wrong place for cap?

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ProfessorRoach
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wrong place for cap?

Post by ProfessorRoach » Mon Aug 09, 2004 4:44 am

I built the following very simple FSM and don't detect any RF, even a few feet away from my rig's antenna(thought I had noise p/u prior, but it tunred out to be miscalibration). Suppose the 50000 pF cap could should not be placed before the 1N34's? All RF meters or crystal sets I've seen don't have a cap in this position. A cap in this position would shorten the electric wavelength of the antenna, but this unit not frequency discriminant, (level change only) so what is it's purpose?<p>Image

ProfessorRoach
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Re: wrong place for cap?

Post by ProfessorRoach » Mon Aug 09, 2004 4:51 am

Corrections:<p>The cap = 50000nF
I used 1N34's as opposed to 0A95's.

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Re: wrong place for cap?

Post by dyarker » Mon Aug 09, 2004 5:52 am

yo prof, is the cap 50nF or 50uF?
(50000nF = 50uF)<p>Or maybe you put in 50pF by mistake. That would be MUCH to small to drive the meter.<p>The place is right. Helps keep you from measuring 60Hz power and other low freq sources.<p>What freq range is the rig you're trying to measure?<p>C U L -<p>[ August 09, 2004: Message edited by: Dale Y ]</p>
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jollyrgr
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Re: wrong place for cap?

Post by jollyrgr » Mon Aug 09, 2004 8:14 am

I might be wrong but I believe that the meter should be more like a 50uA device. Here is a link that provides the exact circuit you want and it uses the 1N34 diode you used:<p>http://w4zt.com/fsm/
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Re: wrong place for cap?

Post by Tommy volts » Mon Aug 09, 2004 8:37 am

Jolly Roger,<p>That is a nice FSM design project. What is the frequency range? I may build this.<p>
Why? "because it's there"

ProfessorRoach
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Re: wrong place for cap?

Post by ProfessorRoach » Tue Aug 10, 2004 4:18 am

The 50nF acts like a High-pass filter, that makes sense. I didn't think that the electric wave length was very significant (lack of freq. discrimination).
For those interested, I am using this to sketch patterns and do level checks, I have two yagi's, 2m (145MHz) and 6m (51MHZ).

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Re: wrong place for cap?

Post by ProfessorRoach » Tue Aug 10, 2004 4:27 am

Correction:<p>My 2m yagi is centered at ~143.0955MHz <p>(@145 the FCC might be knocking at my door!)
Sorry, I was reading something while making last post.

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Re: wrong place for cap?

Post by Dean Huster » Wed Aug 11, 2004 8:04 am

That second post with the "correction" of the cap value to 50000nF seems a bit off. 50000nF = 50µF and that's not a good value for that spot. I think the original 50nF (0.05µF) is correct.<p>By the way, 50nF is a non-standard value. 47nF is what should be presented on any up-to-date skems. And a 25K ohm pot will be difficult to find. Standard values for pots these days tend to fall as 10, 20, 50, so 20K ohm or 50K ohm would be better to list, although the value isn't critical as long as it isn't too low.<p>Changing out the meter sensitivity alters the sensitivity of the FSM. A FSM using a 50µa movement will respond to smaller signals than will one using a 100µa or 200µa movement. The 50µa movement will also be a lot more expensive. If you don't mind having to use battery power, you can always use a cheaper, more readily available and more rugged 1ma movement and drive it with an FET-input op amp. This will also increase sensitivity.<p>No matter how you implement the design, a FSM is an excellent example of what to NOT digitize. A FSM using a bargraph display would be bumping against the next-to-worthless category since you could never see small changes in signal.<p>Dean
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Edd
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Re: wrong place for cap?

Post by Edd » Sat Aug 14, 2004 1:02 pm

At that high of freq (2M), a like value of even as low as 470pf, (like its other internal disc ceramic RF companion’s value), would still be in order with adequate RF coupling of the input as well as providing DC blocking in the event of its errant wire probe lead or telescopic antenna element makes a stray move into hot RF power circuitry (No Kilowatts at 2 or 6 M..of course). The next thought would be to ask if the unit was built inside a plastic housing
This RF doubler circuit is somewhat dependant on its RF ground bypassing.
Even if it be the connective ground counterpoise presented by the users hand holding of the instruments conductive case….with either resistive physical…or hand capacitance to its casing.
Notice that in this circuitry, the metal housing is providing hand contact……with its internally grounded components being connected to the cases metal housing.
http://w4zt.com/fsm/Pc301315m.jpg <p>ooopss host is not forwarding that solo pic, so cut 'n paste in the above URL or go to:
http://w4zt.com/fsm/ and note the central cluster of 6 symmetrical photos and pick right bottom corner pic and zoom in on its component groundings to the metal housing ........any chance your case did not provide hand contact to chassis ground?<p>73's de Edd
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