One of the comments posted on the video shared on yesterday's blog essentially said: What no AGC?
That observation was based on seeing me adjust the audio gain pot as I tuned across some really loud signals. Typically, I do not include AGC in my rigs although several have that feature using circuitry from W7ZOI.
In keeping with the season, I wished we had built in AGC for use on several of the political candidates as they speak about immigrants eating your pets.
To implement AGC takes a whole raft of electronics to do it properly. So, for today's discussion let's talk about AGC.
Let us start by theorizing that maybe AGC was invented for use in AM automobile radios. It would be hard to drive down the road and to keep having to adjust the volume which could present a distraction. Just my theory not necessarily the real how or why. Sitting in front of a rig presents less of a safety problem.
In essence, AGC levels out the final audio gain so that the volume coming out of the speaker is within a small range regardless of a strong or not so strong signal. The process starts with detecting the very strong signals and adjusting the overall system gain so that we do not have blasting signals coming out of the speaker.
That is a simplistic view as behind the scenes is the detection and measurement to some preset level and then enacting the gain reduction at the speed of light so our ear detection system cannot tell what is happening.
AGC takes many forms including at the RF level, the Audio level only and a combo of both Audio and RF. If you check Hayward's SSDRA you will see a combo of the RF and AF AGC in one of his receiver projects.
The "audio only" approach tends to present a signal that sounds like it is "pumping" -- a shortcoming manifest in the Sideband Engineers SBE-33. Some simple Audio AGC circuits have a FET switch in the audio chain where biasing the FET reduces the signal passing through to the final AF amp stage.
The RF approach would typically apply gain reduction in the RF Amp and 1st IF Amp stages. Think of it -- the strong signal hits the antenna jack and in literally zero time you are trying to reduce the gain of the RF amp. Not an easy task. Some approaches simply bypass the RF stage and use the 1st IF amp stage as the crowbar to knock down the gain.
One "untying of the Gordian Knot problem" is the enactment of the AGC process without introducing distortion in the signal. A second problem especially in transceivers using a common IF is how to have AGC on receive and No AGC on transmit, or how to have AGC on Receive but ALC in the same circuit on transmit. This is more than a single FET and two resistors!
One thing I noted when working with "making" Dual Gate MOSFET's using two J310's is that you could add a trim pot to Gate 2 to adjust the stage gain. Extending this idea, you could have a variable voltage instead which depends on the signal level from some preset standard (AGC). A small relay could switch between the AGC voltage and a Fixed Gain wide open DC voltage on Transmit or the application of an ALC signal.
If you make R3 a 3.3K resist0r in series with a 10K trimpot wired as variable resistor you can adjust the stage gain Now substitute a variable DC voltage for the 10K trimpot and you have an AGC controlled stage. You would have to have a negative going AGC to reduce the gain on strong signals. By negative going means that a weak signal would be a higher voltage whereas a very strong signal would be a much smaller voltage.
Again, all of the added stuff is a high wire balancing act and unless it is done right is nothing short of a disaster. Thus, an Armstrong approach of manually adjusting the volume is not a severe handicap while operating.
As usual there are commercial devices now available that likely will find their way into future homebrew rigs. Microsemi has a voltage-controlled attenuation device good from DC to 50GHz that can provide up to 27dB of attenuation.
TYGNYBNT. Do not eat your pets.
73's
Pete N6QW