[Collins] 51J4 panadapter tap?

[Dr. Gerald N. Johnson]

On Tue, 2006-04-25 at 02:49 +0000, Eugene Hertz wrote:
> Hello all, 
> 
> Recently acquired a Panoramic SB-15a panadapter (ultrasonic spectrum 
> analyzer) that covers ranges of <100kHz to >500kHz which is (in theory) 
> great so I can use it for both my 455kHz radios and my 51J4. Sweep width 
> is 0-200KHz. It has a nice large 5" scope!
> 
> I was wondering if anyone has an opinion of the best way to tap the 
> 51J4 for the widest bandwidth and least loading etc., etc.? One 
> gentleman I located offered the following, and I was wondering if 
> anyone had any opinion or different solution. 
> 
> The suggestion was to connected it to the plate of V106 through a 
> series combination of a 0.001uf/630V cap and a 47K resistor 
> into RG-58 coax. According to the gentleman, this minimizes loading 
> and did not require retuning of the first IF transformer T101. He 
> also noted that bandwidth was 40-100kHz at that point, which is 
> probably pretty darn good. 

Seems workable. The plate of the mixer has the greatest bandwidth
available. A common connection uses a few pf capacitor and no resistor.
The tuning C of an IF transformer is typically 100 pf so the few pf of
the few pf coupling capacitor or the couple pf of that 47K resistor
doesn't detune the IF transformer primary much.
> 
> Any other opinions? Contrary or supporting? Any thoughts about 
> the series cap and resistor values? I am not completely sure of 
> the input impedance of the analyzer other than to say at DC with 
> the unit on but unplugged, I measured infinite ohms on my flue 
> 8840a/af. Would there be any benefit/disadvantage to tap after 
> T101 but before the crystal filters? 

If the analyzer is an open circuit at DC, then there's already a
coupling capacitor so one may not be needed in the radio. The .001 value
is very uncritical, the reactance is far smaller than that of the 47K
resistor. 7 pf has 47K reactance at 500 KHz. The input Z of the analyzer
is probably fairly high paralleled by the shunt C of the coax cable at
29 pf per foot.

There would be disadvantage to coupling after T101. Narrower bandwidth.
The primary impedance bandwidth of the IF transformer is always wider
than the transmission bandwidth.

A more optimal connection for the analyzer might be off the cathode of
the mixer tube, but removing the by pass capacitor so there's detectable
signal could have a significant effect on receiver gain. But bandwidth
for the analyzer should be a lot greater if the mixer didn't then
oscillate.

Most optimal would be a separate mixer with the same signal and LO
inputs and a resistor for the plate circuit. But that would be
inconvenient to add to a receiver.
> 
> Options? Ideas?
> thanks
> Eugene

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer