[ARC5] S+N/N ratio results.

[Glen Zook]

The input impedance on the S-Line receivers is somewhat more consistant than the A-Line receivers including the 75A-4.  However, it is very easy to believe that the impedance could range from 30-ohms to 150-ohms depending on the band.  That is one of the reasons an antenna trimmer was included in the A-Line receivers.  The general coverage receivers including the 51-J, R-388 series, etc. definitely have input impedances that can vary all over the place.  The nominal antenna impedance of the 75A-4 was 50-ohms unbalanced whereas the nominal antenna impedance on the 75A-1, 75A-2, and 75A-3 receivers was 300-ohms balanced.  Especially the 75A-1 and, to a somewhat lesser extent the 75A-2 and 75A-3, are much happier with either a balun or antenna tuning unit matching 50-ohm coaxial cable to the receiver.

The primary reason for the matching pad on the older signal generators was that the impedance of the signal generator was definitely not constant.  Therefore, a pad, usually 6 dB, was implemented to present a constant impedance of the input signal.  Many signal generators, including the Measurements and most military generators (i.e. TS-497 series) had 6 dB pads available for both 50-ohm and 75-ohm loads.

A matching pad is not usually required on "modern" signal generators with a true 50-ohm output and the signal level, in microvolts, applied to the unit under test should be the same.  However, the "reading" on the calibrated attenuator (before the matching pad) will be twice that of the applied voltage before the 6 dB pad.  But, during the time frame in which the Collins receivers were designed, the vast majority of signal generators did require a matching pad when feeding a 50-ohm input impedance.  Therefore, Collins definitely did use a matching pad when aligning the receivers.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Geoff <geoffrey at jeremy.mv.com>
To: Glen Zook <gzook at yahoo.com> 
Cc: ARC5 <arc5 at mailman.qth.net> 
Sent: Sunday, June 16, 2013 1:26 PM
Subject: Re: [ARC5] S+N/N ratio results.
 

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.