I've got the latest version of the sync detector completed. When I called
Future Electronics for some quotes on the SA637, I discovered that this chip
had been replaced by the SA647. Both of the chips are discontinued. I've got
about 1400 of the 637s in my private stock, but that won't be enough to put
this into production, so the next step is to design a sync detector, using
an Analog Devices AD607. The folks at AD were nice enough to send me a
workable application circuit, but unfortunately, this is going to require a
four layer board, so I am not going to be able to etch the boards at home
with this one. Not a bad thing in itself, but I am going to have a board
house do the prototype boards.
Pete
Pete KE9OA wrote in message
...
Thanks John! I did finish the MC1350-NE602 design, but took it a step
further, integrating that circuit into an SA637 chip. The SA637 does an
even
better job than the first circuit, eliminating the need for that coupling
transformer that is needed for the MC1350 for best performance.
Once I got that circuit successfully developed, I decided to go with a
true
PLL type of sync detector. I just got through with the board layout
yesterday, using an SA637 as a limiting stage, feeding the limited I.F.
signal into the phase detector input of a 4046 PLL chip. The I.F. signal
is
split into two branches............the 1st branch is applied to the I.F.
input of the SA637, while the 2nd branch is applied to the RF port of an
NE602. The VCO output of the 4046 is applied to the LO input of the NE602,
and the audio output is taken from the I.F. port of the NE602. I decided
to
upgrade the circuit a bit, including a ceramic bandpass filter between the
I.F. output and the limiter input of the SA637. It should work pretty
well,
but I still want to be able to vary the phase of the 4046's VCO signal.
This
way, I will end up with zero degrees phase shift through the system,
affording good performance. It is important to keep the absolute system
phase shift with plus or minus a few degrees.
It has been quite a bit of fun so far. It will be cool, once our software
guy writes the code for the spectrum analyzer display.
We are still a few months off, but things are looking good.
The active loopstick circuit is working well enough to put on the market
as
a stand-alone unit. The gain is quite high, so I may need to put a
variable
attenuator on the output. Because I am using a source follower ahead of
the
final RF amplifier, the unloaded Q of the tuned circuit can be taken
advantage of.
I am not sure it I will put any regeneration into the active loopstick
design yet. None of the regeneration circuits that I have found on the
Internet have worked very well.
Pete
John Crabtree wrote in message
...
"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