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

[Hopperdhh at aol.com]

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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