[ARC5] S+N/N ratio results.

[Geoff]

When I did all the mods to the 75A4 at National 1965-66 one of the first 
things the engineers wanted to see was the input impedance. Using a new GR 
1606A they recently acquired it was anything but 50 Ohms across each band 
and ranged from 30 to almost 150. No problem in use since the antenna 
trimmer did the matching.
I would have to believe their general coverage sets of the era were even 
worse.

Carl
KM1H



Carl
KM1H



Not exactly!  Collins alignment procedures for receivers was pretty 
specific.  After signal generators became available with a true 50-ohm 
output, then the signal generator was connected directly to the receiver. 
With older signal generators including the HP-606- series, Measurements 65, 
military TS-497- series, and so forth, that did not present a 50-ohm 
impedance over the entire coverage, a resistive network was connected 
between the signal generator output and the receiver.  The values of the 
resistors chosen formed a 6 dB pad.   The signal generator output (in 
microvolts) for an S-9 "S" meter reading was given as 100-microvolts. 
However, this reading was before the 6 dB pad and, therefore, the actual 
signal level applied to the receiver for an S-9 reading was actually 
50-microvolts.


For "field" alignment, when the crystal calibrator was used for the signal 
source, a 47-ohm resistor was to be inserted into the receiver antenna jack 
which "terminated" the input impedance to approximate a 50-ohm load.

Glen, K9STH


Website: http://k9sth.com



From: Geoff <geoffrey at jeremy.mv.com>
To: Roy Morgan <k1lky at earthlink.net>; Richard Knoppow 
<1oldlens1 at ix.netcom.com>
Cc: ARC5 <arc5 at mailman.qth.net>
Sent: Saturday, June 15, 2013 3:42 PM
Subject: Re: [ARC5] S+N/N ratio results.


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.

** Interesting, do you have any citations to that?

> ...  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.


**  Chuckle


> 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

** Completely agree but easily accomplished over several decades now with
proper procedures and equipment. For an ARC-5 it would be strictly a labor
of love but since this group drifts all over the universe there are
legitimate applications.





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