Thanks, Eric, and all the other folks! I just got through building up the
2nd pass of the prototype, and it looks good. The MDS is less than .1uV, and
the system doesn't go into clipping until 50,000uV, so I would still like to
bump that up a bit. The sensitivity figure is with no RF amplification ahead
of the mixer. I am using a doubly-balanced diode ring mixer, followed by a
diplexer, crystal filter, and another matching network, followed by a
TDA1572. Audio distortion is .5%, with a 100uV signal.
I still would like to be able to implement the 1572's on-board oscillator,
but it just doesn't seem to work with a fundamental mode crystal. Using the
recommended values from Philips only yields an oscillator with the LC
network dominating the circuit.
Next week, I will start the Sync detector design. This weekend, I will be
winding some prototype loopsticks. I was able to purchase a few samples of
ferrite rod stock from Elna, but unfortunately, the minimum order from them
is 3,000 pieces @ $1.50 each. Sure beats Palomar's prices, at $15.00 each!
There are still a few other issues with the design................with a
1kHz tuning step size, the loop filter needs to have a natural frequency of
about 15Hz. This gives a settling time of around 100mSec, which is okay, as
long as you don't make large tuning changes (memories). There are some
solutions..........one of them is to use a DAC to pretune the VCO to the
desired frequency, and let the PLL take over afterwards. National
Semiconductor does have some chips in their LMX-XXXX series of synthesizer
chips, but unfortunately, the fractional N types with the speedup circuit
that I had in mind don't go low enough in frequency. Then, you have the
fractional N sidebands to contend with, unless you use a Modulated
Fractional N Divider (MFD). This technique used Sigma_Delta Modulation to
translate the noise sidebands far out from the carrier, so they are much
easier to filter. Hewlett-Packard used that design in their 8642 series of
signal generators, but in my web scouring, I did discover that Philips
supposedly had a Sigma-Delta type of synthesizer. Now, that sounds
interesting...............

Pete

Eric F. Richards wrote in message
...
Thanks for keeping us in the loop on this, Pete, I'm *really* looking
forward to this one hitting the market.

Eric

--
Eric F. Richards,
"Nature abhors a vacuum tube." -- J. R. Pierce, Bell Labs, c. 1940