On 12/01/2011 10:39 AM, Brian Lloyd wrote:
> On Thu, Dec 1, 2011 at 8:20 AM, w1hkj<
w1hkj-uwgUiYJEXI4AvxtiuMwx3w@public.gmane.org> wrote:
>
>> **
>>
>> For a moment imagine that you are still using a radio with crystal (or
>> worse mechanical) filters. The apparent gain over the passband of the
>> filter can vary by several dB and not seriously effect the voice
>> quality. It is a killer for controlling digital modes. The Tx-level
>> (really an attenuator) control provides a fine tuning of the transmit
>> signal level to maintain minimal ALC action and therefore best signal
>> quality.
>>
> Minimal ALC action only measures what is happening at the output of the PA,
> not at the intermediate stages. Is there always sufficient headroom in all
> the lower stages that PA compression (ALC inflection point) is always the
> limiting factor? (I should know all this stuff. I have been doing it for 35
> years.) It is certainly the likely case with older tube gear but not sure
> with solid state. I had problems in packet radio with overdriving the
> first-stage as the gain control would come after the first stage.
The problem is not indigenous to tube transmitters. Well designed
transmitters have sufficient audio and IF dynamic range to avoid signal
distortion due to compression. It is the driver/final stages that cause
problems with digital signals. Older rigs do not provide the variable
output drive as your flex does. Most of the newer transceivers do
provide a similar control. Even my 11 year old Kachina 505DSP has that
type of control, as does both of my TenTec transceivers. They still
have ALC indicators that need to be observed. All of my transceivers
have DSP implemented IF filtering and are very flat across the
passband. They do differ however in the amount of drive needed over the
range of 3.5 to 50 MHz.
Dave
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