VCO without a varactor?

Thread Starter

BlackCow

Joined May 11, 2009
65
I want to build a stable FM modulated transmitter and receiver with a VCO controlled with a PLL (some new concepts I am learning and quite excited about).

My issue is all the VCO circuits I have read about use a varactor (which makes sense). My issue is that Jameco (where I usually get my parts) doesn't seem to have varactors and I don't know where else to buy them.

So are there any VCO designs that don't require them or, better yet, can someone tell me where to get my hands on some varactors?

Thanks!
 

bertus

Joined Apr 5, 2008
22,270
Hello,

Many common diodes also have the capacitance change.
See this quote from the wiki: http://en.wikipedia.org/wiki/Varactor

Substitutes for varicap diodes

Experimentation with the varicap effect need not remain in the realms of advanced electronics or solid state physics labs. All semiconductor junction devices exhibit the effect, some to a surprising degree. Although many common devices exhibit the effect, they were never designed for that purpose so the effect can vary widely between one batch of a certain device and another. This prevents manufacturers from using any old device as a varicap, they need devices with reliable, tightly specified characteristics from one batch to the next, however for small scale experimenters this is generally not an issue and individual devices can be measured and selected for the desired characteristics before insertion into a project.

The Philips BA 102 varicap and a common rectifier diode the 1N5408, exhibit similar changes in junction capacitance with the exception that the BA 102 possesses a specified set of characteristics in relation to junction capacitance whereas the 1N5408 does not. However individual 1N5408s can be tested for their suitability for use as varicaps prior to insertion into a circuit. Both the specific devices exhibit a reduction in capacitance from around 110pF at 0v bias to 60pF at -5v bias. This is almost a halving in capacitance, allowing construction of a V.C.O. which is adjustable over a full octave of frequency. The voltage-capacitance relationship of the 1N5408 is quite different from that of the BA 102 and it is quite easy to forward bias the 1N5408, so the amplitude of the applied A.C. must be maintained below 200mV Pk. Placing multiple devices in parallel will increase the effect by the number of devices used, so 3 1N5408s will vary in total capacitance from around 180pF at -5v bias to 330pF at 0v bias.

Popular makeshift varicaps include L.E.D.s, 1N400X series rectifier diodes, shottky rectifiers and various transistors used with their collector-base junctions reverse biased, particularly the 2N2222 and BC547. Reverse biasing the emitter-base junctions of transistors also is quite effective as long as the A.C. amplitude remains small. Larger current devices with greater junction area tend to possess larger capacitances.
Bertus
 

panic mode

Joined Oct 10, 2011
2,715
any standard reverse biased PN junction can be used as varactor. before i could get my hands on proper varactor i was even using BC of general purpose BJT transistor (BC109b) as a substitute.
 

Thread Starter

BlackCow

Joined May 11, 2009
65
any standard reverse biased PN junction can be used as varactor. before i could get my hands on proper varactor i was even using BC of general purpose BJT transistor (BC109b) as a substitute.
Sounds like this will be fine for my experimentation needs.
I'm still wondering, where do you get your hands on a proper varactor?
 

Wendy

Joined Mar 24, 2008
23,415
A simple 555 is a VCO. It could be frequency converted after the fact. I list this as one possible example.

I have seen simple FM transmitters that use the PN junction of the transistor. This is another.

Define what you consider RF? 100Khz is RF under the right circumstances.
 

panic mode

Joined Oct 10, 2011
2,715
Bill, that is true, few articles such as this:
http://en.wikipedia.org/wiki/Radio_frequency
also consider mere 3Hz to be RF, but we all know this is much of a stretch and hence not answering my question. i think we can say 0.3Hz is also RF, it's just that nobody ever built an antenna for it.

to me 'real RF' is what is commercially and commonly used in radios - 0.5MHz to tens of GHz (and beyond of course - even if it is not common).

for example, i can walk into ANY electronics store and:
- get various 'radio' products (common),
- most if not all using PLL (common)
- all of which are in 'my' RF range

that includes receivers for AM, FM, TVs, cell phones, WiFi products, you name it. and i'd say it is a safe bet that none of them are using 555 as VCO. (any takers? ;) )

one of earlier comments suggested that varactors should not be used because all real VCOs don't use varactors. i am still waiting to see an example of such real and commonly used VCO. everything i came across that says 'radio' and 'PLL' or 'VCO' is varactor based.
 

THE_RB

Joined Feb 11, 2008
5,438
Here's a variable freq oscillator using magnetic frequency control;
http://www.hanssummers.com/huffpuff/fast.html

I quote his page;
"When a toroid is placed in a magnetic field, its permiability decreases, causing a decrease in inductance. A varying magnetic field can be generated by applying a varying voltage to a relay coil. In this way a correction can be applied to the VFO by the stabiliser circuit, with the relay coil and magnetic field replacing the conventional varicap diode which you see in all the usual Huff & Puff circuits."
 

panic mode

Joined Oct 10, 2011
2,715
thank you,

this looks better. it may not be as compact as VCO with varactor but this is viable solution. i like the fact that it is linear.
 

THE_RB

Joined Feb 11, 2008
5,438
You could probably also adjust frequency by puttin a second winding on an inductor and applying DC to the second winding (magnetic amplifier style).

Or one that has interested me; if the oscillator is subject to change frequency as PSU voltage changes, then you could trim freq by making small adjustments to the PSU voltage. That gives VCO effect and also eliminates instability caused by normal PSU fluctuations.
 
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