Response to bctrl ;
Single Sideband FM has been around for a long time and is a favorite mode of modulation used by the military for many years. Most consumer products are not capable of using this mode. SSB FM is a hybrid of SSB Amplitude modulation used to drive and FM transmitter. There are many modes of modulation that are not available commercially and are reserved for proprietary services. The military is notorious for special modes of modulation for security reasons. During the Viet Nam war the army was using this mode smack dab inside some of the ham bands for local ground communication to confuse the enemy. This hardware was never available to the public. I am not sure if ssb fm is legal to use in the Ham bands but I suspect if it is not encrypted it may be ok but may attract unwanted attention. Hams are allowed to do a lot of experimenting in some of the upper bands. Most are becoming obsolete because digital encryption is more secure. I’m not an active Ham.
SSB-FM systems use the Hilbert transform to generate a signal which is simultaneously amplitude and frequency modulated, both components being single-sideband.
See web sites here for explanation of SSB FM mode
Single-sideband FM - Patent 4542534
Narrow band, SSB, FM transmitter - US Patent 4625319 Claims
As to “An IF SPECTRUM DISPLAY - displays energy from the radios discriminator circuit.”
Many radio including scanners FM type is multiple if stages. The first if stage which is very wideband but maybe a high freq IF Stage. So by design, detecting the first IF through a wideband discriminator will yield wideband signals in the audio (easier to design and cheaper parts used in audio range) range when mixed with a sweeping oscillator will yield an IF spectrum display. Keep in mind there a different kind of discriminators so here it is used as a generic term. Taking the signal directly from the last stage of the IF requires more complex design so as to not load the IF or inject noise. A special kind of buffer mixer design is used which in FM terms is a special kind of discriminator. Scanners because of the expense do not generally have this circuitry embedded it the design.
The last discriminator on a scanner will give you an unmodified wideband signal that could be used to demodulate special kinds of signals where the internal demodulators on not designed to detect. Commonly used to detect Packet or FSK etc. This is probably what you are thinking of.
As to
“I was under the impression that the AGC in these scanners was on the audio side. How does this relate to signal level? Is the AGC control we have really controlling the RF or IF stages? “
.
All amplifiers especially have 3db gain or more are subject to going into oscillation. It’s physics. The output signal can get into the input stage and start regenerative feedback which will blow up the amplifier because it is an in band oscillation. The amplifier can also go into out of band oscillation in a high frequency mode called spurious oscillation. It may not cause the amp to blow but will consume energy making the amplifier overheat or inefficient. Amplifiers need to be controlled to prevent this from happening. Feedback circuits are used to control this from happening. They also use these control loops to control the gain of the amplifier. There are complex design issues in using these feedback loops. Incorrect design can cause the amplifier to go into oscillation. Delay timing and phasing of these loops is critical to prevent oscillation and distortion.
The rub is that the analog feedback must be associated to the type of signals used in the stage. A special kind off feedback sometimes called scale down or AGC loops can be used to control feedback gain control loops across different stages that are using different kind in signals being amplified. That means you can have a feedback loop from the audio section all the way back to the rf section if you want to do that. These are DC Voltage control loops where the Ac signal is detected and uses a dc converter to make a dc signal relative to the output signal. CPU designs use (AC to DC convertors) to control gain in the loop. The loop has to be measured against a standard dc reference to detect variations so the loop can be changed to make dynamic compensations. Properly designed AGC loops can be designed to control gain, dynamic range, Shut down amp during overload, And control timing to prevent low freq gain oscillations. The simple answer is AGC is used to control multiple stages of amplifiers in a large loop mode. Since it is DC controlled it is very resistant to amplifier oscillation. Multiple loops can be used even freq cross over loops which can change over all gain when different bands are used.
In the BCD996XT is CPU controlled which digitally controls dc agc which is adjustable in firmware. I am not familiar with the design so I have limited knowledge about its specifics. The audio pot is not in the analog or agc audio gain loop at all that much I know. When you turn the pot the cpu detects digital signals from the pot and adjusts gain accordingly. Volume can also programmed via the rs232 port by sending numeric data to the micro. . Digitally designed AGC control loops/ are used. As well as analog control loops including bias loops to control thermal run away. Much of this are inside IC’s themselves.