Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

This page collects AGC test data I've taken on four receivers:

The purpose of the collected data is to identify the threshold signal level at which each receiver begins to apply AGC action and also to assess the AGC's effectiveness at maintaining constant audio output with changes in input signal level. Attack and release times were not measured in this test.

As a quick overview, AGC--or automatic gain control--automatically reduces the total gain of a radio receiver to maintain a more-or-less constant audio output as the input signal varies in strength. If AGC is applied to all signals, regardless of strength, however, the result is to degrade reception of weak signals. Consequently, good design practice calls for an "AGC threshold" below which signal strength no AGC action is applied--the receiver runs at maximum gain. When the signal exceeds the threshold level, AGC is applied and the output is leveled.

In an ideal receiver, once the threshold point is passed, there will be little or no change in audio output with changes in signal level. Below the threshold point, the receiver's audio output will be linear with respect to the signal input, i.e., a 1 dB change in input level should result in a 1 dB change in output audio.

And, of course, good design practice calls for an AGC on/off switch, allowing the operator to disable the AGC when desired, substituting instead manual RF and/or IF gain control instead.
 

All data was collected with the same automated setup, as illustrated below.
 

The collected data is written to a text file which I then reformatted in Excel and plotted with Origin 7.5.

The test data was taken over the range -130 dBm to 0 dBm, with either 0.5 or 1.0 dBm steps.

The absolute receiver audio output level is not the important data in this analysis, as the audio level is arbitrary, related to the audio gain setting in the K2 and related to the Line Level setting in the RA6790/GM and to the line level output in the R7. Rather, the important thing to observe is the change in audio level versus change in RF level.

I should also note that the RA6890/GM has an operational mode that allows the user to set the threshold from a front panel control. This gives the user three AGC modes:

• AGC off
• AGC on, with preset threshold
• AGC on, with user adjustable threshold

The RA6790/GM is designed, like many commercial and military receivers, for optimum high signal performance and hence does not include an RF amplifier ahead of the mixer.

The Drake R7 is of similar vintage to the RA6790/GM and was intended to be a state-of-the-art amateur grade receiver to replace the R4C. I bought mine new around 1978, if I recall correctly, and paid around $1400 for the receiver and a complete set of auxiliary filters. The R7 has an optional pre-amplifier stage, quite similar to the K2's design, as a matter of fact.

I collected test data from the R7 in four operational modes:

• USB (2.8 KHz bandwidth) pre-amplifier off
• USB (2.8 KHz bandwidth) pre-amplifier on
• CW (500 Hz bandwidth) pre-amplifier off
• CW (500 Hz bandwidth) pre-amplifier on
   

• The threshold is not a sharp break, but rather AGC takes effect over a range of a few dB with a rounded threshold. Presumably, this is a decision by the Drake engineers to smooth the transition between no AGC and full AGC. An alternative explanation is that the AGC loop has less gain.
• There is a small upward slope to the audio output, around 1 dB for a 100 dB increase in RF level above the threshold. The RA6790/GM has essentially no change over this range, but in truth the difference between 1 dB and 0 dB change over this range is not perceptible to the ear.
• The curve most closely matching the RA6790/GM  test conditions (300 Hz bandwidth, no preamplifier) is the green curve. Drake set the AGC threshold around -105 dBm for this operational mode, about 8 dB above the threshold chosen by Racal's designers.
• The curves have some artifacts (bumps) that are due to local RFI caused by the dehumidifier in my basement shop cycling on/off whilst collecting the test data. This effect is most visible in the black curve, but also can be seen in the blue curve as well.

The Elecraft K2 is a ham-band only transceiver of relatively modern design and is in current production. Depending on options, its kit price (it is available only as a kit) runs from $600 to $1200. It has an switchable pre-amplifier. The AGC is applied only to one stage, the K2's 4915 KHz IF amplifier, a Motorola MC1350 integrated circuit. AGC is not applied to the RF amplifier stage or the post-mixer amplifier. Motorola specifies the MC1350's AGC range as 60 dB minimum, 68 dB typical.

I collected test data from the K2 in four operational modes:

• USB (2.2 KHz bandwidth) pre-amplifier off
• USB (2.2 KHz bandwidth) pre-amplifier on
• CW (400 Hz bandwidth) pre-amplifier off
• CW (400 Hz bandwidth) pre-amplifier on

• There is no defined threshold. Rather, there is a wide range over which AGC is slowly introduced.
• Even with strong signals (-60 dBm and more) the AGC does not hold the output constant to the same degree as either Racal or Drake.
• Cubic's R3030 also implements a soft AGC curve, but with significantly better control than the K2; 120 dB change in signal level causes the R3030 to change audio output about 10 dB, whilst the same change in the K2's input causes an audio output change of 25 to 40 dB, depending on mode and pre-amplifier status.

In my opinion, the K2's AGC characteristics are sufficiently different from a "normal" receiver to cause it to  sound different. When operating with AGC enabled, the Drake or Racal (and in fact Drake and Racal are typical of other receivers I've examined) many, if not most, signals are audio leveled to the same loudness, with only relatively weak signals falling below the AGC threshold. The background noise is reduced proportionally, e.g., a signal that's 20 dB above the AGC threshold results in 20 dB gain reduction; the audio beat note is just as loud in the headphones as a signal right at the AGC threshold, but the background noise is reduced 20 dB.

 With the K2, in contrast, many more signals will either not be AGC leveled, or if AGC is effective, it is only partially effective, in that it may, for example, translate a 6 dB change in RF level into a 2 or 3 dB change in audio level.  If the signal increases 6 dB, this gain action results in the audio level going up 3 dB and the background noise going down 3 dB.

Thus, tuning across the band will produce a different audio experience with a K2 than with a Drake R7, or a Racal RA6790/GM or most any other amateur receivers or transceivers.

The Cubic R3030A is a dual receiver, with two complete receivers packaged in one 19" rack panel configuration. All elements of each receiver are modularized, with highly shielded plug-in modules. This another multi-kilodollar receiver made for government and commercial facilities. It is synthesized, with analog IF chains.

The following graph is rather busy, as it shows all four receivers for all test conditions.

As a final reminder, the absolute audio levels are unimportant; rather the change in audio versus the change in RF signal level is what should be examined.