[ARC5] Informative postings (Was S+N/N ratio results)

[Bruce Long]

Make a nice "50 ohm" load for the receiver end of the cable with 50 and
> 0.5 ohm resistors, take the signal off the 0.5 ohm point and factor in
> the 100:1 voltage reduction.  Almost *any* receiver input impedance or
> resistance will get one hundredth of the voltage at the load end of the
> cable.


I've done something like this in the past but accurate voltage dividers and accurate low ( or high) value resistors are difficult to implement even at HF frequencies.
I think any attempt at an accurate .5 Ohm resistor using conventional leaded - non-wirewound resistors at HF is doomed to failure due to the resistor lead and body inductance.  As a rough ( perhaps very rough) rule of thumb assume 20 nH per inch for lead/body inductance. High value RF resistances and at upper HF frequencies i think 100 Ohms starts to qualify as a high value resistance are doomed by the stray capacitance across the resistor body.


You could do significantly better using surface mount resistors and a custom double sided pcb implemented as a microstrip transmission lineusing a symmetrical resistor layout and copper foil around the edges of the PCB to implement the shunt low value resistor connection to the ground plane.  A grounded lid close above the pcb would greatly reduce leakage.  The pcb layout is simple enough in my judgment that the pcb need not be etched but instead could be layout out with a vernier calipers or even better a vernier height gauge, a straight edge and a hobby razor knife.  Make several cuts to deeply score the top side copper foil, then heat with a soldering iron, pick at the end of the scored trace with a sharp point to lift it from the glass epoxy substrate and then use a tweezers and the heat from a soldering iron to remove the copper trace from the ( originally unetched) pcb stock.  With practice this can be done quickly and easily and using a
 vernier height gauge I have gotten within 2-3 thousands of an inch of the desired conductor width,  accuracy which is useful at microwave frequencies but not essential for this application.


________________________________
 From: Leslie Smith <vk2bcu at operamail.com>
To: ARC-5 List <arc5 at mailman.qth.net> 
Sent: Saturday, June 15, 2013 4:48 PM
Subject: [ARC5] Informative postings  (Was S+N/N ratio results)
 

  I followed the ideas on how to measure signal to noise ratios, and
  leakage from RF generators from Roy, Richard (and others) and found
  them instructive.

  Making an accurate measurement (of any physical quantity, not just
  electrical) is always somewhat of an art, and it's therefore
  interesting to read how others have done it (or tried and failed).
  Success AND failure are both informative.


  73 de Les Smith
  vk2bcu at operamail.com


On Sat, Jun 15, 2013, at 12:42, Roy Morgan wrote:
> 
> On Jun 14, 2013, at 7:31 PM, Richard Knoppow <1oldlens1 at ix.netcom.com>
> wrote:
> 
> > …  one can just feed from a low impedance and assume the voltage will be close. I mean a terminated output from a 50 ohm generator is 25 ohms and will not be much affected when connected to a receiver with 200 or 400 ohms input.
> 
> I've thought about this and thought of taking the idea further:  
> 
> Make a nice "50 ohm" load for the receiver end of the cable with 50 and
> 0.5 ohm resistors, take the signal off the 0.5 ohm point and factor in
> the 100:1 voltage reduction.  Almost *any* receiver input impedance or
> resistance will get one hundredth of the voltage at the load end of the
> cable.
> 
> >   Leakage is a real problem.
> 
> The 100:1 divider would not help this: you'd be putting 100 times the
> voltage on the cable (and out some of the leakage points) than would be
> needed with an unterminated cable (receiver input 50 ohms) or 10 times
> the voltage if a 10 db pad is used.
> 
> I've read that the Collins lab procedure to check sensitivity was to hook
> the (50 ohm) generator output directly to the receiver with no
> terminating resistor or pad.  Included in what I read was the statement
> that the receivers had actual input impedances close enough to 50 ohms to
> make this work out right.
> 
> > ...   At any rate, getting an approximate measurement on a boat anchor is possible.
> 
> Oh, good!  I'd rather not thing that all this is in vain.
> 
> > A meaningful measurement on a receiver with fractional microvolt sensitivity is quite difficult.
> 
> I think that many reports of fractional microvolt sensitivity are
> suspect.
> 
> Roy
>  
> Roy Morgan
> k1lky at earthlink.net
> K1LKY Since 1958 - Keep 'em Glowing!
> 
> 
> 
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