x) Testing Gain, S+N/N, AGC Threshold and Figure Of Merit
By Walt Cates, WD0GOF
Note: While this article uses the Hallicrafters SR-400 transceiver as an example, the
concepts should apply equally well to other transceivers and receivers (other than
specific numbers).
Receiver gain, signal + noise to noise ratio ((S+N)/N), AGC linearity and AGC figure of
merit are the prime indicators of receiver performance. These four items can be very
difficult to determine accurately without the proper equipment. But it doesn't take much
and it is not expensive. First you need a reliable, calibrated signal source. I use a 65-
year-old URM-25D. It needs to cover the HF bands and all the IF frequencies. It should
have metered output and built in step attenuator. In addition to a calibrated signal
source, an audio output meter such as the General Radio 1840A (my choice) is all that
is needed. There are many that are up to the task. The key features are: Internal
variable load (at least 3 ohms to 1.5k), variable full-scale power, 2mw to 20w and a
meter calibrated in watts and DB. Fully self-contained no power no batteries. Just about
everything you need to “test to spec” a receiver is covered with these two test
instruments.
Figure 5. General Radio 1840A Audio Output Meter
For my bench setup I have a standard ¼ inch phone plug which goes to a switch. The
switch selects either a speaker or the audio power meter. Most of the equipment I work
on (Hallicrafters SR series) has a front panel phone jack that disconnects internal
speakers. I have an adaptor phone jack with clip leads for working on various other
radios. A side benefit of using an audio power meter with built in load is you do not have
to listen to the constant drone of the receiver. Switching in the speaker allows you to
quickly find the signal when you change frequencies.
Note: Unless it has been modified, the headphone jack on the Heathkit transceivers is
designed for high-impedance headphones and should not be used for audio output
measurements. Instead, use the speaker jack on the rear of the chassis.
Testing Gain, S+N/N, AGC Threshold and Figure Of Merit Page 58
To Next Page
Loading...
Need help?
General
