[1000mp] Noise on MKV RX Audio

[Steve Fraasch]

A large enough choke may cause a voltage spike that will also toast the transistor.  Be careful applying reactive filtering networks.  I have had MUCH better success with RC filtering ILO reactive LC filtering.  All of the systems I develop at work employ RC filtering to the extent possible.

I would suggest the following:  Run the fan power as a twisted pair.   Attach the following components as close to the fan as possible.  From the transistor side, run a 10 ohm resistor in series, a .47 uF cap in shunt to the black fan wire, and another 10 ohm in series.  The RC filter will establish a pole (low pass filter cutoff) around 3 KHz, and with the second resistor it will act as a bidirectional lowpass filter.  Last, run the pair through a high permeability toroid, and wrap as many turns as possible.  You can test the choke by first wrapping a single wire around the core and measuring it with an Autek or MFJ impedance analyzer.  You are looking for an impedance peak well below 1 MHz.

I hope this helps.  The issue is whether the fan will run OK with 20 ohms of current limiting; I am making the assumption that it will.  The resistors will need to be large enough to handle the power dissipation.

Another solution is to run the fan power thru a linear regulation IC.  If the the power supplied is 12V, set the regulator (like a LM317) for an output of 10 volts.  That should provide sufficient isolation.  

73,

Steve, K0SF

-----Original Message-----
From: Paul Baldock <pbaldock at verizon.net>
Sent: Dec 15, 2005 9:48 AM
To: All about Yaesu 1000mp <1000mp at mailman.qth.net>
Subject: RE: [1000mp] Noise on MKV RX Audio

At 10:21 PM 12/14/2005, you wrote:

>I don't think this a good idea. The fan is switched by a transistor.
>That is of no matter. The capacitor will just charge a little slower.

The cap across the fan will cause a large current spike when the 
transistor turns on and probably kill it. If you look at the 
schematic you'll see why. A cap to ground from the positive side of 
the fan should work fine.


> >If that does not work, try installing a small choke in series with the
> >fan.
>A choke without a capacitor is useless.

The choke alone works fine because there is already a significant 
decoupling cap on the 13V line.

73s

Paul

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