receivers. Peak the receiver, set the RF gain to max, Set the AF gain to 500mw out.
Decrease the input signal until the audio power out drops 1dB (down to 400mw or
decrease of 20%). That is the AGC threshold point.
Some receivers like the SR-400A have an adjustment for setting the threshold. There
are differing opinions on where the threshold should be set. In most cases I use 3uv. I
want max gain for weak signals and I also want the AGC to be in the linear range of
AGC action by 5uv. This is an arbitrary set point. Each manufacturer, engineer and tech
have their own opinion. If the manufacturer specifies a set point it is best to use that
point for assessing their products performance. When comparing receivers from
different manufacturers use whatever set point works best for you.
x.1.4. AGC Figure of Merit
The AGC figure of merit is the span of the input signal where the audio output power
remains within a specified limit. In the case of the SR series of receivers that specified
limit is 10dB audio change over a signal input range of 60dB.
If, the receiver gain, the (S+N)/N, and the AGC threshold are all in spec figure of merit
will meet spec. If any of the three of those are at fault then the figure of merit will most
likely not meet spec. So, the figure of merit is a good way to test the overall
performance of a receiver.
For the SR series and most other receivers the spec is: No more than 10 dB of audio
output change from 5uv to 5000uv.
x.2. Summation
Understanding the relationships between the different specifications is key to
determining or comparing the overall performance of receivers. Always keep in mind
that early receivers, 1950’s and before were not built to the established standards of
today. One important thing to remember is there was no standard for input Z
o
of the
receiver and could vary from 20 to 6000 ohms. So, determining the input power to
compute the overall gain can get complicated. From the 60’s on “MOST” receivers’ input
Z
o
has been 50 or 300 ohms. First and foremost, you need to determine what you have
and how to properly interface your test instruments to it. Sometimes you just got to do
the math.
Testing Gain, S+N/N, AGC Threshold and Figure Of Merit Page 60
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