Basics of Varactor Diodes - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: May 11, 2014, 12:06:55 pm
content: My latest video is a tutorial on vacator, or varicap diodes. The basic principle of operation is given. A test fixture is shown, and used to measure the capacitance vs. reverse voltage. As an example of how these diodes can be used, a voltage controlled oscillator (VCO) is built, and measurements shown.Enjoy!

author: w2aew
author_rank: Super Contributor
timestamp: May 12, 2014, 01:28:12 am
content: Quote from: wilfred on May 11, 2014, 02:11:31 pmThanks - I'm glad you liked it!

author: juani_c
author_rank: Contributor
timestamp: May 12, 2014, 01:49:42 am
content: awesome as alwayscould I use this oscillator with your "Simple 3 transistor sawtooth generator" to make a sweep generator? it would be a useful tool for filter testing...Could this VCO be made with BJT instead of FET?Thanks!

author: retrolefty
author_rank: Super Contributor
timestamp: May 12, 2014, 02:05:21 am
content: One thing to keep in mind is that the Voltage Vs Capacitance is pretty non-linear over it's full rated capacitance range. This can be important or not depending on the circuit application the varactor is used in.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2014, 02:39:06 am
content: Quote from: juani_c on May 12, 2014, 01:49:42 amWhy... would you even try? Vary the CCS or the threshold voltage, far more linear and far better range!The best I can claim is 1Hz to 4.5MHz in a single range:http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Triangle.gif(note: the drawing has the 2N4403 connected to the wrong side of the diff pair!)Tim

author: ConKbot
author_rank: Super Contributor
timestamp: May 12, 2014, 12:10:22 pm
content: What are the finer differences between PIN diodes and the varactors? Obviously the varactor is meant for control/adjustment and the PIN diode is meant for switching. Both have the same general overall operation, applied voltage adjusts capacitance of the junction, but surely there is notable differences.

author: han
author_rank: Frequent Contributor
timestamp: May 12, 2014, 12:24:46 pm
content: i haven't see the video(but i will), i find varactor in old tv tuner, with 0-30V biasing..so i will click play and

author: w2aew
author_rank: Super Contributor
timestamp: May 12, 2014, 01:07:50 pm
content: Quote from: ConKbot on May 12, 2014, 12:10:22 pmThe basic difference is that varactors are used in reverse bias, where tunable capacitance is needed for things like voltage variable resonant circuits, FM modulation, VCOs, temperature compensation of crystal oscillators, etc. They are typically not used under forward bias or for switching, except in specific circumstances (see hyper-abrupt diodes).PIN diodes are typically not used in voltage variable capacitance applications. PIN diodes are most often used under forward bias. Most applications take advantage of the excess stored charge in the intrinsic region under forward bias, and the voltage variable resistance of the device (switching and variable attenuator / phase shifter applicaitons). Some specific applications, like PIN Photodiodes, use reverse bias to take advantage of the exceptionally low reverse bias capacitance. I did a video on PIN diodes in RF switching applications - maybe you saw this:

author: ConKbot
author_rank: Super Contributor
timestamp: May 12, 2014, 01:31:51 pm
content: Quote from: w2aew on May 12, 2014, 01:07:50 pmDerp, I saw that but brain farted the details. Been playing with too many photodiodes forwards/reverse biasing them at work, characterizing them, all this bias on diodes stuff is blending together. Switching from an in-amp and no bias to a transimpedance amp with a couple volts of reverse bais on the photodiode = all sorts of magical bandwidth for 'free'

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 12, 2014, 02:06:21 pm
content: That's a coincidence I'm just building a VCO that using a varactor ,Thank you Alan I have a understanding of them now

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2014, 03:11:02 am
content: Quote from: M0BSW on May 12, 2014, 02:06:21 pmGreat! I'd love to hear how your VCO turns out. What band? What tuning range? etc.

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 13, 2014, 07:42:49 am
content: Quote from: w2aew on May 13, 2014, 03:11:02 amIt's a receiver I'm building with my daughter as part of her science project at school, so 40 metre's, is the chosen band, range not quite sure yet, I wanted to tackle the VCO with her to start with, the rest of the receiver she has to partly design, so we are having fun, with a pile of text books spread out., the most busy part of 40 in the UK is 7.100- 7.200 receiver will probably the easiest, your welcome to put an idea in the mix if you want to.Paul M0BSW

author: babysitter
author_rank: Frequent Contributor
timestamp: May 13, 2014, 07:52:21 am
content: (Paul, that sounds like fun. But I guess I have to wait a few years before doing that, at ~22 months and ~4 months olds...But the big one already knows how to turn on the frequency counter on my desk and has her own battery-supplied, mil-spec-hardened Voltage source+voltage divider+LED-DVM toy)Hendrik, callsign see below

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 13, 2014, 01:11:36 pm
content: Quote from: babysitter on May 13, 2014, 07:52:21 amThat's really nice hendrik, their the future of our hobby, Isabel my youngest daughter who is doing this has already at 13 years old wants to take her Ham Radio exam, and I really hope will, in time take on my call-sign when I've shoved off.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2014, 07:47:10 pm
content: How would one select a PIN or varactor diode for step recovery applications?

author: KJDS
author_rank: Super Contributor
timestamp: May 13, 2014, 08:27:20 pm
content: Macom has a huge library of RF diode application notes,Go to here,http://www.macom.com/supportAnd then select diodes as the product catagory group, varactor multipier diodes as the product catagory and cliick the applicatin note tabHappy reading

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2014, 08:43:29 pm
content: Quote from: KJDS on May 13, 2014, 08:27:20 pmAgreed - the major manufacturers are the best source for application specific information on the specialty devices like these.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2014, 09:17:29 pm
content: Maybe there is something wrong with MACOM's web site but none of their application notes listed under "varactor multiplier diodes" and "High Order Step Recovery" discuss step recovery applications at all. I just went through all of them in detail.

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2014, 09:19:50 pm
content: Quote from: David Hess on May 13, 2014, 09:17:29 pmYou may want to check out Microsemi, Aeroflex, and maybe even Skyworks.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2014, 10:22:51 pm
content: I will go through their sites again but so far the best information I have comes from books describing basic semiconductor physics of the devices which has not been very helpful when PIN and varactor diodes are not characterized by the manufacturers for step recovery operation. I have also run across a few existing products where apparently the designers faced the same problem and qualified other types of diodes for step recovery operation which is pretty much what I am resigned to doing.I may have figured out the Macom web site problem but that has not helped me find *any* of their application notes covering what I am interested in.

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2014, 11:20:28 pm
content: Quote from: David Hess on May 13, 2014, 10:22:51 pmIf you google step recovery diodes, you'll get some links from the popular manufacturers regarding applications, such as comb generation.

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2014, 12:50:02 am
content: Quote from: w2aew on May 13, 2014, 11:20:28 pmSure. I do that periodically (this is an ongoing project) but nothing I have found covers the selection of varactor or PIN diodes for step recovery applications. I know what I want to do but am faced with qualifying the diodes as well.

author: w2aew
author_rank: Super Contributor
timestamp: May 14, 2014, 01:50:13 am
content: Quote from: David Hess on May 14, 2014, 12:50:02 amMaybe the apps engineers at these particular manufacturers can help with the selection criteria, etc.