ALC settings (wersja polska)

Pictures below show spectrum of SSB transmitter driven with human voice (not two tone signal generator) with three settings of MIC-GAIN and three ALC level deflections.


1. MIC-GAIN set to no/minimal ALC zone deflection:




2. MIC-GAIN set to 30-50% of ALC zone deflection:



3. MIC-GAIN set to 100% of ALC deflection:





Both 5KHz/DIV

Difference between 5KHz/DIV spectra:

Centre horizontal line is 0dB. The curve around centre point and vertical F line shows that average power increase of main usable transmited signal (area marked ) is quite low (around 2-3dB). Difference in power around main signal (+/- 5~20KHz away, zone marked 2) exceeds 20dB ! It is a source of splatter around main frequency when ALC is forced to work.


It is clear that any higher than occasional ALC deflection may cause transmitter overdrive and make bandwith of SSB signal wide like barn door ...
ALC reduces overdrive peak of RF signal so drive your transmitter without or occasional ALC deflection. Bigger MIC-GAIN and higher ALC deflection increases average power but not increase "S" readings at the receiver (because signal strenght meters are not enought accurate), it increase bandwidth and interferences much more (even 20dB) than average power (a few, 2-3dB). It is not a good deal. Signal may be louder (may be ...) but will be distorted for sure (see pictures how wide it became).
High ALC deflection is much worst than HiFi SSB and produce much stronger splatter around transmitter operating frequency.

The best way to make own signal clean and strong is use efective antenna, speech processor, minimal ALC deflection and low IMD power amplifier.

Do not worry about low power meter readings - it is too slow to show true power of your transmitter. If you want to check your power use osciloscope, probes, dummyload or spectrum analyzer with coupler or attenuator.

If speech processor is ON, MIC GAIN may need to be readjusted (or speech processor level) to keep ALC deflection occasional.

It is important that ALC level may be false increased by reflected power due to elevated SWR of antenna system. Use dummy load to set MIC-GAIN according to ALC deflection to avoid ALC-SWR error.

Most of user manuals advice that ALC should be no more than 2/3 of ALC scale BUT in PEAKS, not constant 2/3 of ALC scale. It is maximum allowable ALC deflection. In that peaks transmitter will be a source of splatter on close frequences (5-30KHz).

AB4OJ's review of IC7600 contain test of speech processor with MIC-GAIN set to 100% ALC deflection. In that situation ALC plays role of dynamic compressor itself and generating huge splatter around main frequency. AB4OJ did not tell about it. Speech processor test in such conditions does not make any sense because ALC plays role of compressor and effect of engaged speech processor is not visible at all. Unfortunatley often we can hear "shity signals" on ham bands from overdriven transmitters with wide splatter. Somebody reads that test may think that it is OK. It is not OK.



ALC is engaged (deflection observed) when transmitter is overdriven. ALC reduces drive with delay (that produces power overshot when low output power is set). If transmitter is overdriven it produces interferences that increase much faster than average power gain. Simply DO NOT overdrive your transmitter - keep ALC readings on minimal level. Set MIC-GAIN little below ALC just start.


Thanks for reading.


ALC settings and external power amplifier signal spectrum



Measurments were taken by ANRITSU MS710C and FTDX9000D (transmitter at class B) with voice recorded on internal voicekeyer. Signals/spectrums were captured using MAX HOLD function by 15-20 times transmited message from voicekeyer record at 50 and 200W output power.

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