Padder Playground (Tracking Curve)
Written by Bryce Ringwood   

Description

This is a continuation of the "Padder and Trimmer" program. Here, you enter data, calculate the "perfect trimmer and padder combination" and see what the tracking error in kHz will be. First enter the data in the top section and press "Calculate", then press "Plot" to plot the tracking error.

Next, you can see what happens when you change the value of the padder. (The padder value is the only one you can change).  At first sight, it seems pretty finnicky and unforgiving - but you have to take into account the huge bandwidth of the front-end RF circuits in a superhet.  In addition, you can sometimes trim out the errors by not aligning the band extremities exactly.

You may run into trouble if you are building a set with two RF stages, and have to stick a bit more closely to the ideal padder value - but generally you should get away with the nearest "preferred value" of silver mica capacitor.

As with all these programs, data must be entered in SI units. This means that it is up to you to enter the correct multiplier in all of the fields (p for pico, u for micro and so on.) If you get negative values for the trimmer and padder capacitors - ponder what it might mean! Sorry about the abbreviated input fields - I'm trying to get everything on one screen.

Tracking Error

 

Cmin 

Swing

f1 -

f3 -

if -

 L1  

 L2 

 Ct1

 Ct2 

Cpad

f2  

 

 

 

 

 

The example provided (sample data) is for a typical medium wave superhet receiver. You might use 0-100pf trimmers and a 680pf silver mica padder capacitor. Note that for short wave bands, the padder will become larger and larger. Single band radios often tried to improve tracking by bending the plates of the variable capacitors in the various tuned circuits. (I'm not a big fan of this practice).

Its quite instructive to re-examine existing communication receivers, such as the Eddystone 940. Quite often padder capacitors were determined by "rule of thumb" - the calculations would have been very laborious, although FORTRAN was around in the 1950's (introduced in 1956 by IBM) .


References

I developed this little program - so you must complain to me if it doesn't work.

The graphics library used for the plot  is "Raphael" by Dmitry Baranovsky.

 
 
 
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