Hi,
Ive got a radio that has rotary switch selection of crystals for the
1st local oscillator.
I want to have electronic selection, where I can change the crystal
selected by use of control lines.
Can I use something like the 74HC4316 to do this without stuffing up
the capacitance of the oscillator and pushing the crystals far off
freq?
The oscillator is a Colpitts type and the crystals are approx 3MHz.
The crystals are made for 20pF holder capacitance. 4 crystals used
presently, so only need to switch these 4.

A circuit of the oscillator is here ...
http://www.box.net/shared/52vxldm8yl

Id prefer to have individual control lines available, rather than feed
in a binary address to the chip.

Id appreciate any comments and/or suggestions for a better device /
circuit to use. Perhaps diodes ?
thanks,
JEFF

Too bad the trimmer caps are on the ground side.  If they had been on the 
hot side you could just sequentially ground one end of the crystal with a 
1N4148 and about a mil of current through it to turn each individual rock 
on.

Jim

-- 
"It is the mark of an educated mind to be able to entertain a thought 
without accepting it."
        --Aristotle


> A circuit of the oscillator is here ...
> http://www.box.net/shared/52vxldm8yl
>

On Mar 11, 1:17=A0pm, sparks <jeff_...@hotmail.com> wrote:

> Ive got a radio that has rotary switch selection of crystals for the
> 1st local oscillator.
> I want to have electronic selection, where I can change the crystal
> selected by use of control lines.
> Can I use something like the 74HC4316 ,,,
> The oscillator is a Colpitts type and the crystals are approx 3MHz.
> The crystals are made for 20pF holder capacitance.

Probably it won't work well; the power supply isolation of the
74HC4316 isn't great, the 50 pF/3 MHz suggests about 1k
ohm impedance, and your ovenized oscillator seems to
indicate the (variable, about 100  ohms) series resistance
could be an issue as well as the channel capacitance (35 pF).

A bunch of reed relays driven by ULN2003 would give you logic control.
Pay attention to the reed orientation, you want the long end toward
the crystals.

On Wed, 11 Mar 2009 14:59:13 -0700, RST Engineering (jw) wrote:

> Too bad the trimmer caps are on the ground side.  If they had been on the 
> hot side you could just sequentially ground one end of the crystal with a 
> 1N4148 and about a mil of current through it to turn each individual rock 
> on.
> 

How about a 1:10 analog data selector on the high side?

Cheers!
Rich

Didnt think of reed relays. I'll have to look up what speed they can
switch at. Ive got a need to switch channels sometimes at rates faster
than 2Hz. Can reed relays do this?
 JEFF
> A bunch of reed relays driven by ULN2003 would give you logic control.
> Pay attention to the reed orientation, you want the long end toward
> the crystals.

Thanks. This was sort of the idea I had, and came up with the
74HC4316.
Dont know whether a data selector would still have the problems that
were mentioned for the above IC.
JEFF


On Mar 12, 1:24=A0pm, Rich Grise <r...@example.net> wrote:
> On Wed, 11 Mar 2009 14:59:13 -0700, RST Engineering (jw) wrote:
> > Too bad the trimmer caps are on the ground side. =A0If they had been on=
 the
> > hot side you could just sequentially ground one end of the crystal with=
 a
> > 1N4148 and about a mil of current through it to turn each individual ro=
ck
> > on.
>
> How about a 1:10 analog data selector on the high side?
>
> Cheers!
> Rich

Silly question I asked really about speed ....I looked up some example
specs.
Ok, theres the DIL package type, and the ones with an externally
encompassing soleniod coil.
Does anyone know if theres much in the way of difference in using each
of these in the circuit. I presume the stray capacitance will be
different?
JEFF

On Mar 12, 1:34=A0pm, sparks <jeff_...@hotmail.com> wrote:
> Didnt think of reed relays. I'll have to look up what speed they can
> switch at. Ive got a need to switch channels sometimes at rates faster
> than 2Hz. Can reed relays do this?
> =A0JEFF

On Thu, 12 Mar 2009 00:24:14 +0000, Rich Grise wrote:

> On Wed, 11 Mar 2009 14:59:13 -0700, RST Engineering (jw) wrote:
> 
>> Too bad the trimmer caps are on the ground side.  If they had been on
>> the hot side you could just sequentially ground one end of the crystal
>> with a 1N4148 and about a mil of current through it to turn each
>> individual rock on.
>> 
>> 
> How about a 1:10 analog data selector on the high side?
> 
> Cheers!
> Rich

I very much doubt that would work.  Diode switching is probably the way 
to go.

-- 
http://www.wescottdesign.com

On Wed, 11 Mar 2009 13:17:38 -0700, sparks wrote:

> Hi,
> Ive got a radio that has rotary switch selection of crystals for the 1st
> local oscillator.
> I want to have electronic selection, where I can change the crystal
> selected by use of control lines.
> Can I use something like the 74HC4316 to do this without stuffing up the
> capacitance of the oscillator and pushing the crystals far off freq?
> The oscillator is a Colpitts type and the crystals are approx 3MHz. The
> crystals are made for 20pF holder capacitance. 4 crystals used
> presently, so only need to switch these 4.
> 
> A circuit of the oscillator is here ...
> http://www.box.net/shared/52vxldm8yl
> 
> Id prefer to have individual control lines available, rather than feed
> in a binary address to the chip.
> 
> Id appreciate any comments and/or suggestions for a better device /
> circuit to use. Perhaps diodes ?
> thanks,
> JEFF

See if you can find any circuits that show such switching using diodes.  
I think that's your best bet.

What sort of physical constraints do you have?  Do you want to just 
replace the switch, or can you replace the whole section of the circuit 
with the crystals?

Or ponder this:  The '+V' is your positive supply, all resistances should 
be equal and enough to flow a couple of mA in the diodes, and you should 
ground just one of the control lines at a time.  Each diode will act as a 
switch for RF -- no current = open, current = closed.

Plan on retuning.

                                           +V
                                            o
                                            |
                                           .-.
                                           | |
                                           | |
                                           '-'
                                            |
                                            |    ___
   .-------------o-------------o------------o---|___|--.
   |             |             |                       |
   |             |             |                      ===
   V             V             V                      GND
   -             -             -
   |             |             |
   |             |             |     ___
   |             |             o----|___|-o  +V or 0V
   |             |             |     ___
   |             o-------------+----|___|-o  +V or 0V
   |             |             |     ___
   o-------------+-------------+----|___|-o  +V or 0V
   |             |             |
   |             |             |
   o             o             o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)



-- 
http://www.wescottdesign.com

sparks wrote:
> Hi,
> Ive got a radio that has rotary switch selection of crystals for the
> 1st local oscillator.
> I want to have electronic selection, where I can change the crystal
> selected by use of control lines.
> Can I use something like the 74HC4316 to do this without stuffing up
> the capacitance of the oscillator and pushing the crystals far off
> freq?
> The oscillator is a Colpitts type and the crystals are approx 3MHz.
> The crystals are made for 20pF holder capacitance. 4 crystals used
> presently, so only need to switch these 4.
> 
> A circuit of the oscillator is here ...
> http://www.box.net/shared/52vxldm8yl
> 
> Id prefer to have individual control lines available, rather than feed
> in a binary address to the chip.
> 
> Id appreciate any comments and/or suggestions for a better device /
> circuit to use. Perhaps diodes ?
> thanks,
> JEFF
   BROWSER HOSTILE!
1) What is it about that reference that crashes NetScrape 7.2?
2) Why does NOTHING show even with Insanity Express 5?
   I have not seen any Colpitts CMOS diagrams, so i would say have N 
seperate Colpitts oscillators feeding CMOS transmission gates (analog 
gates is another term) and drive the gates as you see fit.