The AM Forum

I recently purchased an REA modulation monitor. It works great! I wanted to connect it to my 75A-4... in order to monitor the modulation of reasonably strong received signals. I already have a precision AM detector (i.e., precision rectifier + low pass filter) connected to the 455 kHz output amplifier stage (using an emitter follower) of my 75A-4, and it was easy to increase the level of this dc-coupled AM detector to a high enough value to drive the REA modulation monitor.

However, the AVC circuit in the 75A-4 (a.k.a. AGC circuit) is a peak detector. This makes sense for SSB and CW, but that type of AVC circuit tracks the modulation on an AM carrier. One obtains a better detected AM signal for both listening and for driving the modulation monitor by using an average (carrier) level AVC detector (i.e., a precision rectifier followed by a low pass filter with a high frequency cutoff of less than 25 Hz).

Ref: http://mysite.verizon.net/sdp2/id17.html

I modified my existing precision AM detector by adding an extra stage that inverts the detector output (the 75A-4 uses an AVC control signal of approximately minus 5 volts), and also performs a 0.7Hz low pass filtering (0.22 second time RC constant) function. I inserted this new AVc control signal into the "AVC Test Point" jack on the chassis of the 75A-4.

It works like a charm!

With the 75A-4's AVC switch set to "off", the AVC is controlled by the signal I inserted. The output of my precision detector now tracks the average (carrier) level of the received AM signal... and not the modulated envelope of the AM signal.

To use the 75A-4 on SSB or CW... disconnect the new AVC control signal, and turn the 75A-4's AVC switch from "off" to either "fast" or "slow"

Best regards
Stu

Cool! I bet the audio, especially the low freqs sound cleaner now since the AGC isn't interacting with them as much/at all.

Steve

The peak detector in the 75A-4 has an AVC release time constant of ~0.33 seconds (in slow AVC setting). The AVC attack time constant is much faster... but I believe that, to a good approximation, it is the release time constant that would have the most noticible impact on the demodulated audio in a receiver with a peak detector-based AGC circuit. This means that, in the slow AVC position, the AVC circuit rolls off modulation components of the received signal at frequencies below around 0.5 Hz ... [i.e. 1/(2pi x .33 seconds). So the effect of the AVC on the low frequency components of the received signal is not noticible. It does, however, mess up the behavior of the modulation monitor.

On the other hand.. this effect is quite noticible when I use my FT1000MkV... even in its slow AGC setting (~5 milliseconds AGC release time). The MkV's AGC rolls off low frequencies below 32 Hz (3 dB down point). If I turn off the MkV's AGC and "ride" the rf gain control... the restoration of the low frequency components of the received signal's modulation is particularly noticible. Maybe some day I'll put a circuit like this in the MkV ... if I can find a powerful enough magnifying glass.


While some people I have talked with on the air didn't quite believe me... I have always pointed out that peak detectors are, in effect, low-bandwidth negative feedback circuits... that act as high pass filters... and which roll off modulation frequencies on received AM signals (also on SSB signals) which are lower than the 1/[2pi x the release time constant] of the peak detector. In this regard, they are analogous to ALC circuits, in some modern transceivers, that cause downward modulation.


Happy New Year!
Stu

Good info. Even some of the average type detectors have time constants that will cause intermod with low freqs. Most receivers made after the 1950s weren't designed for audio below 200 Hz or so. Often the AGC reflects this fact. When we AMers try to push lots of audio below 100 Hz through some of these receivers, some bad things happen.

Stu.
A slow AGC release is at least ten times longer these days. I think my Racal is about 6 seconds and it does sound a lot better in slow mode. I notice better low frequency response but never knew why.

The best sounding receiver I've ever used on AM is the Watkins-Johnson 8716.  There is a very noticible difference in low end audio quality between the fast and slow AGC.  Lots of IMD in the fast position while the slow position produces very clean audio.

The AGC on the 8716 is fast attack/selectable decay.  The fast decay is prbably around 50 mS while the slow decay is a couple of seconds.  Essentially the AGC is a peak detector and in fast mode the AGC is decaying faster than the voice peaks can pump it back up. I believe.  Slow decay solves this problem.

I've recently been messing with a 75A2.  Stock, this thing has a pretty bad AVC circuit with too slow of an attack and too slow of a decay.  Putting a diode and 2 megohm resistor in parallel, then putting it in series with the 500K resistor going to the grid of the AVC amplifier tube made the thing a peak detector rather than an average detector.  Then, increasing the AVC time constant cap between the grid and plate of the AVC amplifier tube from .01 to .33 increased the decay to about a second.  It's hard to tell if there's any effect on the low end as the thing has typically crappy Collins audio, but SSB thru the product detector mod I installed sounds outstanding now with very nice undistorted lows.