"GRe"
"Phil Allison"
To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/-
1Hz.
Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.
Stability issues...
The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation takes
place looks very similar to the corresponding part of the BDW141
schematic.
The manual has a description of the sync option (page 7/8):
http://www.slashdocs.com/kwqytp/lag-...on-manual.html
Unfortunatly, other than the original manual I have, there's no schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).
I don't know about possible pitfalls of implementing such a sync input for
the BDW but maybe it's worth looking into it.
** Thanks for you interest and effort.
One problem is that feeding synch pulses into the bowels a low distortion
oscillator is BOUND to result in contamination of the sine wave signal.
Probably serious contamination.
The smallest things matter when aiming for a 0.001% THD residual !!
The -100dB notch filter I use for THD measuremenents has somewhat better
stability than the oscillator - since it uses MF resistors and polystyrene
caps for the bridge, plus a couple of WW pots made with Ni-Chrome wire.
Luckily, one only needs to achieve full null for a few seconds to get a
reliable reading on a scope or millivolt meter.
This is **** simple for THD percentages up to 0.01 % - but becomes a tad
tedious at 0.001 %.
My best result was circa 0.0005% for my Sony CD101 CD player, playing a test
disk, with a digitally generated sine wave.
.... Phil