Welcome to Ham Radio DX!

Amateur Radio Blog Posts

Want to learn more about Ham Radio?

SUBSCRIBE to the Ham Radio DX channel

youtube facebook new-instagram-logo-png-transparent-light twitter2



KL70 Audio Response

22 March, 2017 - Reading time: 2 minutes

Further to my simulcasting post. Initial testing indicated some distortion, especially with CTCSS tones in overlap. Voice was reasonable, but did "tear" in some areas and was distorted. To eliminate a few things I decided to bring back the transmitter on the mountain to compare with the bench unit to make sure that they were in phase and deviating the same/had the same audio characteristics. 

Here are the results of comparing the two units. Again, I set the HPF to 50Hz to get an identical comparison. 


Ignore values below about 800Hz for now. As you can see, the audios were not matched. At 1800Hz the main TX was deviating almost 300Hz more than the bench unit. I then decided to adjust the LPF in the audio lines on both transmitters to see if I could get them matched a bit closer - as you can see I also changed the HPF in the 8920 test set to 300Hz for a more accurate measurement.


That's more like it. The audios are more equally matched. Both transmitters could be run in this fashion. Notice a slight drop above 2000Hz. This is the LPF rolling off the higher frequencies in the splatter filter to eliminate harmonics passing through over deviating the transmitter. As the RTCM/VOTER board does all the limiting, this isn't really too much of a problem.


Above is a completely flat audio input on lifted R403 on the VCO. Again they match. The audio is flat up to about 6.5KHz. Now that we know the audios are matched, we can move onto other things.