[HBR] Initial test of 1st and 2nd IF coils

[Hopperdhh at aol.com]

Cal,

I forgot to mention anything about measuring  bandwidth.  The half power 
points are really 3dB down, (1/SQR(2)) or .707  times the voltage, the half 
voltage points are 6 dB down.  These points are  convenient for comparing 
similar transformers, but for knowing how an IF  transformer behaves in a 
receiver you would probably be more interested in  40 (or more) dB down.  This 
would tell you how it is going to  reject the image frequency.  Those points 
are much harder to measure  because -40 dB is only 1/100 of the voltage.  The 
nice thing is that the  bandwidth at -40 dB will be related to the bandwidth 
at -6 dB.

A  great reference on this subject is the Radiotron Designer's  Handbook.

Dan K9WEK


In a message dated 7/10/2010 6:20:47 P.M.  Eastern Daylight Time, 
Hopperdhh at aol.com writes:
Hi Cal,

Great  suggestion from  Pete.

I would like to add that tube input and  output impedance (resistive  part) 
is typically 1 megohm.  So you  should put a 1 meg resistor across the  
input 
and output windings of  the IF can.  This value is in the tube  manuals, so 
you can verify  it.  This affects the Q of the coils and  therefore the  
coefficient of coupling.

This test does not exactly reflect  the  real world due to things that 
happen in tubes -- space charge and  Miller  effect, to name a couple.

Sometimes I use a 1 meg in series  with the  signal generator instead of 
across the input and then the  small cap would not be  used unless you want 
to 
simulate the tube  output capacitance (also in the tube  manual).  If the 
scope has 1  megohm input resistance, that will take care  of the output 
loading,  
but the IF can should be placed very close to the scope  input to  
eliminate 
cable capacitance.

Please let us know your   results.

73,

Dan K9WEK


In a message dated  7/10/2010  4:43:42 P.M. Eastern Daylight Time,  
radioconnection at gmail.com writes:
Hi  Cal

How are you  coupling the scope and the low impedance output from the   
signal
generator to the IF transformers?  You need to use a very  small  (a few pF 
at
most) coupling cap in series with the sig gen  output.  The  scope will add 
a
few pF capacitive loading that will  affect the tuning  slightly as well.

Pete K1ZJH

On Sat, Jul  10, 2010 at 2:54 PM, C  Eus <catman351 at gmail.com> wrote:

>  All:
> My Hitachi V-355  scope just came in and after fiddling  with  it and my
> Elenco  SG-9500 signal generator, I was able  to obtain some interesting
> results  with the initial testing.   I only tested one coil per IF circuit
>  and
> a Miller 262 kcs  IF coil was actually resonating at 187 kcs while a  
British
>  1600 kcs IF coil resonated near 1080 kcs.  Definitely off  frequency  
but I
> suspect after tweaking, I can get them at the correct  op.  freq.
>
> BTW, when measuring bandwidth on these coils, are  they  measured at the 
-6dB
> points? I take it that -6dB down  would be at the  half power/voltage 
value
> of
> the peak  resonant frequency. Is that  right?  If so, I put in about 3v  
p-p
> on
> the Miller 262 kcs  IF coil and it looks like it  goes down to 1.5 v p-p 
at
> 184 kcs and 191  kcs respectively, or  about a 7 kcs bandwidth (which 
seems a
> little wierd  since I  was expecting a symmetrical bandpass but it turns 
out
> to be   slightly assymmetrical instead). Looking for comments on these
>  findings  and thanks. Cal, N6KYR/4.
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