALC that doesn't cause splatter (spectral regrowth) is not an easy design task and many commercial manufacturers don't succeed in that goal.


Algenon M0JIO
 

This is the circuit I used and the trace is attached below. 


I couldn’t see any overshoot and the height of the waveform corresponds to the 5W of power the radio was set to. My conclusion is that the IC-7100 does not produce excessive ALC Overshoot. 

 


Andrew Cartwright
 

Algenon,

Out of interest how did you get on? have you been using the transverter with your IC7100?  I ask because I am considering the very same route.

Regards

Andrew - G7MNS


Frank, G0GSR
 

Algenon,  I don't have a 7100 to check but I very much doubt that you have zero power overshoot, on SSB due to ALC action, although CW maybe just fine.

I think that you need a digital storage scope to capture it as the power overshoot maybe just 5mS duration (as in the case of the IC-7300).
You also need to view the ALC "working" by allowing  the gain to completely recover, then hitting it with a peak of maximum level.

I have done some tests, on SSB with pulsed tone, allowing 4s after each for the ALC to recover.

On my IC-7300, at low power levels, the ALC induced power overshoot was around 2dB but strangely on 4m, the power overshoot is much worse..
At higher power levels, gain compression in the PA reduces the overshoot but those brief "over-drives" don't help with the general crud that surrounds most signals.

Frank