** Really??
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
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.
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.
Maybe for the base level Honda Civic and the Yugo. These days you get an X M radio if you want it or not and the same electronics receives AM/FM witho ut 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 ban d.
-- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
Some of which are quite low power and some are operated only once a month.
The movement started in some countries (especially in Germany) after WWII when most of the MW frequencies were taken away after WWII.
Why do you need a large range, when there are a huge number of transmitter sites due to the cellular networks. ?
Do they have their own emergency generators capable of running for at least a weak ?
-------------------
** Pure speculation - not fact.Yawnnnnn....
..... Phil
**This is NOT a comprehensive list.
You're going to need a bigger transistor. Try a BC847/2N3904 ?
piglet
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
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
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 1.42pFCceb (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 1.75pFA 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 2.16pFA 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 0.79pFA 4.3V 400mW zener was:
0V 211pF 0.5V 183pF 1V 168pF 2V 149pFA 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
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.
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
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
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
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
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
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