Glenn VK3PE has built up one of the 9 component anti thump circuits described by ND6T (but it is in fact a design by VA7AT ). He has yet to actually try it in his rig.
The PCB is about 26 x 10mm in size. Only difference to ND6T’s version is he placed the 10uF’s on the PCB also. ie remove from uBITX board and fit to this board. Otherwise it follows ND6T’s web page for installation. It is made from SMD 0805 parts.
Bill K9HZ who we have already reported had developed a relay control system driven by the Low Pass Filter I/O lines. In addition, Glenn VK3PE has also come up with a circuit that could be adapted using a digital potentiometer module to be driven by the Raduino using a digital I/O line.
Relay drive control
Bill K9HZ has now drawn up the circuit he used to allow for relay switching of drive levels by band grouping, driven by the LPF band grouping I/O lines of the Raduino. No firmware mod is required to deliver even drive power across all of the bands.
RV1 in the yellow circle is the existing drive pot in the uBITx. Remove it and connect the wires from the relay as shown. Q17C, Q18C, and Q19C references connect to the transistor Q17, Q18, and Q19 collectors. Then set RV1A, RV1B, RV1C and RV1D on the basis of the appropriate band grouping selected to keep the power out flat.
Digital drive control
Glenn VK3PE has come up with an old circuit that could be used to work with a digital potentiometer module to use as adrive control set by the NANO firmware on a band by band basis.
The original drive control, RV1, would need to be removed and a few parts added, along with a digital pot (I2C control) in place of R3 below to form an attenuator in the RF path.
With some careful work it should be possible to design a small PCB that fits into the holes vacated by RV1, either vertically or horizontally.
The schematic is from page 62 of the book by Randy L. Henerson on designing a Transveiver. Its a very old book (1997) ISBN 0-07-028263-3
Glenn VK3PE is attempting to flatten the power curve from the µBITx by playing with the driver and pre-driver stages.
He has completed his mockup of the PA driver stages and
- used MPSH10’s in all stages.
- Used FT37-43 toroids all wound with 10 turns 0.25mm wire either tri- or bi-filar according to original Sch.
- Bypassed all emitter resistors with a 220pF cap.
- swept from C80 input to T10 secondary (as wound)
The result is a very flat response: better than 1db from 3Mhz to 54MHz.
Glenn plans to mock up the finals now to see what the net impact is on the power response curve. If it is nice and flat he will replace the parts in the µBITx and we will have a solution that everybody who wants to flatten the power curve can use.
Glenn, VK3PE, suggests that his measurements of DC levels and RF levels at various points in the PA driver section from Q90 onwards (with a 12.5v supply. ~8W out) might be of interest to some.
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.”