The following diagram from Henning DK5LV provides an insight into the final four stages of the µBITx PA chain. This stretches from Q90 through to the finals.
Warren WA8TOD has measured the gain and drive requirements of the transmit chain at operating frequency from Q90 through the finals. A sweep generator was inserted at C80 and the level adjusted first for a nominal 1 watt indicated on the wattmeter (Purple trace) and then five watts indicated (Blue).
– Gain of the chain varies from 56 dB at 3.7 MHz to 52 dB at 28.2
– Input levels at C80 were -24 dBm (purple) and -15 dBm (blue); note 9 dB input increase generates 7 dB output increase….. 2 dB com
pression and resulting unacceptable IMD (not shown)
Henning Weddig DK5LV says he is in the process of using an op amp and SSM2166 on his “original” BITX40 as a way of achieving an audio AGC function.
This op amp has a gain of 10 (20 dB) and an audio AGC system. He purchased a pcb from ZL1CVD with the DIL chip via ebay years ago. Unfortunately ZL1CVD does not sell this pcb any more…
The SSM2166 has a dynamic range of 60 dB, the opamp in front of it is intended to replace the first discrete amplifier after the demodulator and will amplifiy the audio into a range the SSM2166 can handle. The SSM2166 has a provision for outputting an AGC voltage (an RMS output) and this may be able to be used for driving an S-meter.
Others are thinking along the same lines. For example, Glenn VK3PE says:
“I’m thinking along the same lines. I built a Mic amp version using the SSM2167 and thought it might work also for the Receiver. It’s essentially the module seen on ebay but I’ve added a level pot on the output side.”
Meanwhile Alex PA1FOX comments:
“I am using the AGC from the original uBitx design, but found the time constant of 1uF and 100k to be far too low. This makes it a very fast AGC, not producing a nice sound with voices. I changed to 1.5 uF and 4M7 and now stronger stations are nice and clear (they ‘push’ the noise level down ) and the gain comes up nice and easy when the QRG is clear. I think I’ll keep it this way.”