J M Noeding wrote:

"Rick Karlquist N6RK" wrote:

Why bother with MOSFETs? You can get varactor diodes used for tuning
AM broadcast receivers with that much capacitance.


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

I really doubt these varactor diodes are much good for tuning a
transmitters PA? I don't want to go into retyping all the pages from
G3VA, but the circuits are still shown as noted in the first place at
http://home.online.no/~la8ak/c97.htm , you need some amount of mosfets
to build a 10nF variable capacitor, but it is realisable. Don't know
about other types mosfets than the mentioned ARF446/7 and APT10050, I
got some hundreds IRF630 for nothing on a Scandinavian VHF
meeting......
Technical topics, Radcom December 2003 pg 71 (G3VA): The note is based
on "Frequency-Agile Class-D Power amplifier, by Frederick H Raab and
Daniel Ruppe of Green Mountain Radio Research Company, and was
presented at recent IEE HF Conference at Bath University. It describes
(IEE Conference Publication No.493, pp81-85) an electronically-tuned
class-D power amplifier that addresses the simultaneous needs for
efficiency and frequency agility.

Since the yanks probably wouldn't accept any ideas from the ozzer
world, I can enlighten you that Dr Raab is WA1WLW

73
from LA8AK in Europe
--
remove ,xnd to reply (Spam precaution!)

I have no problems with good ideas! Its the weird political stuff I
don't want to hear.
--
I say, the boy is so stupid that he tried to make a back up copy of his
hard drive on the Xerox machine!

Michael A. Terrell
Central Florida

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

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