varicaps?

[Phil Allison]

On Thursday, April 2, 2020 at 3:10:50 PM UTC+11, Gerhard Hoffmann wrote:

Am 02.04.20 um 02:41 schrieb Phil Allison:


** So misses out on 4000 or so stations in the US alone.

AM broadcast will never disappear, it's just too damn useful.

Here in Europe it has disappeared without a trace.

** Really??


https://en.wikipedia.org/wiki/List_of_European_medium_wave_transmitters

The masses moved to FM 40 years ago and now to a digital mode
which is harder to sell to the people.

** AM has major technical advantages in relation to range and simplicity/ reliability of receiving equipment.

Here in Australia we have thousands of AM transmitters, private and Govt owned and rely on them at times of emergency to get crucial info out to the public.

Example the recent, severe bush fires.



...... Phil

 

On Wed, 1 Apr 2020 16:30:55 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

Joerg wrote:

-------------

Up to about 30pf they'll likely remain. Skyworks and such. Those with
large capacitances lost their market.



** AM radio has disappeared ??

In many countries in Europe the domestic MW/SW stations have been
closed for more than a decade ago and also foreign services have also
been shut down a few years ago.

Domestic services using ground wave (and NVIS) was useful when the
distance between transmitter sites was large. These days when
cellular phones need quite close distances. there are also a large
number of sites for low or medium power FM/DAB transmitters and more
and more radio programs are received directly through the cellular
network.

News to me.


... Phil

 

[Chris Jones]

On 02/04/2020 10:30, Phil Allison wrote:

Joerg wrote:

-------------

Up to about 30pf they'll likely remain. Skyworks and such. Those with
large capacitances lost their market.



** AM radio has disappeared ??

News to me.


... Phil

If someone were designing an AM radio these days, quite likely they
would use a chip for the radio receiver circuitry, and those quite
likely would have a PLL with the VCO integrated on the chip, e.g.
TDA7786. So, the market for discrete varactors suited to the AM band may
have gone away regardless of whether AM radios stay popular.

 

[Rick C]

On Thursday, April 2, 2020 at 2:23:36 AM UTC-4, Chris Jones wrote:

On 02/04/2020 10:30, Phil Allison wrote:
Joerg wrote:

-------------

Up to about 30pf they'll likely remain. Skyworks and such. Those with
large capacitances lost their market.



** AM radio has disappeared ??

News to me.


... Phil



If someone were designing an AM radio these days, quite likely they
would use a chip for the radio receiver circuitry, and those quite
likely would have a PLL with the VCO integrated on the chip, e.g.
TDA7786. So, the market for discrete varactors suited to the AM band may
have gone away regardless of whether AM radios stay popular.

Maybe for the base level Honda Civic and the Yugo. These days you get an XM radio if you want it or not and the same electronics receives AM/FM without an extra chip. That is likely why AM was left out of the Tesla radio. It's not worth the bother of adding the passives required to filter the band.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

 

On Wed, 1 Apr 2020 22:42:00 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

On Thursday, April 2, 2020 at 3:10:50 PM UTC+11, Gerhard Hoffmann wrote:
Am 02.04.20 um 02:41 schrieb Phil Allison:


** So misses out on 4000 or so stations in the US alone.

AM broadcast will never disappear, it's just too damn useful.

Here in Europe it has disappeared without a trace.


** Really??


https://en.wikipedia.org/wiki/List_of_European_medium_wave_transmitters

Some of which are quite low power and some are operated only once a
month.

The masses moved to FM 40 years ago and now to a digital mode
which is harder to sell to the people.

The movement started in some countries (especially in Germany) after
WWII when most of the MW frequencies were taken away after WWII.

** AM has major technical advantages in relation to range and simplicity/ reliability of receiving equipment.

Why do you need a large range, when there are a huge number of
transmitter sites due to the cellular networks. ?

Here in Australia we have thousands of AM transmitters, private and Govt owned and rely on them at times of emergency to get crucial info out to the public.

Example the recent, severe bush fires.

Do they have their own emergency generators capable of running for at
least a weak ?

 

[Phil Allison]

On Thursday, April 2, 2020 at 5:32:41 PM UTC+11, upsid...@downunder.com wrote:

On Wed, 1 Apr 2020 16:30:55 -0700 (PDT), Phil Allison
pallison49@gmail.com> wrote:

Joerg wrote:

-------------

Up to about 30pf they'll likely remain. Skyworks and such. Those with
large capacitances lost their market.



** AM radio has disappeared ??

In many countries in Europe the domestic MW/SW stations have been
closed for more than a decade ago and also foreign services have also
been shut down a few years ago.

**This is NOT a comprehensive list.

https://en.wikipedia.org/wiki/List_of_European_medium_wave_transmitters

Domestic services using ground wave (and NVIS) was useful when the
distance between transmitter sites was large. These days when
cellular phones need quite close distances. there are also a large
number of sites for low or medium power FM/DAB transmitters and more
and more radio programs are received directly through the cellular
network.

** Drivel.

News to me.


... Phil

 

[Phil Allison]

Chris Jones wrote:
-------------------

** AM radio has disappeared ??

News to me.


... Phil



If someone were designing an AM radio these days, quite likely they
would use a chip for the radio receiver circuitry,

** Pure speculation - not fact.

Yawnnnnn....

...... Phil

 

[piglet]

On 01/04/2020 11:34 pm, John Larkin wrote:

On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

 

[Chris Jones]

On 02/04/2020 19:27, Phil Allison wrote:

Chris Jones wrote:
-------------------



** AM radio has disappeared ??

News to me.


... Phil



If someone were designing an AM radio these days, quite likely they
would use a chip for the radio receiver circuitry,


** Pure speculation - not fact.

Well perhaps we should speculate instead that the chip companies make
the integrated radio ICs as some kind of decorative ornament, rather
than something that is sold in quantity for use in radio receivers. No,
we shouldn't speculate that, it is surely fact!

 

[Jeroen Belleman]

On 2020-04-02 13:11, Chris Jones wrote:

On 02/04/2020 19:27, Phil Allison wrote:
Chris Jones wrote:
-------------------



** AM radio has disappeared ??

News to me.


... Phil



If someone were designing an AM radio these days, quite likely they
would use a chip for the radio receiver circuitry,


** Pure speculation - not fact.

Well perhaps we should speculate instead that the chip companies make
the integrated radio ICs as some kind of decorative ornament, rather
than something that is sold in quantity for use in radio receivers. No,
we shouldn't speculate that, it is surely fact!

Times have changed for sure. Look at the Silicon Labs Si4735
for example. Just one chip for LW, SW, AM and FM bands. Oh yes,
it does RDS too. No filter coils, no varicaps. A radio with one
of these looks nothing like what radios used to look like inside.
The only giveaway they still retained is a ferrite antenna rod.

Jeroen Belleman

 

On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Piglet]

On 02/04/2020 15:28, George Herold wrote:

On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:
On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

Fun, If I can riff on the physics. The variable C is caused by
changes in the depletion width. So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.

Yes, one can get the depletion zone narrower (higher C) by approaching
forward bias but that quickly stops getting useful above 0.6 - 0.7V.

Heh - that reminds me try that Red LED in the pre-forward bias
direction. Thanks.

piglet

 

[Phil Hobbs]

On 2020-04-02 09:29, jlarkin@highlandsniptechnology.com wrote:

On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.

An epitaxial PN junction makes an 'abrupt' varactor. Hyperabrupt
devices have increased doping near the junction, declining as you get
further away. Those typically have capacitance ratios of 10:1, but can
reach 30 or more.

Cheers

Phil Hobbs




--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:

On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

Fun, If I can riff on the physics. The variable C is caused by
changes in the depletion width. So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.

 

[Piglet]

On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:

On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.

Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

 

[Phil Hobbs]

On 2020-04-02 10:41, Piglet wrote:

On 02/04/2020 15:28, George Herold wrote:
On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:
On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a
varicap.  I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool.  I should have googled it first, there's a few examples
out there.

There are pots of low-pF ones on Digikey.  The 1000-pFish ones
(MVAM108
etc.) are long gone, and the tens-to-hundreds range is
declining.  My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about
0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of
fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp
could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

Fun,  If I can riff on the physics.  The variable C is caused by
changes in the depletion width.  So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.


Yes, one can get the depletion zone narrower (higher C) by approaching
forward bias but that quickly stops getting useful above 0.6 - 0.7V.

Heh - that reminds me try that Red LED in the pre-forward bias
direction. Thanks.

piglet

Besides C-ratio, the other important FOM for a varactor is Q.
Highly-doped diodes are the ticket for that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-04-02 10:51, Piglet wrote:

On 02/04/2020 15:45, Phil Hobbs wrote:
On 2020-04-02 10:41, Piglet wrote:
On 02/04/2020 15:28, George Herold wrote:
On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:
On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a
varicap.  I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool.  I should have googled it first, there's a few examples
out there.

There are pots of low-pF ones on Digikey.  The 1000-pFish ones
(MVAM108
etc.) are long gone, and the tens-to-hundreds range is
declining.  My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from
about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of
fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp
could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They
have
delta-c catios like 6:1 over some voltage range. Looks like
transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at
zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee
break I
will try those too.

piglet

Fun,  If I can riff on the physics.  The variable C is caused by
changes in the depletion width.  So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.


Yes, one can get the depletion zone narrower (higher C) by approaching
forward bias but that quickly stops getting useful above 0.6 - 0.7V.

Heh - that reminds me try that Red LED in the pre-forward bias
direction. Thanks.

piglet


Besides C-ratio, the other important FOM for a varactor is Q.
Highly-doped diodes are the ticket for that.

Cheers

Phil Hobbs
n

Aren't emitters highly doped? Does that mean I could explore B-E
junctions as poor-man's varicaps?

piglet

That was commonly done BITD. Unfortunately Rbb' is generally much
higher than the emitter or collector resistances.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Piglet]

On 02/04/2020 15:45, Phil Hobbs wrote:

On 2020-04-02 10:41, Piglet wrote:
On 02/04/2020 15:28, George Herold wrote:
On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:
On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a
varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples
out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones
(MVAM108
etc.) are long gone, and the tens-to-hundreds range is
declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from
about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of
fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp
could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee
break I
will try those too.

piglet

Fun, If I can riff on the physics. The variable C is caused by
changes in the depletion width. So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.


Yes, one can get the depletion zone narrower (higher C) by approaching
forward bias but that quickly stops getting useful above 0.6 - 0.7V.

Heh - that reminds me try that Red LED in the pre-forward bias
direction. Thanks.

piglet


Besides C-ratio, the other important FOM for a varactor is Q.
Highly-doped diodes are the ticket for that.

Cheers

Phil Hobbs

Aren't emitters highly doped? Does that mean I could explore B-E
junctions as poor-man's varicaps?

piglet

 

On Thu, 2 Apr 2020 15:12:00 +0100, Piglet <erichpwagner@hotmail.com>
wrote:

On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

Very nice data.

I could use a high-capacitance part and pad it down with a series NPO,
but that's messy. I want a high C ratio part, like a real varicap, to
avoid the high baseline value of a low-slope diode. The baseline
capacitance just sits there wrecking my tempco and slowing down my
control loop.





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

On Thu, 2 Apr 2020 15:41:48 +0100, Piglet <erichpwagner@hotmail.com>
wrote:

On 02/04/2020 15:28, George Herold wrote:
On Thursday, April 2, 2020 at 10:12:28 AM UTC-4, piglet wrote:
On 02/04/2020 14:29, jlarkin@highlandsniptechnology.com wrote:
On Thu, 2 Apr 2020 10:24:49 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 01/04/2020 11:34 pm, John Larkin wrote:
On Wed, 1 Apr 2020 16:56:48 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-04-01 16:20, sea moss wrote:

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't
seen it done.

Standard technique in ASIC design

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Cool. I should have googled it first, there's a few examples out there.

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108
etc.) are long gone, and the tens-to-hundreds range is declining. My
fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

According to my Boonton, a BFT25 c-b junction only goes from about 0.7
pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture
capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could
go to 20 volts.


You're going to need a bigger transistor. Try a BC847/2N3904 ?

piglet

I'm guessing that a varicap is doped to be, well, a varicap. They have
delta-c catios like 6:1 over some voltage range. Looks like transistor
junctions have much flatter c:v curves. So the tempco contribution
will be a lot higher if I use a transistor.

I've ordered some singles, Skyworks parts in the horrible SC79
package, and my 4-layer proto board should be here soon. If the board
house doesn't shut down.




Yes I think you're right about the flatter c:v curves. You only want
0.2pF swing so will have quite a lot of padding down.

I hooked up an SMD BC848C to the Boonton and got these:
Ccb (E open*)
0V 4.0pF
0.5V 3.2pF
1V 2.8pF
2V 2.36pF
3V 2.05pF
4V 1.85pF
5V 1.71pF
6V 1.60pF
7V 1.50pF
8V 1.42pF

Cceb (E tied to B)
0V 4.29pF
0.5V 3.54pF
1V 3.16pF
2V 2.69pF
3V 2.40pF
4V 2.20pF
5V 2.05pF
6V 1.92pF
7V 1.83pF
8V 1.75pF


A thru-hole version was Ccb (e open):
0V 5.35pF
0.5V 4.55pF
1V 4.12pF
2V 3.56pF
3V 3.19pF
4V 2.95pF
5V 2.72pF
6V 2.52pF
7V 2.33pF
8V 2.16pF

A thru hole RF part MPSH10 Ccb (e open):
0V 1.23pF
0.5V 1.09pF
1V 1.02pF
2V 0.94pF
3V 0.89pF
4V 0.86pF
5V 0.84pF
6V 0.82pF
7V 0.8pF
8V 0.79pF


A 4.3V 400mW zener was:
0V 211pF
0.5V 183pF
1V 168pF
2V 149pF

A junk box T1 red LED:
0V 28.4pF
0.5V 25.9pF
1V 24.4pF
2V 22pF
3V 20.4pF
4V 19.1pF
5V 18pF
6V 17.3pF
7V 16.6pF
8V 16pF

* I tried the NPN bjts Ccb-e and Cb-ce i.e. involving the base-emitter
junction - as expected the capacitance was higher at 12 to 16pF at zero
bias falling to 6-8pF at 6V (didnt go higher to avoid breakdown) but I
noticed that the low bias capacitance values looked jittery and only
settled down above about 2V.

My junkbox has some wire ended varicaps somewhere so next coffee break I
will try those too.

piglet

Fun, If I can riff on the physics. The variable C is caused by
changes in the depletion width. So what sort of devices would have
a large variation?
PIN photodiodes?
maybe c-b junc of high voltage transistors?
What else?

George H.


Yes, one can get the depletion zone narrower (higher C) by approaching
forward bias but that quickly stops getting useful above 0.6 - 0.7V.

I'm trying to get a lot of swing out of my oscillator to slam an ECL
comparator and get low jitter, maybe 1.5 volts p-p sine, so I need a
lot of back-bias on the varicap. Most are rated for 15 volts, so I'll
run up there, which is usually a pretty linear c:v region.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard