[TenTec] #218 filter

Dr. Gerald N. Johnson geraldj at storm.weather.net
Tue Oct 3 14:09:14 EDT 2006

On Tue, 2006-10-03 at 07:21 -1000, Ken Brown wrote:
> The easiest way to check a filter (if having the expensive lab equipment 
> is easy) is to sweep it with a spectrum analyzer / tracking generator. 
> If you don't have access to one of those ( not the kind of gear 
> accountants typically can bring home from the office for a weekend ) 
> Then you can do the same thing manually by tuning across a CW carrier 
> and logging the audio output level at perhaps 50 Hz intervals. The AGC 
> needs to be turned off, and you do need an AC voltmeter or other 
> instrument such as an oscilloscope to tell you the audio level. Plot the 
> data on a cartesian plane. You need to make a audio output level versus 
> frequency plot.  You also need to have the  BFO frequency far enough 
> from the skirt of the filter so that the low end audio rolloff of the 
> audio stages does not mess up you measurements at that end of the plot. 
> You probably also want to do the same to another known good (not 
> necessarily the same bandwidth) filter.
> I think you need a basic analog, or at least not too digital, radio to 
> do this. The Orion will probably not be the radio to use for this 
> measurement, unless you have control of the BFO frequency, and can 
> really turn off all forms of automatic gain control.

Another technique that works is to use spectrum analyzer software on a
PC with sound card and let it display the noise spectrum of the received
audio with the noise of a dead band at the receiver antenna connector.
The longer the averaging, the smoother the response curve will be.
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer

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