Transmitter remodulation – what's wrong with that?

Watch the following video of SN9D operation

Quite a nice operation, however? But you noticed an indication of ALC on the transceiver? Yes, the indicator significantly exceeds half. It means, that the control circuit of the amplifier must reduce the gain, as there is too much excitation coming from the driver stage.

This effect may be welcome for the operator – because it raises the average performance, but at the same time it is highly undesirable, because in this region the transmitter produces intermodulation (IMD) products.

Compressor effectWhat is ALC??

ALC is the transmitter circuit, whose function is to limit overmodulation, the occurrence of interference in the vicinity of the transmitted signal (entanglement) and transmitter circuit protection. If ALC registers overmodulation, it will automatically reduce the gain of the transmitter amplifier stages. The second function of ALC is reflectometric protection, which will also reduce the gain of the amplifier stages of the transmitter so as not to damage the output circuits.

ALC thus creates some signal compression. It manifests itself like this, that the hands of the power indicator are more to the right and the counter stations are talking, that the signal is stronger and stronger.


… the average performance is related to this

Every voice, its intensity and individual sounds have different intensity. We have the possibility to set the transmitter to maximum power as follows, so that no tip crosses it. However, relatively few voices reach such a peak. If we were to average the performance, we would get medium (average) performance. While the maximum can be, for example, 100W, medium power only 30W. And everyone knows, that 30 is less than 100.

What about that? The compressor is most often used, either low-frequency or high-frequency. The latter allows spikes (which are relatively few) limit.


What happens, if to any – usually the final one – amplifier, we supply too much excitation power?

Such an amplifier cannot work beyond its limits. It starts to behave similarly to a compressor, but with that difference, that will create new products on the nonlinearities of the active element, which the original excitation signal did not contain. Thanks to the compression, the operator will increase the average performance again, but the signal spectrum will be expanded. Usually some nearby station starts complaining, that your signal is wide. Do not under any circumstances use ALC to "amplify" the signal!

What such a signal might look like? This is how a very high-quality uBITX transceiver signal looks after modifications

IMD products with MSA-1105 and SNA586 MMIC amplifier and 45 MHz filter at a power of 2W
IMD products with MSA-1105 and SNA586 MMIC amplifier and 45 MHz filter at a power of 2W

uBITX is a very simple low-power transceiver. Therefore, it should not reach such quality parameters. However, if we increase the excitation, so the spectrum can look like this:

IMD products with MSA-1105 and SNA586 MMIC amplifier on 45 MHz filter at a power of 5W
IMD products with MSA-1105 and SNA586 MMIC amplifier on 45 MHz filter at a power of 5W

A quick look at the spectrum shows, that there are many of those signals, much more. And while in the first picture you can see only two prominent peaks below and above 44,992 MHz, so in the second picture you can see the tip on 44,994 MHz or weaker peaks around 44,996 MHz. The originally narrow signal has become wide.

The tone, as i am “treated” You can read the IMD of the uBTX transceiver at and how to set the modulation to SSB in the article Setting the modulation of the SSB transmitter

So if you next observe the ALC on the transceiver, care about it, so that the modulation level is set correctly. The quality of your signal is your calling card!