Oscillator AGC supply voltage sensitivity

[Clifford Heath]

I've built the following circuit for an HF oscillator (though it should
work ok at VHF too), but the bias for the output make the AGC very
dependent on supply voltage.

Without adding more silicon, how would you make this work over a wider
supply voltage range, say 2.6-4V?

Two files below - the circuit, and ca3000.lib, with --- CUT HERE ---

Clifford Heath.

--- CUT HERE ---
Version 4
SHEET 1 1132 708
WIRE 304 -160 208 -160
WIRE 384 -160 304 -160
WIRE 464 -160 384 -160
WIRE 688 -160 464 -160
WIRE 800 -160 688 -160
WIRE 912 -160 800 -160
WIRE 1040 -160 912 -160
WIRE 384 -128 384 -160
WIRE 800 -128 800 -160
WIRE 912 -128 912 -160
WIRE 304 -112 304 -160
WIRE 688 16 688 -160
WIRE 912 16 912 -48
WIRE 944 16 912 16
WIRE 304 32 304 -48
WIRE 384 32 384 -48
WIRE 384 32 304 32
WIRE 560 32 384 32
WIRE 592 32 560 32
WIRE 304 80 304 32
WIRE 464 80 464 -160
WIRE 912 80 912 16
WIRE 1040 80 1040 -160
WIRE 208 128 208 -160
WIRE 240 128 208 128
WIRE 560 128 560 32
WIRE 560 128 528 128
WIRE 736 128 560 128
WIRE 800 128 800 -48
WIRE 848 128 800 128
WIRE 304 192 304 176
WIRE 384 192 304 192
WIRE 464 192 464 176
WIRE 464 192 384 192
WIRE 688 208 688 96
WIRE 688 208 512 208
WIRE 704 208 688 208
WIRE 384 224 384 192
WIRE 688 240 688 208
WIRE 512 272 512 208
WIRE 512 272 448 272
WIRE 912 288 912 176
WIRE 912 288 752 288
WIRE 928 288 912 288
WIRE 912 320 912 288
WIRE 512 336 512 272
WIRE 800 336 800 128
WIRE 384 432 384 320
WIRE 512 432 512 400
WIRE 512 432 384 432
WIRE 688 432 688 416
WIRE 688 432 512 432
WIRE 800 432 800 416
WIRE 800 432 688 432
WIRE 912 432 912 400
WIRE 912 432 800 432
WIRE 1040 432 1040 160
WIRE 1040 432 912 432
WIRE 1040 464 1040 432
FLAG 1040 464 0
FLAG 592 32 Vtank
IOPIN 592 32 Out
FLAG 704 208 Vagc
IOPIN 704 208 Out
FLAG 928 288 Veout
IOPIN 928 288 Out
FLAG 944 16 Vcout
IOPIN 944 16 Out
SYMBOL voltage 1040 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3.4v
SYMBOL cap 288 -112 R0
SYMATTR InstName C2
SYMATTR Value 180pF
SYMBOL ind 368 -144 R0
SYMATTR InstName L3
SYMATTR Value 10uH
SYMBOL npn 240 80 R0
SYMATTR InstName Q1
SYMATTR Value CA3046
SYMBOL npn 528 80 M0
SYMATTR InstName Q2
SYMATTR Value CA3046
SYMBOL npn 448 224 M0
SYMATTR InstName Q3
SYMATTR Value CA3046
SYMBOL res 672 0 R0
SYMATTR InstName R5
SYMATTR Value 15k
SYMBOL npn 848 80 R0
SYMATTR InstName Q4
SYMATTR Value CA3046
SYMBOL cap 496 336 R0
SYMATTR InstName C1
SYMATTR Value 220pF
SYMBOL npn 752 240 M0
SYMATTR InstName Q5
SYMATTR Value CA3046
SYMBOL res 672 320 R0
SYMATTR InstName R7
SYMATTR Value 470
SYMBOL res 896 304 R0
SYMATTR InstName R8
SYMATTR Value 220
SYMBOL res 784 -144 R0
SYMATTR InstName R4
SYMATTR Value 47k
SYMBOL cap 800 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 10p
SYMBOL res 784 320 R0
SYMATTR InstName R6
SYMATTR Value 68k
SYMBOL res 896 -144 R0
SYMATTR InstName R10
SYMATTR Value 330
TEXT 208 264 Left 0 !.tran 0 50uS 0 1nS
TEXT 200 464 Left 0 !.INC "ca3000.lib"
--- CUT HERE ---
*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3046 PSpice MODEL
*REV: 2-24-97
** ----- BJT MODEL -----
*
..model CA3046 NPN
+ (IS = 10.0E-15 XTI=3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=145.7E+00 ISE=114.286E-15 NE=1.480E+00
+ IKF=46.700E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.005E-15
+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=991.71E-15
+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.02E-12
+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=277.01E-12
+ ITF=1.750E-00 XTF=309.38E+00 VTF=16.37E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00
*
*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3086 PSpice MODEL
*REV: 2-24-97
** ----- BJT MODEL -----
*
..model CA3086 NPN
+ (IS=10.0E-15 XTI= 3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=156.6E+00 ISE=114.886E-15 NE=1.470E+00
+ IKF=36.700E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.005E-15
+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=991.79E-15
+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.02E-12
+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=278.55E-12
+ ITF=.770E-00 XTF=91.38E+00 VTF=18.37E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00
*
*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3127 PSpice MODEL
*REV: 2-13-97
** ----- BJT MODEL -----
*
..model CA3127 NPN
+ (IS=3.20E-12 XTI= 3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=95.2E+00 ISE=20.586E-12 NE=1.990E+00
+ IKF=61.500E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.805E-9
+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=281.1E-15
+ MJC=0.138E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=651.9E-15
+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=122.61E-12
+ ITF=1.600E-00 XTF=2.050E+03 VTF=307.00E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00



*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3096 NPN PSPICE MODEL
*REV: 3-13-97
** ----- BJT MODEL -----
*
..model CA3096N NPN
+ (IS=10.0E-15 XTI=3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=466.5E+00 ISE=74.286E-15 NE=1.660E+00
+ IKF=14.000E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.005E-15
+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=786.51E-15
+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.28E-12
+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=490.01E-12
+ ITF=.270E-00 XTF=5.38E+00 VTF=28.39E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00 NK=.468
*
*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3096 PNP Spice MODEL
*REV: 3-13-97
** ----- BJT MODEL -----
*
..model CA3096P PNP
+ (IS=10.0E-15 XTI=3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=94.5E+00 ISE=976.47E-15 NE=1.990E+00
+ IKF=1.1100E-03 XTB=0.000E+00 BR=.1000E+00 ISC=10.005E-15
+ NC=2.000E+00 IKR=10.00E-03 RC=10.000E+00 CJC=3.84E-12
+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.45E-12
+ MJE=.336E-00 VJE=0.750E-00 TR=10.000E-09 TF=24.3E-9
+ ITF=1.25E-00 XTF=10.05E+00 VTF=9.79E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00 NK=.53
*
*COPYRIGHT © 1997 INTERSIL CORPORATION
*ALL RIGHTS RESERVED
*
*CA3083 NPN PSPICE MODEL
*REV: 3-13-97
** ----- BJT MODEL -----
*
..model CA3083 NPN
+ (IS=10.0E-15 XTI=3.000E+00 EG=1.110E+00 VAF=1.00E+02
+ VAR=1.000E+02 BF=112.8E+00 ISE=99.086E-15 NE=1.410E+00
+ IKF=120.900E-03 XTB=0.000E+00 BR=16.0E+00 ISC=116.12E-15
+ NC=1.700E+00 IKR=29.800E-03 RC=10.000E+00 CJC=991.71E-15
+ MJC=0.333E-00 VJC=0.7500E-00 FC=5.000E-01 CJE=1.02E-12
+ MJE=.333E-00 VJE=0.750E-00 TR=10.000E-09 TF=275.61E-12
+ ITF=.3750E-00 XTF=91.950E+00 VTF=8.90E+00 PTF=0.000E+00
+ RE=0.0E+00 RB=0.00E+00

--- CUT HERE ---

 

[[email protected]]

On Monday, October 7, 2013 9:36:46 PM UTC-4, Clifford Heath wrote:

Short answer: replace R5 with a current source.

 

[Clifford Heath]

It'd be good to know how to run such a range of tests using LTSpice.

Perhaps Jim can offer advice on how to automate that kind of modeling?

Note that the AGC voltage is developed by comparing the emitter voltage
with the AGC transistor's diode drop. That diode drop changes with
temperature, so the oscillator's output voltage will change with
temperature, too. If that's an issue, you need to find a different way
to develop your AGC (if it's critical, you probably want an op-amp in
there somewhere).

It's not critical. I just want two things: to stabilise the amplitude
fairly quickly, and to keep following stages linear. If you disconnect
the AGC in that Colpitts circuit, you'll see that the amplitude is
still stabilising after 100us, instead of closing within 30us, because
C1/C2 are still charging to the emitter's DC point.

The purity was dramatically improved by the addition of the base
capacitor, which also allowed removing a much larger cap in series with
the inductor. That cap was also a cause of slow startup, because it
takes a while to reach its final DC state of charge.

I will be adding varactors to tune the oscillator, which is why I
don't want to rely on post-filtering (though the varactors add more
non-linearity themselves).

Regarding extra phase noise from adding AGC, surely that depends on the
kind of filtering applied? Here I have a simple R/C combination, but it
could go 2nd order.

Thanks for your thoughts. Oh, I just saw your final response. I'll send
this, then play with your proposal and respond to that.

Clifford Heath.

 

[Clifford Heath]

With your circuit as shown, I don't think the AGC is doing anything at
all. Rather, it looks like the amplitude is limited because Q1 is
saturating.

I think the AGC transistor's BE diode is turning on hard enough to turn
Q1 full on. I played with different resister values and the AGC seems to
be doing something, but there's definitely two methods of action.

In a normal common-collector Colpitts, you don't have the collector
resistor (R3 in your schematic). If I set R3 very small, then the AGC
does something -- it slams down to zero volts, which means its out of
regulation.

Right - I had that at one time, and I'm sure I had it regulating too,
but it's fiddly as you say.

Try this. It's a proper grounded-collector Collpits. You can see that
AGC is really working because the bias line does get pulled down, but
never gets pulled all the way down to zero. Starting is a hair slower,
because the higher emitter resistance limits transistor current (and
hence gain) -- you may be able to influence this a bit by dinking with
the bias network.

I had this topology at one time but didn't get it as clean as you have
it. Probably discounted it because its slower to start, but the payoff
is there - impressively clean result.

This is in no way optimized or even checked to make sure that it's not
just a fixture for burning up LM3046 chips -- that's your job.
And finally -- why am I doing this? And why am I doing it for free?
Shouldn't I be doing paying work?

If it's any consolation, it's not my job either I'm just a hobbyist
learning how to design DF gear, in the hope of attracting some of the
very fit Sydney geeks into a radio hobby, through sprint ARDF.

Be sure to check it with a variety of inductor Q's...
Also be sure to check its operation with a pick-off in place...

Good advice, will do.

Clifford Heath.