Using inductors as loads in the collectors of Q1 and Q2 forces them
to operate at a fixed (about 3.5V) Vce regardless of quiescent
current. Also L1 resonates with C2 (and about 5 or 6 pF of "stray"
capacitance from L1 and Q2-cb) at the 40MHz operating frequency.
Likewise L2 resonates with collector-base capacitance of Q3
multiplied by it current gain.
It is clearly stated in the LTspice help file (Waveform Viewer =>
Trace Selection) that, "It is also possible to point at voltage
differences with the mouse. You can click on one node and drag the
mouse to another node. You will see the red voltage probe at the
first node and a black probe on the second. This allows you to
differentially plot voltages: [screen graphic]."
Why didn't you bother to look at the help file? In the section on
waveform arithmetic it also states that alternatively you could do
this by right-clicking on the graph window or on the label of an
existing trace and manually editing the expression (e.g. edit "V(8)"
to be "V(8,7)" or "V(8)-V(7)").
No. Q3 is an emitter follower used as an active rectifier (to reduce
loading on the prior stage, Q2). Q4 is biased under exactly the same
quiescent conditions as Q3 in order to form a compensating reference
point to which the meter can be differentially connected. This is
what makes zero input yield zero output.
Of course the meter *coil* will have some inductance by the nature
of its construction. Its relevance is that it serves somewhat to
filter out RF.
Probably responding to all the points raised here will take care of
that. Let me know if you are still clueless about anything else.
Looks like D1 protects the input Q against reverse Vbe from strong
signals though I've usually seen back to back diodes for that.
Correct, but only one diode is needed in antiparallel to Q1's already
existing base-emitter diode.
Ditto for D2 but it's also part of a doubling detector. Look at
what JT gave you and redraw this new circuit substituting a diode
for Q3. Cool, eh?
Yes, D2 limits strong signals, but, no, it doesn't really serve any
other function. Detection is performed solely by Q3.
L1, L2 for DC bias while blocking RF.
Not really (well maybe sort of, for L2). L1 has the most signal
precisely at the RF operating frequency (where is part of a resonant
network).
C1-C3 [are] for DC blocking.
Yes for C1 and C3, but not for C2 which is part of a resonant network.
Q3 in emitter follower since you want to drive a low impedance with
current. It gives current gain, not voltage gain.
Spot on.
D3 as a detector if you want to call it that.
Sorry, it protects, rather than detects (since its only function is
to limit the voltage applied to the meter coil).
It makes the meter read DC current.
No. Applying the meter differentially between Q3 (the detector) and
Q4 (the reference) does that.
Pulsating DC, mind you, C5 provides filter action to fix that and
it bypasses R5. C6 bypasses R6.
Yes and yes.
Q4 is a NULL adjust to zero yer meter, i.e., make nodes 7 & 8
equal. C4 keeps stray RF from messing with Q4.
Right again (Q4 should be presented with impedances at both dc and
ac as seen by Q3).
Where do you get 2.8V batteries or is that a Thevinin equiv?
Maybe it's a couple of 1.5V batts run down.
That's about right. Batteries only start out at 1.5 volts per cell.
I'd want to use rechargables and NiCd's are 1.25V each or 2.5 per
pair. Oughtta still work.
Yes, the exact voltage isn't too important just so long as it is at
least several times greater than a Vbe drop and the gain stage bias
resistors (R1 and R2) are adjusted to keep Q1 and Q2 biased up at
at least 2ma so they have enough GBW to function well at 40MHz.
And Paul, this circuit is not borrowed from anywhere. I made it up
over about an hour to amuse myself with the interesting problem you
presented. And because of the ensuing discussion, I learned of that
clever zero forward voltage diode connected transistor (not used
here).
By the way, I've played with the sim some more and have realized
that there is no need for R5 and R6 to be as small as R1 and R2.
The circuit actually works better when they are 10k (cuts battery
drain to under 5ma and provides higher circuit gain and an
additional layer of protection for the meter movement).
Other changes I'd recommend would be to insert a 3k3 resistor in
series with the meter coil a 10nF capacitor directly across it to
further protect it from overdrive and RF (at only a small cost to
sensitivity due to the square law response of the detector).
Gain adjustment is probably best done with a 100 ohm pot placed
right at the antenna input to ground with the wiper going to the
input coupling capacitor, C1. This should be a small, low
inductance type such as a flat, quarter inch, single turn trimmer.
Here is the updated LTspice schematic file to cut and paste. (Look
in alt.binaries.schematics.electronic for a gif of the schematic.)
______________
Version 4
SHEET 1 1120 680
WIRE -288 -176 -512 -176
WIRE 688 -176 688 -96
WIRE 688 -32 688 64
WIRE 688 320 560 320
WIRE 0 320 -416 320
WIRE -672 32 -672 112
WIRE -672 32 -624 32
WIRE -400 32 -416 32
WIRE -288 32 -288 -32
WIRE -288 -176 0 -176
WIRE 0 -176 336 -176
WIRE -288 -112 -288 -176
WIRE -352 96 -352 128
WIRE -512 -112 -512 -176
WIRE -528 32 -512 32
WIRE -512 32 -496 32
WIRE -512 32 -512 -32
WIRE -672 288 -672 320
WIRE -576 96 -576 112
WIRE -592 112 -576 112
WIRE -576 128 -576 112
WIRE -672 112 -656 112
WIRE -432 32 -416 32
WIRE -416 32 -416 208
WIRE 688 240 688 320
WIRE 688 64 752 64
WIRE 0 288 0 320
WIRE -208 32 -192 32
WIRE -192 64 -192 32
WIRE -192 32 -144 32
WIRE 0 -16 0 -176
WIRE -192 144 -192 176
WIRE 0 208 0 176
WIRE 400 32 416 32
WIRE 432 64 432 32
WIRE 432 144 432 176
WIRE 336 -176 560 -176
WIRE -912 -48 -880 -48
WIRE 336 208 336 176
WIRE 336 -16 336 -176
WIRE 432 32 448 32
WIRE -192 176 -176 176
WIRE -112 176 -96 176
WIRE -192 176 -192 192
WIRE 336 288 336 320
WIRE 336 320 0 320
WIRE 336 176 352 176
WIRE 416 176 432 176
WIRE 432 176 432 192
WIRE 336 176 336 128
WIRE 0 128 16 128
WIRE 0 128 0 80
WIRE 336 128 336 80
WIRE 688 64 688 176
WIRE -272 32 -288 32
WIRE -288 32 -304 32
WIRE -416 288 -416 320
WIRE -416 320 -672 320
WIRE -672 112 -672 208
WIRE -848 32 -752 32
WIRE 688 -176 800 -176
WIRE 816 -176 816 -96
WIRE 816 -16 816 64
WIRE 816 320 800 320
WIRE 816 64 816 160
WIRE 752 64 752 80
WIRE 752 64 816 64
WIRE 816 240 816 320
WIRE 320 128 336 128
WIRE -96 32 -96 64
WIRE -96 128 -96 176
WIRE -96 32 -64 32
WIRE -96 176 0 176
WIRE 0 176 0 128
WIRE -144 32 -96 32
WIRE 16 128 112 128
WIRE 224 -96 224 -16
WIRE 224 128 320 128
WIRE 112 -96 128 -96
WIRE 208 -96 224 -96
WIRE 112 128 112 80
WIRE 112 128 144 128
WIRE 208 128 224 128
WIRE 560 -128 560 -176
WIRE 560 -176 688 -176
WIRE 560 16 560 320
WIRE 560 320 336 320
WIRE 448 -64 432 -64
WIRE 432 -64 432 32
WIRE 416 32 432 32
WIRE 528 -64 544 -64
WIRE 560 -64 560 -48
WIRE 112 0 112 -16
WIRE 800 -176 816 -176
WIRE 800 320 688 320
WIRE 544 -64 560 -64
WIRE 112 -16 144 -16
WIRE 208 -16 224 -16
WIRE 224 -16 224 128
WIRE 112 -16 112 -96
WIRE -848 32 -848 48
WIRE -848 112 -848 128
WIRE -688 32 -672 32
WIRE -848 -48 -848 -32
WIRE -912 160 -912 -48
WIRE -880 -48 -848 -48
FLAG -912 240 0
FLAG -880 -48 1
FLAG -672 32 2
FLAG -576 128 0
FLAG -352 128 0
FLAG -192 192 0
FLAG 432 192 0
FLAG 752 80 0
FLAG -512 32 3
FLAG -416 32 4
FLAG -288 32 5
FLAG -144 32 6
FLAG 16 128 7
FLAG 320 128 8
FLAG 416 32 9
FLAG 512 32 0
FLAG 800 -176 11
FLAG 800 320 12
FLAG 544 -64 10
FLAG -848 128 0
SYMBOL ind -528 -128 R0
WINDOW 0 40 32 Left 0
WINDOW 3 40 57 Left 0
WINDOW 39 40 80 Left 0
SYMATTR InstName L1
SYMATTR Value 1µ0
SYMATTR SpiceLine Rpar=2k Cpar=2p
SYMBOL npn -528 96 M270
WINDOW 0 80 48 VBottom 0
WINDOW 3 80 48 VTop 0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL cap -752 48 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C1
SYMATTR Value 1n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL cap -496 48 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C2
SYMATTR Value 10p
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL res -688 192 R0
SYMATTR InstName R1
SYMATTR Value 1k0
SYMBOL diode -656 128 R270
WINDOW 0 -32 32 VTop 0
WINDOW 3 -32 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL voltage -912 144 R0
WINDOW 123 32 64 Left 0
WINDOW 39 32 96 Left 0
WINDOW 0 32 32 Left 0
WINDOW 3 0 128 Center 0
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=33
SYMATTR InstName V1
SYMATTR Value SINE(0 {p} 40.5e6 50u)
SYMBOL npn -304 96 M270
WINDOW 0 80 48 VBottom 0
WINDOW 3 80 48 VTop 0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL cap -208 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 1n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL res -432 192 R0
SYMATTR InstName R2
SYMATTR Value 1k0
SYMBOL npn -64 -16 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL res -208 48 R0
WINDOW 3 36 64 Left 0
SYMATTR Value 10k
SYMATTR InstName R3
SYMBOL npn 400 -16 M0
SYMATTR InstName Q4
SYMATTR Value 2N3904
SYMBOL cap 512 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C4
SYMATTR Value 1n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL res 320 192 R0
WINDOW 0 -5 40 Right 0
WINDOW 3 -5 75 Right 0
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL res 416 48 R0
WINDOW 0 -4 40 Right 0
WINDOW 3 -4 64 Right 0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL cap -112 160 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C5
SYMATTR Value 10n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL res -16 192 R0
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL cap 416 160 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C6
SYMATTR Value 10n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL Misc\\battery 816 -112 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 132 Left 0
SYMATTR SpiceLine Rser=1
SYMATTR InstName V2
SYMATTR Value 2.8V
SYMBOL Misc\\battery 816 144 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 132 Left 0
SYMATTR SpiceLine Rser=1
SYMATTR InstName V3
SYMATTR Value 2.8V
SYMBOL ind -304 -128 R0
WINDOW 0 40 32 Left 0
WINDOW 3 40 57 Left 0
WINDOW 39 40 80 Left 0
SYMATTR InstName L2
SYMATTR Value 1µ0
SYMATTR SpiceLine Rpar=2k Cpar=2p
SYMBOL cap 672 -96 R0
SYMATTR InstName C7
SYMATTR Value 10n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL cap 672 176 R0
SYMATTR InstName C8
SYMATTR Value 10n
SYMATTR SpiceLine Rser=10m Lser=10n
SYMBOL ind 112 -80 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
WINDOW 39 71 56 VCenter 0
SYMATTR InstName Lmeter
SYMATTR Value 10µ
SYMATTR SpiceLine Rser=380
SYMBOL diode -112 128 M180
WINDOW 0 36 40 Left 0
WINDOW 3 19 2 Left 0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 144 112 M90
WINDOW 0 0 24 VBottom 0
WINDOW 3 32 24 VTop 0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL res 544 -80 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R7
SYMATTR Value 330k
SYMBOL res 544 -144 R0
WINDOW 3 36 64 Left 0
SYMATTR Value {100k*a}
SYMATTR InstName R8a
SYMBOL res 544 -80 R0
WINDOW 3 36 64 Left 0
SYMATTR Value {100k*b}
SYMATTR InstName R8b
SYMBOL res 96 -16 R0
WINDOW 0 36 48 Left 0
SYMATTR InstName R9
SYMATTR Value 3k3
SYMBOL cap 208 -32 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C9
SYMATTR Value 10n
SYMBOL res -864 16 R0
WINDOW 3 36 64 Left 0
SYMATTR InstName R10b
SYMATTR Value {100*d}
SYMBOL res -864 -48 R0
WINDOW 3 36 64 Left 0
SYMATTR InstName R10a
SYMATTR Value {100*c}
TEXT -1032 -208 Left 0 !.tran 0 100u 0 .1u
TEXT -1032 -240 Left 0 !;ac dec 101 1e6 1e9
TEXT 408 -232 Left 0 ;.step param n list 0 .5 1
TEXT 168 -200 Center 0 ;100ua Full Scale
TEXT 408 -208 Left 0 !.param n=.5015 a=limit(n,.999,1m) b=1-a
TEXT 472 -152 Center 0 ;Meter Offset\nAdjustment
TEXT -912 288 Top 0 !.param p=sqrt(2)*rms
TEXT -1032 -160 Left 0 !.param rms=7.5m
TEXT 168 -264 Center 0 ;FIELD STRENGTH METER
TEXT -760 120 Center 0 ;Meter Gain\nAdjustment
TEXT -1032 -88 Left 0 !.param m=1 d=limit(m,.999,1m) c=1-d
TEXT -1032 -112 Left 0 ;.step param m list 0 .1 .3 1
