Audio input to transmitter - level? #ubitx

Rob French (KC4UPR)


So I'm driving digital audio into the uBITX, and I'm wondering how, in the absence of any real test equipment, to properly set the audio input to avoid transmit distortion.  Now, I do have the ability to set the peak-to-peak voltage of the DAC I'm using.  So, if the DAC is hooked up to the mic input, what would be the proper audio peak voltage level to be (typically) setting? 



in order to control the AF input for voice and digital i add the level input measurement on a vu-meter. You just need to calibrate the level you display on it to prevent any overloading of the AF input amplifier.

on the vu-meter i supervise the AGC and AF input with a switch.

73s from Didier F5NPV

Evan Hand

II do not know of a way to set it with out some test equipment, at least a dvm that can measure the current to the PA section.

Using either a current meter on the PA input, or an RF power meter on feed to the antenna, I would start with the supplied mic, and then do the "HEEEELOOOW" test in the wireup section on the HF Signals web page.  Use that current or power level as the desired level, and then adjust the audio drive to get the same level of output, or lower.  Then go and try the digital mode of choice and get feedback on signal quality. 

I have not used the digital modes with the uBITX, so there maybe flaws in the above.  I did use the method to verify an amplified mic setting to insure I did not overdrive the mic input, and verified as much as possible with the simple spectrum analyzers (SA) that I have (Cheap RF Explorer and a RSPlay RSP2 used as an SA).  As far as I could tell there was no splatter.

Above are just my thoughts. You are still responsible for the signal quality, and there may be better options, like going to a local club that has access to a SA or other test equipment.



Many options are possible to tune your AF level. obviously measurement tools like ocsilloscope tap directly at the output of the Af Amplifier or a spectrum analyzer is also very useful.
in another hand you can use also another RX, a SDR receiver or even a Websdr online in order to monitor quit accurately your transmission. When you find the correct level without distorsion spikes you just need to calibrate a vumeter with the suitable value and threshold. The UBITX does not have a AF level input monitoring 
and without an additional device to monitoryour level it is simply impossible to know if the level applied from the Microphone preamp is not too high or too low. 

in the picture below you will notice a little switch just below the vu-meter in order to monitor the AGC and AF input level. 
73s F5NPV / Didier


Big fan of analog meters. For bitx levels and especially in SDRs—to balance the chi. ;-)

Jack, W8TEE

Would this count?

Inline image

Jack, W8TEE

On Sunday, October 13, 2019, 8:51:16 AM EDT, bobolink <rwhinric@...> wrote:

Big fan of analog meters. For bitx levels and especially in SDRs—to balance the chi. ;-)

Jack, W8TEE

Don - KM4UDX

Is it as simple as adding the(appropriate) resister in series with the meter + side and ground the meter - side?

And where would I tap the a V4 board for the resister going to the + meter?




You can monitor your AF intput with this device . You can tap directly at the input from the mike plug though a 600/600 transformer and in serie with a 10nf capacitor.

vu meter


Rob French (KC4UPR)

Do you use the whole circuit board? Or just the meter, connected to the transformer and capacitor?  Also, where do you put the capacitor?  Would you use the capacitor to block the DC bias from shorting to ground through the transformer? Or would the capacitor be closest to the meter?


Hello ,

The capacitor is tap directly on the mike plug and the AF signal (in parallel in fact) The main purpose is to insulate the mike AF circuit from the vu-meter device.
I am using the full device in my other ubitx for the Measurement of the RX level tap from the R70 Resistor and the other channel for AF input measurement
73s Didier F5NPV