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> Itt BB521 uhf varactor diode 2..18PF (qty.10)
Itt BB521 uhf varactor diode 2..18PF (qty.10)
ITT BB521 UHF Varactor/Variable Capacitance Diode 2..18pF (Qty.10)
On offer are several lots, each of 10 new (old stock) pieces of a variable capacitance diode by Siemens, on factory bandolier.
These are in a DO35 package, with wire leads (so, easy to work with) but you'll want to trim them down at higher frequencies. Designed for tuning UHF VCOs. Also useful for voltage controlled filters, timebase trimming and many other places a variable capacitance is needed.
* Low min capacitance: Vr= 28v for ~2pF
* High Capacitance Ratio ~8 (Cv1/Cv28)
* Low loss: series resistance of 0.55 Ohm typ @ 470MHz
* DO35 package with wire leads, easy for hand soldering
Once popular in TV tuners, now getting harder to find ...
A useful quantity here to experiment with (or save for a rainy day) at a fine price while they're here. One for the experimenters parts drawer ...
[NB. Most folks reading this listing will already know this stuff, so please skip if your intelligence feels insulted.] A property of most diodes is that when reverse biased they form a 'depletion layer' which acts as a capacitor. The thickness of the layer, and thus the capacitance depends upon the voltage applied. Variable capacitance diodes (aka varicaps or varactors) are designed to optimise this characteristic.
The most common application for varactors is as a replacement for mechanical variable capacitors, which are expensive to manufacture. Varactors have displaced the mechanical variety almost completely in commercial electronics over the past couple of decades.
As with mechanical variable caps, their main application is tuning oscillators or filters, usually alongside an inductor in a tuned ('tank') circuit. Sometimes they're used to trim the frequency of a crystal oscillator (aka 'rock bending').
The tuning range is a function of the ratio between the highest and lowest capacitance obtainable. These diodes achieve a 8:1 ratio with a 27 volt swing making them useful in broadband VCOs at UHF.
A stable voltage supply is very important in order to achieve a stable capacitance (and thus oscillator frequency). Usually a small dedicated regulator is used to isolate the tuning voltage from the main equipment supply. By superimposing an audio signal on the DC control voltage, the frequency of the oscillator is modulated and an FM signal results.
Varactors usually retail around the 0.50 each mark, often more for wire leaded ones ... here at a fraction of that ...
