Very few receivers today have an "off" position for the AGC but I've found it is possible to effectively turn it off by simply backing off on the RF gain control. I'm almost ashamed to say that in all my years hamming I have just discovered this. Here's how it works.
Figure 1 is the AGC response curve made at Clifton Laboratories for a commercial grade receiver . With the RF gain full on there is region from the noise floor up to a point called the "AGC knee" where receiver gain is linear. At the knee the AGC kicks in and maintains a more-or-less constant audio output for an ever increasing RF input. But, as the RF gain is backed off the knee moves to the right so that the linear region covers a much wider range and that's the fact we take advantage of to defeat the AGC. If the knee is chosen carefully then all the signals likely to be received will be in the linear region. It is always possible that a really strong signal will appear inside the bandpass to activate the AGC but for QRSS band conditions this is very unlikely.
|Figure 1. Receiver AGC Response Curve|
Not all receivers have the same AGC response curve and the linear region may not actually be all that linear. To check my TS-440 I inserted attenuators of known values at the antenna terminals and looked for the corresponding drop in output as read via Spectrum Lab. Actually I have only one attenuator I consider calibrated, a Tektronix 011-0059-01 fixed 20 dB type with BNC connectors. I used this to check the internal attenuator in my TS-440 which is supposed to be 20 dB and checked out within a dB. Thus I have two accurate 20 dB steps of attenuation for a total of 40 dB.
Figure 2 is the test setup. For a signal source I used my Palomar R-X Noise Bridge which provides a strong, stable signal for testing. With the noise source OFF the output as read by SL defines the system noise floor. With the noise source ON the output was adjusted for a convenient level above the noise floor. Attenuators were added to vary the input and the output observed on SL.
|Figure 2. Test Setup|
|Figure 3. Effect of Attenuators With and Without AGC|