"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.

On Tue, 2 Nov 2004 11:58:17 -0600, "Steve Nosko"
wrote:


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?

Yes, it is doable with one fixed capacitor, however, the voltages
across the stray capacitances would be different and their effect
would be harder to predict.


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 meant to say series _capacitance_ (not resistance), sorry for the
confusion.


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

This would be an elegant solution.


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

As well as this.


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.

For a given total capacitance, putting more varactors in parallel
means that the capacitance each varactor must produce is reduced, thus
the tuning voltage must be increased, which is a good thing, since
this improves the ratio between the tuning voltage and RF voltage.
Dropping the impedance levels and hence increasing the capacitance
required also means that even more varactors can be connected in
parallel.

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

clip

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.

The OP clearly had something similar in mind, since he originally
asked for a 2:1 tuning range, but now he is asking for the 30-500 MHz
range. I do not think that such huge range can be handled by just
switching base capacitors. With such large frequency range, it would
make more sense, to build at least three completely independent
filters, say 30-90 MHz, 90-270 MHz and 270-500 MHz, while each filter
could switch in base capacitances. The two lower filters could be
lumped LC filters, while the last range could be limited to less than
one octave, so that strip line filters could be used.

Paul OH3LWR

"Paul Keinanen" wrote in message
The OP clearly had something similar in mind

Yes, that was my plan.

since he originally
asked for a 2:1 tuning range, but now he is asking for the 30-500 MHz
range. I do not think that such huge range can be handled by just
switching base capacitors. With such large frequency range, it would
make more sense, to build at least three completely independent
filters, say 30-90 MHz, 90-270 MHz and 270-500 MHz, while each filter
could switch in base capacitances.

I was thinking of two... 30-125MHz and 125-500MHz. I.e., each one tunes a
range of about 4:1. Something like 3-5 base capacitors would provide the
broad tuning, with varactors doing the fine tuning.

The two lower filters could be
lumped LC filters, while the last range could be limited to less than
one octave, so that strip line filters could be used.

The problem is that even though the 125-500MHz filter only needs to create a
bandstop region somewhere in the 125-500MHz range, it must otherwise still
pass the complete 30-500MHz range. As far as I can tell, this still
eliminates all distributed-style filters from consideration.

---Joel

"Paul Keinanen" wrote in message
...
On Tue, 2 Nov 2004 11:58:17 -0600, "Steve Nosko"
wrote:


Why both? It is the total series cap which is of concern. ...

Yes, it is doable with one fixed capacitor, however, the voltages
across the stray capacitances would be different and their effect
would be harder to predict.

Yes, at the higher freqs, strays become significant, as I remarked.
....

For a given total capacitance, putting more varactors in parallel
means that the capacitance each varactor must produce is reduced, thus
the tuning voltage must be increased, which is a good thing, since
this improves the ratio between the tuning voltage and RF voltage.
Dropping the impedance levels and hence increasing the capacitance
required also means that even more varactors can be connected in
parallel.

.... Let me ponder this... By golly, running several parallel varactors at
fifty volts does sound good, doesn't it. You are away from the high
non-linear region near zero. Never had that option, I guess.


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