"Paul Keinanen" wrote in message
...
On Mon, 1 Nov 2004 17:24:10 -0600, "Steve Nosko"
wrote:
The "back to back" configuration I am familiar with is like this:
+ Control voltage
|
R
|
+----||-----+---||---+
| |
| |
+--LLLLLLLL+LLLLLLLLL--+

There could also be a series cap in this configuration--...

You would need fixed capacitors on _both_ anodes and also resistors to
ground from both anodes to get the DC bias. The small series
capacitors are essential, since they take the most part of the RF
voltage.

Hi Paul,

Why both? It is the total series cap which is of concern. One cap
would simply be half the value of each of the two. 2 x 5pf = 1 x 2.5pf,
no?

Of course, the series resistor will reduce the tuning range.

I don't see this. It would be just like the one shown. 100k or 1M. It
is for DC and is large enough to be neglegible at RF. (strays acknowledged)

I think there are many two diode configurations.
One option:
+ Control voltage
|
R
2.5pf |
+-||-+-||---+---||---+
| | |
| R |
| | |
| gnd |
| |
| |
+--LLLLLLLL+LLLLLLLLL--+ DC gnd down here

THough I don't think it is necessary, if you require symmetry.
Another option:
+----+ Control voltage
| |
R R
| |
+----||-+-||-+--||---+
| 2.5pf |
| |
| |
| |
| |
| |
+--LLLLLLLL+LLLLLLLLL--+

Another option:
+-----------+ Control voltage
| |
R R
| |
+-||-+-||-+-||-+--||-+
| 5pf | 5pf |
| | |
| gnd |
| |
| |
| |
+--LLLLLLLL+LLLLLLLLL--+


[[I am obviously ignoring parasitics of the diode or cap body to ground.
More "stuff" of any kind in tthe in the circuit = more paracitic capacitance
to fight. That is a mechanical RF layout issue and, of course, not to be
ignored.]]

You could get away with the series capacitor and put multiple (maybe
10) varactors in series. Thus, the RF voltage across each varactor
would be low. You will need a high tuning voltage, perhaps 100 V.

I agree, Clearly a complex arrangement. However, is there really an
advantage to having low voltage if you now have so many contributors to the
problems...


... I also don't get this talk about the filter Z. Since you'd need to Z
match
in/out of the filter, it seems to me the varactor voltages will be the
same
for any Zin/out since this will be determined by how "tightly" they are
coupled into the resonant circuit and not the Zin/Zout, no? The Z match
will just change the Vin/out.

[describes z matching in/out of a filter...]

That's my point. Once you select an inductor type, you have fixed a number
of future decisions. The transformation can get you to wherever you need to
be *IN* the resonator....see next

To reduce the Z in a resonator, you will have to reduce both the
inductive Xl and capacitive Xc reactance by reducing the inductance
and increasing the capacitance (e.g. by multiple varactors).

So now the varactor must be _more_ of the overall capacitance to get the
desired tuning range, no. If you want minimal side effects, then the
varactors need to be just barely inthe circuit, so to speak, which means
that they will have small voltages.

Rhode wrote an article in QST a few years ago about running the HF
varactor tuning front end at a lower impedance level to avoid the high
RF voltages on the varactors.

Probably necessary reading, but I guess that I am trying to point out that
this is one of those "no free lunch" situations. That a wide tuning range
AND good IM, temp performance are each pulling the design in opposite
directions? I mean:
Wide tuning - means - varactors need to be a larger fraction of the total
capacitance...
But this means that the non linearity and temp drift of the diode has more
effect.
If you put series caps to reduce voltage, although you are reducing the
effect of cap non linearity and temp drift because the diode is now
"decoupled" from the tuned circuit, you are reducing the potential tuning
range.

I have no significant comments on most of the rest...except...

...However, I
have never seen parallel "coils" in any practical circuit, apparently
there are some parasitic capacitance problems.

Side bar: My first "short wave" receiver did this.
Take one 50's tube AM radio. Parallel both the RF
and LO coils with outboard coils to get 75 Meter
phone band (it was mostly AM then). I think it was
in Popular Electronics or some such mag.

However, I think that the OP should also study of making a shortened
1/4 (stripline or microstrip) resonator, with very wide resonators

Been there, done that (not varactor tuned, though). The needed
direction.

(and thus low impedance levels) and do the impedance transformation at
the input and output coupling. If multiple stage filtering is needed,
look for interdigital filters

Sure, Interdigital or combline... 220, 221. Whatever it takes...


If the tuning speed is not very large, look for some mechanical tuning
at the end of the stripline resonator,

Something you should not reject is a "ranged" system. Tune a smaller
range with the varactors and switch in/out other caps for larger shifts.

73,
--
Steve N, K,9;d, c. i My email has no u's.