EMRFD Message Archive 7809
Message Date From Subject 7809 2012-09-22 20:31:15 g0kla T2 Audio response not flat I built a micro T2 phasing transmitter for 40m a while ago, but I seem to have a problem with it that I don't understand. I occasionally get poor audio reports, usually telling me that my audio is missing the high tones or that it sounds like it has too much base.
I measured the RF output for different tones fed into the microphone input and that does seem to be the case - Here is a graph of RF output for a selection of audio tones fed into the mic input: http://www.g0kla.com/workbench/gw_rfout_audioband.gif
I traced this through the transmit chain. Everything seems to be fine in the audio sections. The output of the audio processor for I and Q seems fine, but once the audio passes through the diplexer, it starts to be attenuated. This is then amplified after the mixers in each stage of the transmitter. The input and output of the diplexers are shown in this graph http://www.g0kla.com/workbench/gw_diplexer_levels_audioband.gif
Any thoughts on what is going on here? Is this normal? I thought the audio response of the diplexers should be flat.
Otherwise the rig works well. I have worked 95 dx entities on 40m with it, so it's not exactly a terminal problem. I just want to understand why it behaves this way.
7810 2012-09-22 21:15:03 KK7B Re: T2 Audio response not flat Hi Chris,
I expect you have an early version of the microT2. The frequency response in the first version has a gradual constant downward slope from 300 Hz to 3300 Hz. That ends up nearly flat with some electret microphone elements I used during the development that have a peak near three kHz. With a flat microphone, the gradual downward slope will emphasize the bass.
I did some careful measurements using a flat noise source and made a slight circuit adjustment to obtain a flat response on the 2nd and later versions of the circuit board. The reports are on the R2pro yahoo group website here:
in the pdf files labeled microT2modulator and microT2performance.
The change described in the reports is simple: add a 47 ohm series resistor between the modulator outputs and low-pass filter inputs. That resistor pair was incorporated on the circuit board of the 2nd and later revisions of the microT2. Depending
7811 2012-09-24 05:58:09 g0kla Re: T2 Audio response not flat Rick,
Thanks for the response. That sounds like it could be the issue. I'll try it and report back with my results.
7812 2012-09-24 20:04:37 g0kla Re: T2 Audio response not flat Rick,
I inserted 47 ohm series resistors between the modulators and the low pass filter inputs. I measured the response again and it looks much flatter, as you can see here:
I had to increase the mic gain as you indicated. Everything else looks good, still with a clean two tone signal. I'm going to see if I can get some audio reports on the air.
thanks for the help
7821 2012-09-29 12:55:52 chrisethompson Re: T2 Audio response not flat Reports 7823 2012-09-29 19:10:47 kb1gmx Re: T2 Audio response not flat