Over the last few years I have not had to think too hard about ensuring I’m driving my FT-2000 at a good level.  i.e. its not being overdriven.  It has an Automatic Limit Control (ALC) meter and its been a matter of adjusting the audio amplifier gain such that the ALC triggers occasionally and only on the highest signal peaks.  If a good quality transmitted representation of the input signal is to be produced its essential that the equipment stages are not overdriven.  I guess we all rely on the ALC and its correct adjustment is second nature, however, take away the ALC function and what do you do?  How do you ensure you don’t overdrive the radio or downstream amplifiers?  This is the problem I had 🙂

I’m using the FT-2000 as an exciter for my Kuhne Electronic VHF transverter.  In this mode the ALC circuitry of the FT-2000 no longer works and the transverter does not output a drive level signal which I could connect to the FT-2000 or to a homebrew ALC meter.  But the transverter does have a red ALC LED which illuminates when the drive level becomes excessive.

The signals I’m presenting to the radio are audio and are either voice from the microphone (voice chain) or data from a SignaLink USB audio interface (data chain). Each of these chains has several amplifiers which can be adjusted by an operator:

• Voice chain:  3 x microphone and 2 x RF amplifiers
• Data chain: 2 x audio and 2 x RF amplifiers.

Ensuring each amplifier is set to a level such that downstream equipment is not  overdriven producing distortion and splatter is essential!

So, how did I adjust everything?  Well, the transverter documentation says to start by setting the radio to full power (0.1mW when using the transverter connector) in RTTY or CW modes and key the transmitter, then adjust the TX Power control of the transverter to output  20 Watts which should be prior to ALC LED illumination.  Having done that using a constant CW modulated signal I had the transverter generating maximum power for a maximum signal drive level from the radio.  That set the upper threshold which could not be breached by the voice or data chains without distortion or the transverters ALC activating.

Next I fed the constant CW, RF output of the radio directly into my digital oscilloscope rather than the transverter and noted the signal power.  Interestingly, the power levels seemed much higher than the figures stated in the FT-2000 Operating Manual.  But from the previous step I knew the transverter was ok with this.

CW Transmission

This allowed the accurate identification of the RF power level driving the transverter and thus the baseline levels for the voice and data chains.  The next step was to generate a constant RTTY tone using the computer and adjust the amplifiers in the data chain such that the levels identified in the first step where not breached.

RTTY tone generated by DM-780

Once done, I performed a similar exercise on the voice chain for J3E (SSB).  By selecting a cumulative display mode it was possible to record the varying power level over several iterations of a CQ call.  As can be seen, most elements of the call resulted in an average level, but interestingly, the ‘G’ or Golf in G0RVM generated significantly more power.

J3E SSB Voice adjustment

The gain of the voice amplifiers in the voice chain were adjusted so that the upper threshold identified in the first step were not breached.  The last step was to adjust amplifier gain for A3E (AM).  I had expected this to be very similar to the J3E level but it needed to be slightly higher.

A3E (AM) Transmission

AM is not a mode I use very often, if at all, so I didn’t spend too much time adjusting amplifier gain but I did change the gain from the factory default to ensure that the upper threshold identified by the first step was not breached.  It could still go a little higher but it will do…

So, the result of all this testing is that I am now confident that using the voice or data chain’s I’m not going to overdrive the radio or transverter.  Downstream of the transverter is an RF power amplifier, but that is specified to generate 1kW for 25 Watts of drive, and as I don’t wish to run it at full power, overdriving should not be an issue.

I’ll just add at this point that the above approach is applicable to amplitude modulated signals only and thus not those that are frequency modulated.

Andrew
G0RVM