AGC for Digital Ops: Friend or Foe?

Contributed by Brian Waterworth, VE3IBW.

I belong to the Elecraft reflector discussion group.  While I joined to learn more about my rig (KX3/PX3), I have found a wealth of amateur radio knowledge on various topics.  This group often has side conversations about everything Ham radio; sometimes Elecraft gear isn’t even mentioned.

One such tangent conversation centred around the use of AGC (Automatic Gain Control) for digital modes.  Being that I predominantly use digital for my HF operations, I was very interested in this topic.  The reflector elmers recommended turning off AGC when using digital modes.   

My research, prior to joining the Elecraft reflector, seemed to suggest that AGC should be on.  For digital modes, use the fast setting and for voice modes use the slow setting.  This seemed to work for me.  But I don’t think I really had a good grasp of what AGC was doing at that time.  A few years of operating PSK31, RTTY, and JT-modes revealed something that I unconsciously did not make a connection to AGC (fast or slow).  This reflector side conversation opened my eyes to AGC and its effect on my weak signal digital QSOs.

AGC, as many reading this article might know, is the ability of a transceiver to dynamically reduce RF gain when too much radio frequency energy is present in the passband.  The idea behind these adjustments is to avoid saturating downstream transceiver components (local oscillator, audio amp, your ear drums).  

I glibly referenced ear drums in the last paragraph because the reflector discussion turned to why AGC is nice to have when a strong signal enters the passband.  But, how many of us actually listen during digital operations?  I used to, but at a very low audio volume.  Beeps, twerps, blips, and clicks are a little fatiguing after a while.  For my current digital operations, I rarely have the speaker enabled and never listen through headphones.  All the information I need to operate digital is via a digital-mode program’s waterfall.

Narrow digital modes, such as PSK31 and JTx, enjoy many potential QSOs within a typical receiver passband.  A voice mode QSO, by contrast, occupies the entire passband.  During Field Day operations, PSK31 can have 20+ active QSOs in progress too.  If one of these QSOs happens to be a very strong signal, AGC will reduce the RF gain.  This doesn’t sound so bad.  However, in one passband (say 14.076MHz), you might have 20-30 active JTx QSOs and one of these might be yours with a weak station.

I use FLDigi to work PSK31 and RTTY during Field Day.  To compensate for strong signals in the passband, I created macros to reduce the receive filter bandwidth around a signal I was interested in trying to establish a QSO.  This worked great to filter out stronger adjacent signals in the passband.  But, was it necessary?  I am beginning to think it wasn’t.

Let’s look at an example using WSJT-X.  It doesn’t have the convenience of macros to adjust and re-instate filter settings like FLDigi does.  

I got interested in JT modes over the past two years as I wanted to work QRP and be able to make contacts around the world with very little power (5 watts or less).  I also wanted to do this in a portable fashion (on a sailboat, in a park, on an island, in my backyard on a nice day).  JTx modes seemed like the best candidate.  There are some JT-mode operators that use 30+ watts.  There are also band conditions that will present a low power signal as 0db or more above the noise floor; i.e. a strong JTx signal.  With AGC enabled, these stronger signals will reduce the RF gain.  Weak signals will become weaker.

Each trace, (JT65 between 0-2,500hz and JT9 above 2,500hz) in figure 1, is a CQ or an active QSO within one passband.  The darker red and deeper yellow colours represent stronger signals.  You can see there are some very weak signals where there is no red at all (usually between -15db and -25db below the noise floor).

Figure 1: Typical WSJT-X waterfall with AGC turned off, Preamp off

Imagine you are halfway through a JT9 QSO, which takes about six minutes to complete, and a strong signal comes through for some other QSO or a CQ call; could even be at the opposite end of the passband.  Your transceiver’s AGC will reduce the RF gain and your weaker signal QSO may be artificially reduced beyond the JT decoder’s capability.  At this point, you may or may not be able to complete the QSO.  

Figure 2: WSJT-X waterfall with AGC-F and Preamp off

From figure 2, the darker bands through time slot 14:38 is the effect of the AGC reducing the RF gain in the presence of the strong signal at about 1,375Hz.  To level set, blue is the typical WSJT-X waterfall colour displayed when there is enough audio drive from the rig, through the computer’s sound card, to the WSJT-X program.  Black indicates not enough audio drive.  From the above diagram, you can see that some of the weaker JT9 signals (> 2,500hz) are obliterated on the WSJT-X waterfall.  The JT modes are fairly robust, but a strong signal in the passband may be enough to disrupt a QSO when AGC is turned on.

In figure 3, I turned off the AGC prior to operation.  I received an answer to my CQ from a station in Spain.  I received this station’s hail at -27db below the noise floor.  The diagram shows a very faint trace at about 2,550Hz.  

Figure 3: WSJT-X waterfall, active JT9 QSO with a Spanish station at -27db; preamp off

The presence of the strong signal at approximately 2,200hz in the passband may have caused the AGC to reduce RF Gain to a point where WSJT-X would not have been able to decode this very weak signal.  Fortunately, the AGC was turned off and other strong signals did not affect the RF Gain setting.

By switching off AGC, I have been able to successfully complete QSOs with very weak stations in the presence of stronger stations in the passband.  If not for the Elecraft reflector elmers, I would have likely continued operating digital with AGC turned on.