On Wed, 26 Nov 2003 22:27:29 GMT, "Rick Karlquist N6RK"
wrote:


The tolerance of AM BCB varactors is extremely tight,
since the front end has to track with the LO in the radio.
Certainly far tighter tolerance than unspecified MOSFETs.

Which varactors are you referring to ?

Looking at the specifications for the BB212, which is a common AM
varactor on this side of the Pond, the relative capacitance error
between the two sections at 0.5 V is less than 3.5 %. However, the
limits for the absolute capacitance at 0.5 V is 500 - 620 pF.

The tempco of those varactors is very low. Probably better than
any inductor you are going to resonate it with. In the low
100's of PPM.

BB212: 500 ppm/C at 0.5 V, 150 ppm/C at 10 V

To get such huge capacitance, the control voltage must be quite low.

No it's not. The tuning voltage range is at least 10 volts and often
higher. At least as high as the breakdown voltage of the gate
oxide in a MOSFET.

The maximum tuning voltage is not the issue, the problem is at the low
control voltage area (maximum capacitance) in which the tuning voltage
to get the maximum capacitance can be as low as 0.5 V, which might be
less than the peak-to-peak voltage coming from the antenna.

varactor capacitance, thus, you would have to use two pairs of back to
back (4) varactors of 1500 pF each to get a total 1500 pF capacitance.

You can get several thousand pF in one diode from Toko. 1500 pF is
by no means the limit.

The Toko web page for varactors seems to be under construction. Do you
have the type number for this kind of beast ? This would be really
nice for some LF experiments, if the specifications are really that
good.

Paul OH3LWR