RadioBanter - View Single Post

December 22nd 03, 05:00 AM

"Pete KE9OA" on 12/20/03 wrote:

Hi folks,
I finally got the AM sync detector running. The hardest
part of it is the loop filter. I haven't been able to find any information
on loop filter design, when there is no divider in the feedback loop.
I have discovered that with a 4046, you set your lock range by
setting the Fmin, Fmax of your VCO, while the capture range is
set by the loop filter. With a wideband loop filter, the demodulated
audio sounds fine, but with a + or - 3kHz capture range, there is a
chirping type of distortion on the modulation peaks. In this case,
settling time is 50mSec, while with the broadband loop, settling
time is 100uSec.
Overall, these things aren't too bad to build, except for that loop filter.
Once I get that optimized, I will have a sync detector that is worthy
of being sold as a stand alone unit.
Projected price of this unit will be around the 100 dollar mark.

Pete

Are we to understand that you have moved away from using the MC1350 and NE602
in a homodyne configuration (your update of 2003/12/05) to the synchrodyne
approach with a PLL oscillator ?

I have just dug out an article:
Trevor Wheatley (of Surrey Electronics), "AM synchronous demodulator",
Electronics and Wireless World, Sept 1989, pp858-860
It offers SSB (LSB or USB), ISB, envelope, DSB and quadrature detection.
Reading the article closely shows that a primitive form of passband tuning is
available as well.

The article has one of the most comprehensive sync detector circuits which I
have seen. It also has some interesting comments on the effects of phase
modulated carriers (used on some LW stations in Europe) and allowable loop
bandwidths.

Keep up the good work.

73 John KC0GGH