[R-390] Comments on ER Issue 208 AGC modification

[Larry H]

Charles, This is indeed an excellent write up about how the R-390A AGC
works and mods to it.

Thank you for taking the time to document this area.  I enjoyed your in
depth analysis.

In reference to the 'moment of silence', I think what you are proposing is
to remove the V506A plate multiplier function from the AGC circuit and let
the AGC switch select the appropriate cap (0, 2 or 20 mfd) for the
indicated speed.  I like this idea very much.  I have a good idea how to
easily wire it up that I'll include in my response to David's comments on
this subject.  I think his way is a little more complicated than necessary.

Do you know who originated the '2 diode agc mod for ssb'?

Regards, Larry

On Tue, Dec 24, 2019 at 10:36 PM Charles Steinmetz <csteinmetz at yandex.com>
wrote:

> Many, many moons ago I said I would post an analysis of the ER AGC mod
> (Electric Radio, Issue 208) if I saw the article.  A helpful list member
> sent me the article some time ago, so here goes.  It's long, and pretty
> technical, so grab a cuppa joe and a pile of schematics before trying to
> digest it.
>
> First, I'll go through the stock AGC circuit (including brief
> descriptions of the mods I do when I do simple AGC modifications), then
> proceed to the ER-208 mod.  Readers will need to have a schematic of the
> R390A at hand to follow the first part, plus the mod article from
> Electric Radio, Issue 208 to follow the second part.
>
> AGC OVERVIEW:
> The purpose of AGC in the 390A is to develop a negative voltage that
> varies with the IF signal level, which is used to bias the RF stage, the
> mixers, and the IF stages.  By varying the bias of these stages, the RF,
> mixer, and IF gains are changed (they all have less gain when the AGC
> voltage is more negative).  The AGC voltage is derived by rectifying and
> low-pass filtering (quasi-integrating) the IF signal, much like AC from
> a power transformer is rectified and filtered in an AC-DC power supply.
>
> In the stock circuit, an IF signal is taken from the IF cathode
> follower, V509B, and amplified by the AGC IF amp, V508.  The amplified
> IF signal is fed through a 220 pF capacitor, C546, to diode clamp V509A
> (called the "AGC rectifier").  V509A clamps the V509A end of C546 to a
> potential near ground when the plate of V508 (and its low-Q tuned load,
> Z503) swings positive, and C546 charges up as its V508 end goes positive
> until the plate reaches its peak positive voltage (about 200v).  After
> the IF signal on the plate of V508 crests and starts back down, the
> V509A end of C546 is no longer diode-clamped to ground by V509A, so the
> charge in C546 drives the 100k resistor R545 negative, following the
> voltage on the plate of V508 but some 200-odd volts less positive.
>
> This happens on every cycle of the 455 kHz IF signal, and the end result
> is that a half-wave rectified version of the 455 kHz IF signal appears
> across R545, with the positive excursion clamped near ground by V509A
> and the half-wave pulses extending negative from ground.  Using C546
> this way is a form of "charge pumping" -- C546 is charged on each
> positive half-cycle of the 455 kHz IF signal, and some of that charge is
> transferred or "pumped" (with integration due to R546) to C547 on each
> negative half-cycle.  The clamped, half-wave rectified IF signal on R545
> is fed through 180k resistor R546 to 0.1uF filter capacitor C547, to
> produce a DC voltage that varies with the received signal strength
> (around +12v with no signal input [see below re: R544], and as much as
> 30v negative with strong signals).
>
> IMPORTANT NOTE:  The analysis below assumes that V508 and its plate load
> (Z503) are able to source and sink more current than necessary to charge
> the AGC capacitors at the time constants listed.  This is NOT always
> true in practice, so the actual attack times observed can be
> SIGNIFICANTLY LONGER than the calculations indicate.  This is discussed
> in more detail below.
>
> If there were no IF signal, the non-grounded end of C547 would sit at
> around +12 VDC due to the voltage divider from B+ formed by 2.7M
> resistor R544 and 180k resistor R546 to the anode of the diode clamp,
> V509A.  This positive bias is countered (and overcome, with any
> appreciable IF signal) by the negative-going, half-wave rectified IF
> signal flowing through R546 into C547.  The time constant to charge C547
> through R546 is around 18 mS (T=RC; 180k x 0.1uF = 18 mS).
>
> SIDE NOTE: the "time constant" ("T" = RC) is NOT the time it takes to
> fully charge or discharge the capacitor to the new voltage -- it is a
> mathematical construct used for analysis and is related to frequency as
> expressed in radians.  A capacitor fed by a resistor charges to 63% of
> its new value in 1 x T (= 1 x RC).  After 2 x T (= 2 x RC), the
> capacitor voltage reaches 87% of its new value, and after 5 x T (= 5 x
> RC) it reaches 99% of its new value.
>
> The time constant to DISCHARGE C547 is more challenging to calculate;
> the discharging resistance is 280k (R546 + R545), which would give T =
> RC = 28mS; BUT the discharge is assisted by the bias current through
> R544 (which is close enough to a constant current of 70uA to be treated
> as such).  A constant current charges (or discharges) a capacitor at a
> constant linear rate R = dV/dT = I/C.  For I = 70uA and C = 0.1uF, R =
> 700 volts per second (= 0.7 v/mS).  So, the effective discharge rate is
> significantly faster than T = 28mS, and depends on how far (how many
> volts) the AGC needs to go from its old value to its new value.
>
> The AGC filter is actually two sections -- one comprising R544, R545,
> and R546 along with C547, discussed above, followed by another section
> comprising 220k resistor R547 and some combination of capacitors C548
> (0.1uF) and C551 (2uF).  The AGC Line is fed from the non-grounded end
> of C548.
>
> For Fast AGC, C551 is out of the circuit and the second section
> comprises only R547 and C548.  The time constant of this combination by
> itself is 22ms, although that is only an approximation because the end
> of R547 toward C547 is not grounded for the low frequency AGC signal.
> Together, the two sections give a two-pole response with an effective
> time constant ("T" = RC) of around 35mS for both charge and discharge
> when Fast AGC is selected.
>
> When Medium AGC is selected, C548 is paralleled by 2uF C551.  In this
> case, C547 does next to nothing and the time constant ("T" = RC) is
> approximately symmetrical (charge = discharge) with a single-pole value
> of something over 400 mS.
>
> Finally, when Slow AGC is selected, C548 is again paralleled by C551,
> but this time the "far end" of C551 (the end away from C548) is not
> attached to ground, but rather to the plate of AGC time constant tube
> V506A.  Because C551 is connected from the plate of V506A to its grid,
> its value is increased by the Miller effect.  In this case, the
> effective value of C551 is around 20uF, giving a single-pole time
> constant ("T" = RC) of around 4 seconds.
>
> BONUS!!   EXPLANATION OF THE DREADED "MOMENT OF SILENCE":
> Note that the end of C551 away from C548 gets switched to ground when
> Medium AGC is selected, and to the plate of V506A when Slow AGC is
> selected.  Because the plate of V506A is not at ground potential (it
> sits at around +30 VDC when there is little signal, and can reach ~
> +200VDC under strong-signal conditions), the very large C551 must charge
> and discharge into the AGC network every time you switch between Slow
> and Medium AGC.  This is why the radio goes silent for a while when you
> switch from Slow AGC to Medium AGC!  When you throw the switch, the AGC
> line goes immediately to about 30 volts more negative than it was, and
> you have to wait while it discharges through R547, R546, R545, and R544.
>   Because this series string has high resistance, the discharge (AGC
> recovery) is slow (several seconds).  [The reverse happens when you
> switch from Medium to Slow AGC -- there is a period of distorted audio
> from the overloaded IF sections.  This period is very brief, however,
> because the capacitor discharges through the following grid-cathode
> diode, which has a much, much smaller resistance than the series string
> of R547, R546, R545, and R544.  It generally passes unnoticed.]
>
> To cure this -- and only this -- you can very easily modify the AGC
> circuit by adding a 20uF film capacitor to ground and permanently
> grounding the end of C551 that is away from C548, so that the AGC switch
> adds C551 in parallel with C548 for Medium AGC and the new 20uF film
> capacitor in parallel with C548 for Slow AGC.
>
> Now, apart from the "muting after switching" or "Moment of Silence"
> problem, why would one want to modify the R390A AGC system?  Primarily
> because symmetrical attack and release time constants (equal time to
> charge and discharge the AGC capacitance) are not the hot setup.  We
> generally want our AGC to attack substantially faster than it releases.
>   To do this, one needs to charge the AGC filter capacitor(s) from a
> lower resistance than the discharge resistance.
>
> Another reason one might want to modify the R390A AGC system is to apply
> AGC differently to the various gain-controlled stages to maximize
> headroom and minimize noise.  As a general matter, this is known as
> "staged AGC."  In the particular case often encountered in radio AGC
> circuits, it is commonly known as "delayed AGC."  NOTE that in this
> usage, "delayed" does not (except possibly incidentally) mean "delayed
> in time" -- it means "delayed with respect to the IF signal level."  In
> other words, AGC in NON-delayed stages is directly proportional to the
> IF signal level, while AGC in DELAYED stages is proportional to the IF
> signal level ONLY ABOVE A THRESHOLD (the "delay threshold").
>
> THE "TWO-DIODE" AGC MODIFICATION:  [The "two-diode" AGC mod is often
> (erroneously, in my view) referred to by the name of one person who
> wrote about it, but it had been widely used for decades before that.]
> One very easy way to produce asymmetrical charge/discharge time
> constants in an R390A is simply to parallel R546 and R547 with
> solid-state diodes (1N914/1N4148 or equivalent), with both cathodes
> toward V509A.  This arrangement turns C546 into a classic charge pump
> (with no integration due to R546 and R547).  I have installed this
> modification into scores of R390As since the early 1960s (along with
> adding the 20uF capacitor and switching changes as discussed above to
> eliminate the "muting after switching" problem), with excellent results.
>   (I sometimes change the values of C548, C551, and the new capacitor,
> and/or the value of R547, to tailor the AGC action for particular uses.
>   I also sometimes put some resistance in series with the diodes to slow
> down the attack time.)
>
> NOTE, HOWEVER, that this method cannot produce extremely fast attack
> times in any event, because Z503, V508, and the C546 charge pump can
> only supply limited current to charge the AGC capacitor(s).  To achieve
> really fast attack times, particularly with Medium and Slow AGC, you
> need to add a current amplifier between the plate of V508 and C546.  I
> have used solid state emitter (BJT) or source (FET) followers for this,
> for a much improved AGC attack response.
>
> To achieve a 1mS attack time with Slow AGC, the AGC dectector needs to
> supply 400 mA to charge C551 (!!).  The current to charge the AGC filter
> capacitors ultimately comes from the AGC IF amplifier V508, a 5749,
> which can deliver only 12-15 mA on a good day.  So the best you can do
> without adding a current amplifier is a 40 or 50 mS attack time in Slow
> AGC, and 4 or 5 mS in Medium AGC.  [NOTE: These figures are NOT the time
> constant "T" (= RC).  They are the full-scale attack times, more like 3
> x T.  See the "SIDE NOTE:" above.]
>
> THE ER 208 MODIFICATION:
> With that background, let's turn to the ER mod (have the ER article
> handy as you read the following):
>
> The author of the ER article (whom I do not know) used a solid-state
> diode to rectify/clamp the 455kHz IF signal, and re-purposed V509A as an
> additional, common-cathode AGC IF amplifier.  The plate of V509A feeds a
> 510pF charge-pumping capacitor, and solid-state diode D1 performs the
> clamping function originally performed by V509A.  In this case, the
> half-wave rectified IF signal is clamped not to ground, but to the
> cathode of V509A at a stated +5.2 VDC.  Presumably, this is to give the
> raw rectified IF signal a positive bias, as was accomplished in the
> original circuit by the 70uA current through R544 (see above).
>
> The current that charges the 510pF pumping capacitor on the
> positive-going swing of the V509A plate flows through D1 into the 1k
> cathode resistor and its 0.01uF bypass capacitor, and will therefore
> cause the bias on V509A to wander with the IF signal level.  The 0.01 uF
> bypass capacitor begins to effectively bypass the V509A cathode at
> around 15 kHz.  In other words, audio frequencies are NOT effectively
> bypassed, while the 455 kHz IF frequency IS fairly effectively bypassed.
>   Therefore, if the IF level varies significantly at audio or sub-audio
> frequencies -- which it will not do very much with an AM signal, but
> most certainly will with an SSB or CW signal -- there will be positive
> cathode feedback in the V509A AGC IF amplifier at audio frequencies.
> This may explain why the author found it necessary to attenuate and
> further integrate the IF voltage to the RF amplifier for undistorted CW
> reception (with new R1, R2, and C3, T = RC = ~100mS).  It would be much
> better design practice to obtain the positive AGC bias some other way.
>
> V509A and its 5.6k plate resistor can supply about double the current to
> the charge pump than V508 and Z503 can in the stock circuit, so we
> expect that somewhat faster attack times are possible with the modified
> circuit.  During the negative-going portion of each cycle of the V509A
> plate signal, the charge put into the 510pF capacitor during the
> positive-going portion of the cycle is pumped through D2 to charge the
> various AGC filter capacitors.  Because one of the filter capacitors
> (the 0.005uF capacitor) is charged directly by the 510pF pump capacitor
> through D2, the AGC loop can have a very fast attack time.  However,
> even in Fast AGC mode, most of the filter capacitance -- 0.1uF -- is fed
> through a 47 k resistor, so the bulk of the AGC is integrated with a
> time constant (T = RC) of ~4.7mS.  This means that for transient events
> (faster than a few mS), the AGC can attack very quickly but will also
> release quite quickly (T = RC = ~235uS) as the charge from the 0.005 uF
> filter capacitor flows through the 47k resistor into the 0.1uF filter
> capacitor.
>
> This characteristic may be useful for reception during lightning and
> other transient disturbances, although for this purpose I would be
> inclined to increase the value of the 0.005 uF capacitor a bit and
> consider reducing the value of the 0.1uF capacitor similarly.  The
> Medium and Slow AGC capacitors each have 100k resistors in series,
> limiting the attack time constants ("T") to 50mS and 100mS,
> respectively.  I would be inclined to reduce those resistors to 39k
> (Medium) and 20k (Slow) to give T = 20mS for both.  In this case, the
> full scale attack time is about 3 x T, or 500 mS and 1 S for the
> author's values and 200 mS for my suggested values.  [NOTE that even
> these values are MUCH too slow, IMO.]
>
> Unlike the stock circuit, which discharges the AGC filter capacitor(s)
> through R547, R544, R546, and R545, the modified circuit discharges the
> AGC filter capacitor(s) through new R1 and R2.  With the values given in
> the schematic, the discharge (AGC decay) time constant ("T") in Fast AGC
> mode is about 150mS.  In the text, the editor states that he used 680k
> for R2 and sometimes used 270k for R1.  With those values, the Fast AGC
> decay time constant ("T") is around 100 mS.
>
> For Medium AGC, the attack time constant (T = RC) is about 47 mS and the
> release time constant is about 700 mS (schematic values) or 500 mS (with
> the ER editor's values).  For Slow AGC, the attack time constant (T =
> RC) is about 100 mS and the release time constant is about 1.5 seconds
> (schematic values) or 1 second (editor's values).  In both cases, the
> Fast AGC components provide additional fast attack with fast release on
> transients under ten milliseconds, as discussed above.  In my view, a
> release time constant of 1.5 seconds is much too fast for Slow AGC, so I
> would be inclined to increase C1 to at least 2 uF, and, better, to 4.7
> uF or possibly even 10 uF, depending on one's choice of R1 and R2.  The
> resistor in series with C1 could be reduced to 10k or even lower to
> maintain the attack time closer to 20 mS, but only so much can be done
> because we run into the finite ability of V509A and its 5.6k plate
> resistor to supply current to the charge pump.
>
> I noted above that a second reason to modify AGC systems could be to
> apply AGC differently to different stages in order to optimize headroom
> and noise.  As noted, this often takes the form of "delayed AGC."
> (Again, "delayed" here does not mean delayed in time; rather, it means
> delayed with respect to the RF input level.)  As the input signal level
> is gradually increased from 0, the gain of some stages is reduced
> significantly before the gain of other stages is affected at all.  By
> attenuating the AGC feed to the RF section, the ER modification provides
> quasi-delayed AGC to the front end.  By "quasi-delayed," I mean that it
> isn't "delayed" in the usual sense, where the stages that have delayed
> AGC run at full gain until a certain input level (the "delay
> threshold"), then start reducing gain as the input further increases.
>
> Rather, this "quasi-delayed" AGC starts immediately (with increased IF
> level), but because it is attenuated, the gain of the "quasi-delayed"
> stages doesn't go down AS FAST with increased RF input as it did
> originally, so the RF stage never does get fully gain-reduced (or at
> least not until the IFs are fully cut off).  I see no reason to believe
> that this arrangement optimizes either headroom or noise.  In fact,
> there is good reason to believe it is the OPPOSITE of what one would
> want.  So, my intuition is that this is far from an optimal solution,
> given the known overload problems in the R390A front end.  But one would
>   need to do gain, noise, and headroom analyses of every stage from the
> antenna input to the detector to know for certain.
>
> Best regards,
>
> Charles
>
>
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