[ARC5] S+N/N ratio results.

[Geoff]

>
> Unfortunately, when the load on the input is significantly lower than the 
> actual design impedance, the apparent sensitivity of the unit will almost 
> always require a higher level of signal. That is, a stronger signal is 
> required with the lower impedance than would be required if the input 
> impedance is properly matched.
>
> This fact is quite evident in receivers like the Collins 75A-1 when a 
> coaxial cable fed antenna is attached. Adding a balun with a 6:1 (even a 
> 4:1 TV balun works very well) or some sort of "antenna tuner" between the 
> coaxial cable and the receiver antenna input improves the apparent 
> sensitivity of the receiver greatly.
>
> Glen, K9STH
>
>
> Website:  http://k9sth.com
>
>
>     Glen, I understand this to mean that the receiver impedance is higher 
> than the source impedance, not the other way around. If so, I generally 
> agree when it comes to an antenna. For a signal generator the voltage at 
> the generator output will not be changed a lot by the receiver. The 
> question with the generator is how to know what the actual voltage is at 
> the receiver terminals.  When one has a matched condition, say a 50 ohm 
> generator into a 50 ohm receiver (that actually looks like 50 ohms) the 
> output system of most generators will read correctly.  Where the input 
> impedance is high (say 400 ohms) its probably better to terminate the 
> generator in its characteristic impedance and calculate how much the 
> voltage will be pulled down by the receiver impedance in parallel with the 
> generator and termination.  Since I have a generator which will put out a 
> fairly high voltage and have a sensitive RF voltmeter I just measure the 
> level at some convenient level to determine the correction for lower 
> levels.  This does seem to work.
>    My Hewlett-Packard 606A is calibrated for the voltage into a 50 ohm 
> termination and has a true 50 ohm source impedance. That is, the open 
> circuit voltage is double what the attenuator and meter say.  However I 
> also have a General Radio 1001A generator which has a much different 
> arrangement.   The output attenuator presents a 10 ohm impedance at the 
> panel connector for all except the highest level on the step attenuator 
> where it is actually 50 ohms. For the lower steps its necessary to use a 
> 40 ohm coaxial resistor in series with the output.  The resistor is one of 
> the accessories delivered with the generator. Because of the way the 
> output circuit is arranged it reads open circuit voltage so one must 
> divide the reading for the actual effective voltage. Where its set up for 
> 50 ohms into a termination or matched load the voltage is, of course, half 
> of the indicated voltage. This is a PITA but I got used to it before I got 
> the -hp- generator.  Another problem with the GR generator is that it has 
> rather poor amplitude modulation and a _lot_ of incidental FM at higher 
> frequencies. Its also damn heavy.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com


When I started at National in 63 they were using the GR 1001A and besides 
the problems mentioned they were continuous drifters at the higher 
frequencies.
National wound up building crystal oscillators for the alignment frequencies 
of their product line; one per tech bench and switch selected.

Sometime in 64 they got 606's to cover the NCX-3, 5 and HRO-500 but they 
drifted even worse. Luckily the 1001A's were sitting in storage and were 
pressed into service until HP came back with the 606A about 5-6 weeks later.

Carl