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Offline GoldDigger1950
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« Reply #30 on: March 19, 2013, 07:15:27 am »
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Then use a digital resistor array. Remember that a fully tuned circuit is an LCR circuit, not just LC.

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« Reply #31 on: March 19, 2013, 09:55:02 am »
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Why not just use a Varicap? Shocked

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Offline GoldDigger1950
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« Reply #32 on: March 19, 2013, 10:29:46 am »
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Quote:Posted by homefire
Why not just use a Varicap?

He just said he can't find them.

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« Reply #33 on: March 19, 2013, 10:52:59 am »
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Substitute with Transistors or Other Diodes.

I've never done it but others have.

Just a thought.

Vericaps have other issues too like Injection of Noise .      If he could do the Individual Caps it would be much more stable for sure, Eaaa?

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 are not 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 and the "Q" of the 1N5408 is less. 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 VCO 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 AC must be maintained below 200mV peak. 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 LEDs, 1N400X series rectifier diodes, Schottky 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 AC amplitude remains small. Higher-current devices with greater junction area tend to possess higher capacitance.
 
Before the development of the varicap, motor driven variable capacitors or saturable-core reactors were used as electrically controllable reactances in the VCOs and filters of equipment like World War II German spectrum analyzers.


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« Reply #34 on: March 19, 2013, 11:06:49 am »
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Quote:Posted by homefire
Substitute with Transistors or Other Diodes.

You defeat the purpose of using the DSPic if you do that. One part versus dozens of parts is rather counterproductive.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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Offline xavierTopic starter
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« Reply #35 on: March 20, 2013, 01:35:21 pm »
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OK guys I found a new supplier, actually they are far from new but I had forgotten about them (been away for too long) AP Electronics, found them on the net too so will be spending some time researching there site for components, in the meantime I have made a small development board for testing some of my theories.   

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So many questions so little time

Offline GoldDigger1950
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« Reply #36 on: March 20, 2013, 03:20:01 pm »
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Do they have a web site, Xavier?

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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Offline xavierTopic starter
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« Reply #37 on: March 21, 2013, 02:32:38 pm »
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They sure do GD   

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http://www.onlinebusinesslist.co.za/company/530718/a-p-electronics/website
 



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So many questions so little time

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« Reply #38 on: March 21, 2013, 04:59:39 pm »
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Thanks, Xavier. Looks like you have a fairly good resource there.

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« Reply #39 on: March 25, 2013, 05:12:18 pm »
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I will need to go and see them. I found someone that knows exactly where they are in JHB (don't want to get lost in that place)  I see that kabier2 got Moderated in my absence. I was going to ask you can I program a DSpic with a normal pic programer or do need some thing else? I mean it does work with ICSP don't it?   

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So many questions so little time

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