36HC108 - Short range radio transmitters for hi-fi stereo sound
This project is fully applicable as long the analog broadcast system not is replaced with
a digital system (such as DAB radio), but the design may be interesting for purposes
of study, then the method for delivering a radio signal is a bit special.
This radio transmitter do not consist of operational amplifiers or transistors, since you can use
HC logic for the same purpose. The project would be difficult to design if I did not have access
to my custom built filter simulator (DOS program), which make it possible to produce passive
filters for different purposes.
36HC108 includes a stereo generator and the construction is structured to
function for the system we use when we listen on radio (with stereo support).
The block diagram:
If you want to look closer on this thing you can download it here:
In the zip-file can you find everything required to implement it (manuals, diagrams and PCB-files)
- except to realizing it.
Since it's now easy to order a processor crystal with the value 4864 kHz (that was not the case before
where I live) so awakened the interest in to spice up this construction. I noticed a significant improvement
in stereo separation with the help of the new quartz crystal. Beyond this exists a reason to improve the hi-fi
properties as best as possible. This project contains therefore a new module named ST-AAC, which regulates
the signal level and keeps it constant. The improvements also include some fine-tuning of the old audio filter
circuit on ST-AUDIO.
To begin with: change the capacitor C3 on ST-AUDIO to 27nF. This provides a near optimal stereo separation.
Then it's time to brush up the sound. If we reducing C5 and C6 to 100 nF instead of 2.2 uF so will incoming
low frequency interference be blocked with sufficient effectively. R13 is changed to 22k (previously 47k)
which means that the base is lifted even more. C12 is changed to 10n (previously 22n) which highlights
the lower midrange. Finally shall the treble be lifted slightly by changing C8 to 3.9 nF, instead of 3.3 nF.
R16 can remain unchanged, but R15 should be raised to 220k (previous 100k) so that the pilot tone
not begins to dominate. Implements these changes will we get a richer and more vivid stereo sound
(in concerned radio receivers) and in addition reduces the background noise to a minimum. 2016-02.
2016-08: Further adjustments have now been made with two components. The midrange and treble
was apparently suppressed but has now been raised. The changes have resulted in even better sound
and a problem with the stereo decoding also disappeared. Change R13 to 100k (previously 22k) and
C8 to 3n3 (previously 3n9).