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Old 6th Dec 2022, 3:51 am   #1
Bazz4CQJ
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Default Varicaps which swing from 10-100pF ??

Does anyone know if such a component exists? If not, what's the biggest swing anyone knows of? I've never used a varicap so I'm on the bottom of a learning curve.

Thanks
B
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Old 6th Dec 2022, 4:29 am   #2
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Default Re: Varicaps which swing from 10-100pF ??

Many, many types of varicap (== Varactor) diodes are no longer made. The ones that still exist are very product-specific and are for AM tuner, FM tuner, TV tuner, cellphone. Due to increasing integration and migration to different (SDR) architectures, some of these applications are also falling below the turnovers semiconductor houses see profitable. There are some specialist ones from the likes of MA/com for military/aerospace applications with matching prices.

You're asking for a 10:1 capacitance ratio. This will give you a 3.16:1 tuning ratio if there are no fixed capacitors in play. There usually are, so your tuning ratio is diluted and will be lower.

Medium wave is officially 531 to 1602kHz. A ratio of 3.01:1, therefore a capacitance ratio of 3.01^2 = 9.102:1 is needed to tune it. Because of the need for trimmers and fixed capacitors, the varactors will need to work harder than this, but also receivers won't always do the full band assignment, so this militates the other way.

Your best chance is looking for parts aimed at medium wave AM radios like the MVAM series. Philips, Siemens Toko are worth looking at, but can be tedious to search. Don't expect well organised selection tables.

You may next find that the devices are bigger in capacitance than you need. Actually this is good. Varactors for tuning are best used in back-back common cathode pairs. This halves the capacitance you get, but preserves the ratio. The anodes go to DC ground via RF circuitry, the common cathode gets a positive-going tuning voltage via a resistor or RF choke.

Capacitance changes fastest at the low voltage, high capacitance end. Their Q is at its worst here as well. The high voltage end tunes decreasingly slowly and you get limited either by supply availability or diode voltage rating.

Enough to be going on with?

David
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Old 6th Dec 2022, 6:33 am   #3
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Default Re: Varicaps which swing from 10-100pF ??

Plenty of 1SV149 on the web.

LEDs make good varicaps.
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Old 6th Dec 2022, 8:38 am   #4
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Default Re: Varicaps which swing from 10-100pF ??

Mmmm...thanks for those comments. The 1SV149 gives lots of swing, but I think I'd prefer Cmin a bit lower. OK re LED's - that could offer aesthetic challenges .

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Old 6th Dec 2022, 10:52 am   #5
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Default Re: Varicaps which swing from 10-100pF ??

You might find it helpful to look at the web site of Hans Summers, [www.hanssummers.com]. There is a section on Varicap Diodes under his 'other radio topics' heading on the left hand side of the home page.

Hans carried out measured experiments with 'proper' varicaps, ordinary diodes and LEDs, and plotted the results of voltage versus capacitance swing as a series of graphs, which,- as already noted above-, are anything but linear!

The web pages also give links to similar experiments by other Radio Amateurs together with discussion on the right and wrong way to do the testing.

Like you, I was looking for a varicap with a large swing, [for use in a homebrew poor man's sweep oscillator], but found I was being too ambitious and ended up moderating the requirements.

Hope this helps

Jimbo M0GJD
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Old 6th Dec 2022, 12:36 pm   #6
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Default Re: Varicaps which swing from 10-100pF ??

Wide sweep oscillators are often done as a more modest sweep mixed down with a fixed oscillator.

David
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Old 6th Dec 2022, 2:10 pm   #7
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Default Re: Varicaps which swing from 10-100pF ??

@Jimbo thanks for that reference to Hans Summers. I couldn't use the link you posted (blocked, possibly by my ESET security software) but I found his site just by putting his name in Google.

@David- I maybe should have said earlier, this is not for use as an oscillator, but, potentially, an IF filter.

Thanks
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Old 6th Dec 2022, 2:54 pm   #8
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Default Re: Varicaps which swing from 10-100pF ??

KV1235, KV1236 and BB212 are capable of 100'S Of pF.
You would probably have to search to get them now, Cirkit used to sell them.
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Old 6th Dec 2022, 7:55 pm   #9
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Default Re: Varicaps which swing from 10-100pF ??

I once built a low-frequency VCO [200-400KHz, to be double-balanced-mixered with a 10MHz xtal] which used MVAM115 varicaps; it worked quite well.

Wide-swing varicaps are found in pretty much every current production MW/LW radio; you may need to strap a few in series/parallel to get the necessary swing for a sub-500KHz oscillator but it can be done....
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Old 6th Dec 2022, 8:55 pm   #10
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Default Re: Varicaps which swing from 10-100pF ??

Quote:
Originally Posted by G6Tanuki View Post
I once built a low-frequency VCO [200-400KHz, to be double-balanced-mixered with a 10MHz xtal] which used MVAM115 varicaps; it worked quite well.

Wide-swing varicaps are found in pretty much every current production MW/LW radio; you may need to strap a few in series/parallel to get the necessary swing for a sub-500KHz oscillator but it can be done....
Yes, to stay within 10pF at the low end, running 2 or 3 in parallel does look like it may be the way to go.

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Old 13th Dec 2022, 2:59 pm   #11
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Default Re: Varicaps which swing from 10-100pF ??

This has the swing you are after (10pF to 102pF) provided you don't mind SMT https://uk.farnell.com/nxp/bb201-215...-23/dp/1757785 I have attached the datasheet.
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Old 13th Dec 2022, 5:28 pm   #12
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Default Re: Varicaps which swing from 10-100pF ??

Thanks for that. I have very poor vision in one eye, and I declared the shack a "SMT-free Zone", but I could make the odd exception as that is a useful range.

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Old 16th Dec 2022, 8:53 am   #13
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Default Re: Varicaps which swing from 10-100pF ??

Here are some 10-60pF, 4 for £2 ...
Trimmer capacitors

Postage to the UK should be OK, or I could assist if they were interesting. Would help with your self-imposed exile from your trading partners.

eBay and other auction sites can be useful because this would likely be a vintage component.
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Old 16th Dec 2022, 11:21 am   #14
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Default Re: Varicaps which swing from 10-100pF ??

Thanks for the offer, but what I'm looking to do cannot be achieved with ordinary tuning caps.

I may reveal more about this fascinating project at Christmas time, as my gift to you all ..

B
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