Understanding double balanced mixer

G

George Herold

Guest
I was playing around with a double balanced mixer the yesterday. In
an attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file. (To be attached
later to thread.) This worked fine. I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance. This doesn’t work nearly
as well! The transformers are doing something that I don’t
understand. (I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

LTspice file without transformers below.


Version 4
SHEET 1 880 680
WIRE 320 -64 160 -64
WIRE 416 -64 320 -64
WIRE 528 -64 464 -64
WIRE 160 -48 160 -64
WIRE 528 -48 528 -64
WIRE 320 -32 320 -64
WIRE 416 -32 416 -64
WIRE 320 48 320 32
WIRE 320 48 256 48
WIRE 416 48 416 32
WIRE 464 48 464 -64
WIRE 464 48 416 48
WIRE 528 64 528 32
WIRE 160 80 160 32
WIRE 528 160 528 144
WIRE 768 160 528 160
WIRE 160 192 160 160
WIRE 160 192 -96 192
WIRE 528 192 528 160
WIRE 768 192 768 160
WIRE -96 208 -96 192
WIRE 160 208 160 192
WIRE 768 288 768 272
WIRE 528 304 528 272
WIRE -96 320 -96 288
WIRE 160 320 160 288
WIRE 320 320 320 48
WIRE 416 320 416 48
WIRE 320 400 320 384
WIRE 320 400 160 400
WIRE 416 400 416 384
WIRE 416 400 320 400
WIRE 256 432 256 48
WIRE 528 432 528 384
WIRE 528 432 256 432
FLAG -96 320 0
FLAG 768 288 0
SYMBOL ind2 176 -64 M0
WINDOW 0 84 20 Right 2
WINDOW 3 102 61 Right 2
SYMATTR InstName L2
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL voltage 160 64 R0
WINDOW 123 0 0 Left 2
WINDOW 39 -157 61 Left 2
WINDOW 3 -195 24 Left 2
SYMATTR SpiceLine Rser=10
SYMATTR InstName V1
SYMATTR Value SINE(0 1 100k)
SYMBOL ind2 176 304 M0
SYMATTR InstName L3
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL diode 400 -32 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 400 320 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 336 384 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 336 32 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL ind2 512 400 M180
WINDOW 0 61 71 Left 2
WINDOW 3 57 35 Left 2
SYMATTR InstName L5
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL ind2 512 48 M180
WINDOW 0 40 108 Left 2
WINDOW 3 41 73 Left 2
SYMATTR InstName L6
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL voltage 528 48 R0
WINDOW 123 0 0 Left 2
WINDOW 39 92 72 Left 2
WINDOW 0 79 13 Left 2
WINDOW 3 77 47 Left 2
SYMATTR SpiceLine Rser=10
SYMATTR InstName V2
SYMATTR Value SINE(0 .5 10k)
SYMBOL res -80 304 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL res 784 288 R180
WINDOW 0 -39 78 Left 2
WINDOW 3 -47 33 Left 2
SYMATTR InstName R4
SYMATTR Value 1k
SYMBOL voltage 160 304 R180
WINDOW 123 0 0 Left 2
WINDOW 39 59 24 Left 2
WINDOW 0 94 80 Left 2
WINDOW 3 45 50 Left 2
SYMATTR SpiceLine Rser=10
SYMATTR InstName V3
SYMATTR Value SINE(0 1 100k)
SYMBOL voltage 528 288 R180
WINDOW 123 0 0 Left 2
WINDOW 39 -157 15 Left 2
WINDOW 0 -87 81 Left 2
WINDOW 3 -192 47 Left 2
SYMATTR SpiceLine Rser=10
SYMATTR InstName V4
SYMATTR Value SINE(0 .5 10k)
TEXT -320 416 Left 2 !.tran 400u
 
On Nov 8, 10:05 am, George Herold <gher...@teachspin.com> wrote:
I was playing around with a double balanced mixer the yesterday.  In
an attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file.  (To be attached
later to thread.)
Click to expand...
Here's the file with transformers if you are interested.

George H.

Version 4
SHEET 1 880 680
WIRE -96 16 -304 16
WIRE 608 16 -16 16
WIRE -304 32 -304 16
WIRE -304 128 -304 112
WIRE -128 176 -304 176
WIRE 112 176 -48 176
WIRE 320 176 160 176
WIRE 416 176 320 176
WIRE 480 176 480 128
WIRE 480 176 464 176
WIRE 528 176 480 176
WIRE -304 192 -304 176
WIRE 112 192 112 176
WIRE 160 192 160 176
WIRE 528 192 528 176
WIRE 320 208 320 176
WIRE 416 208 416 176
WIRE 608 256 608 16
WIRE 112 272 48 272
WIRE -304 288 -304 272
WIRE 160 288 160 272
WIRE 208 288 160 288
WIRE 320 288 320 272
WIRE 320 288 256 288
WIRE 416 288 416 272
WIRE 464 288 464 176
WIRE 464 288 416 288
WIRE 528 288 528 272
WIRE 576 288 528 288
WIRE 48 304 48 272
WIRE 528 304 528 288
WIRE 160 320 160 288
WIRE 320 320 320 288
WIRE 416 320 416 288
WIRE 608 368 608 336
WIRE 320 400 320 384
WIRE 320 400 160 400
WIRE 416 400 416 384
WIRE 416 400 320 400
WIRE 256 432 256 288
WIRE 352 432 256 432
WIRE 528 432 528 384
WIRE 528 432 352 432
WIRE 208 448 208 288
WIRE 352 496 352 432
WIRE 576 544 576 288
WIRE 576 544 464 544
WIRE 208 560 208 528
WIRE 576 640 576 624
FLAG -304 288 0
FLAG 48 304 0
FLAG 464 544 Vout
FLAG 608 368 0
FLAG -304 128 0
FLAG 480 128 Vpos
FLAG 352 496 Vneg
FLAG 208 560 0
FLAG 576 640 0
SYMBOL ind2 96 288 M180
WINDOW 0 -7 76 Right 2
WINDOW 3 -7 42 Right 2
SYMATTR InstName L1
SYMATTR Value 200ľ
SYMATTR Type ind
SYMBOL ind2 176 176 M0
WINDOW 0 -7 42 Right 2
WINDOW 3 -5 73 Right 2
SYMATTR InstName L2
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL voltage -304 176 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value SINE(0 2 100k)
SYMBOL res -32 160 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 10
SYMBOL ind2 176 304 M0
SYMATTR InstName L3
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL diode 400 208 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 400 320 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 336 384 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 336 272 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL ind2 624 240 M0
WINDOW 0 -64 56 Left 2
WINDOW 3 -71 8 Left 2
SYMATTR InstName L4
SYMATTR Value 200ľ
SYMATTR Type ind
SYMBOL ind2 512 400 M180
WINDOW 0 36 90 Left 2
WINDOW 3 9 -39 Left 2
SYMATTR InstName L5
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL ind2 512 288 M180
WINDOW 0 40 108 Left 2
WINDOW 3 41 73 Left 2
SYMATTR InstName L6
SYMATTR Value 100ľ
SYMATTR Type ind
SYMBOL res 0 0 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10
SYMBOL voltage -304 16 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value SINE(0 1 10k)
SYMBOL res 224 544 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL res 592 640 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 1k
TEXT 136 176 Bottom 2 !K1 L1 L2 L3 1
TEXT -320 416 Left 2 !.tran 400u
TEXT 704 144 Top 2 !K2 L4 L5 L6 1
 
On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday. In an
attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file. (To be attached
later to thread.) This worked fine. I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance. This doesn’t work nearly as
well! The transformers are doing something that I don’t understand. (I
find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.
Click to expand...
(LTSpice file snipped)

The polarities are wrong. The transformers are arranged as below (note
the dots), which leads to the noted voltage source arrangement.

.----------
|
.--------- /+\
( o ( )
-. ( \-/
)|( |
)||--------- < == > o----------
)|( o |
-' ( /+\
( ( )
'--------- \-/
|
'----------
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


--
www.wescottdesign.com
 
On Tue, 08 Nov 2011 10:27:44 -0600, Tim Wescott wrote:

On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday. In an
attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file. (To be attached
later to thread.) This worked fine. I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance. This doesn’t work nearly as
well! The transformers are doing something that I don’t understand.
(I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong. The transformers are arranged as below (note
the dots), which leads to the noted voltage source arrangement.

.----------
|
.--------- /+\
( o ( )
-. ( \-/
)|( |
)||--------- < == > o----------
)|( o |
-' ( /+\
( ( )
'--------- \-/
|
'----------
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
Click to expand...
Oh -- I'm not sure about your voltage levels, but your input and output
impedances are way high for the usual diode DBM. And some source
impedance should be assumed in the LO: you're really interested in
driving the correct current through the diodes; drive a fixed voltage
through them and the current (which goes exponentially to voltage) will
likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms. That's something like
1.4V p-p (or maybe 2.8 -- it's been a while. Do the math). But you need
that source impedance! Put resistors in series with your inductors on
the drive side, at least.

--
www.wescottdesign.com
 
On Tue, 08 Nov 2011 10:00:39 -0800, George Herold wrote:

On Nov 8, 11:31 am, Tim Wescott <t...@seemywebsite.com> wrote:
On Tue, 08 Nov 2011 10:27:44 -0600, Tim Wescott wrote:
On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday.  In
an attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file.  (To be
attached later to thread.)  This worked fine.  I then replaced the
two transformers with opposite polarity voltage sources at one half
the amplitude, and kept the series inductance.  This doesn’t work
nearly as well!  The transformers are doing something that I don’t
understand. (I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong.  The transformers are arranged as below
(note the dots), which leads to the noted voltage source arrangement.

                             .----------
                             |
      .---------            /+\
      ( o                  (   )
   -. (                     \-/
    )|(                      | )||---------    < == >   o----------
    )|( o                    |
   -' (                     /+\
      (                    (   ) '---------            \-/
                             |
                             '----------
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

Oh -- I'm not sure about your voltage levels, but your input and output
impedances are way high for the usual diode DBM.  And some source
impedance should be assumed in the LO: you're really interested in
driving the correct current through the diodes; drive a fixed voltage
through them and the current (which goes exponentially to voltage) will
likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms.  That's something like
1.4V p-p (or maybe 2.8 -- it's been a while.  Do the math).  But you
need that source impedance!  Put resistors in series with your
inductors on the drive side, at least.

--www.wescottdesign.com- Hide quoted text -

- Show quoted text -

Yeah, I wanted to make one that works below 100kHz. So I ordered some
Audio transformers. 600 ohm impedance.. so the 1 k ohm is not too far
off.

The audio transformers only claim to work up to 15kHz. But here's a
'scope shot of the DBM at 100kHz LO and 1 kHz RF

http://imageshack.us/photo/my-images/850/tek0032.png/ Things get a bit
'squirrelly' above 300kHz.

The same drives but 10kHz on the LO.

http://imageshack.us/photo/my-images/838/tek0033.png/

It's great fun, I've got this SRS spectrum analyzer so I can play with
the drive levels and watch all the harmonics move around.
Click to expand...
Have you checked the MiniCircuits catalog? I seem to recall that they
have mixers that work down to 50kHz or even lower. They're on the web,
at the obvious address.

Things will work at your 600 ohm impedance level, but the distortion-free
power levels will suck. If I recall what you're trying to do that
shouldn't be a big deal -- just don't try to use them for receiving LORAN.

Also check the amateur radio literature. Winding a pair of balanced
transformers for 100kHz service shouldn't be too tedious if you start
with the right core material -- audio transformers need all the windings
so they'll work down at 300Hz (or 30Hz, or whatever); those windings (and
core losses) just get in the way at higher frequencies.

On most of those mixers there's at least one port that works all the way
down to DC, and if there's no bells and whistles there's nothing to keep
the mixer from operating in "reverse". So you do need to inject your
high-level drive into the LO at the rated frequency, and you do need to
deal with signals above the rated frequency at the "RF" port, but there's
nothing to say that the "IF" port (the one that goes down to DC, usually)
has to be an outie -- you can inject your low-level low frequency signal
there and extract your high-frequency signal at the "RF" port.

--
www.wescottdesign.com
 
On Nov 8, 11:27 am, Tim Wescott <t...@seemywebsite.com> wrote:
On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:
I was playing around with a double balanced mixer the yesterday.  In an
attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file.  (To be attached
later to thread.)  This worked fine.  I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance.  This doesn’t work nearly as
well!  The transformers are doing something that I don’t understand..  (I
find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong.  The transformers are arranged as below (note
the dots), which leads to the noted voltage source arrangement.

                             .----------
                             |
      .---------            /+\
      ( o                  (   )
   -. (                     \-/
    )|(                      |
    )||---------    < == >   o----------
    )|( o                    |
   -' (                     /+\
      (                    (   )
      '---------            \-/
                             |
                             '----------
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

--www.wescottdesign.com
Click to expand...
Ahh Duh! Thanks Tim, amazing how 'stupid', I can be sometimes.
I flipped my voltage sources and everything is fine.


George H.
 
On Nov 8, 11:31 am, Tim Wescott <t...@seemywebsite.com> wrote:
On Tue, 08 Nov 2011 10:27:44 -0600, Tim Wescott wrote:
On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday.  In an
attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file.  (To be attached
later to thread.)  This worked fine.  I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance.  This doesn’t work nearly as
well!  The transformers are doing something that I don’t understand.
(I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong.  The transformers are arranged as below (note
the dots), which leads to the noted voltage source arrangement.

                             .----------
                             |
      .---------            /+\
      ( o                  (   )
   -. (                     \-/
    )|(                      |
    )||---------    < == >   o----------
    )|( o                    |
   -' (                     /+\
      (                    (   )
      '---------            \-/
                             |
                             '----------
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

Oh -- I'm not sure about your voltage levels, but your input and output
impedances are way high for the usual diode DBM.  And some source
impedance should be assumed in the LO: you're really interested in
driving the correct current through the diodes; drive a fixed voltage
through them and the current (which goes exponentially to voltage) will
likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms.  That's something like
1.4V p-p (or maybe 2.8 -- it's been a while.  Do the math).  But you need
that source impedance!  Put resistors in series with your inductors on
the drive side, at least.

--www.wescottdesign.com- Hide quoted text -

- Show quoted text -
Click to expand...
Yeah, I wanted to make one that works below 100kHz. So I ordered some
Audio transformers. 600 ohm impedance.. so the 1 k ohm is not too far
off.

The audio transformers only claim to work up to 15kHz. But here's a
'scope shot of the DBM at 100kHz LO and 1 kHz RF

http://imageshack.us/photo/my-images/850/tek0032.png/
Things get a bit 'squirrelly' above 300kHz.

The same drives but 10kHz on the LO.

http://imageshack.us/photo/my-images/838/tek0033.png/

It's great fun, I've got this SRS spectrum analyzer so I can play with
the drive levels and watch all the harmonics move around.

George H.
 
On Nov 8, 1:19 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Tue, 08 Nov 2011 10:00:39 -0800, George Herold wrote:
On Nov 8, 11:31 am, Tim Wescott <t...@seemywebsite.com> wrote:
On Tue, 08 Nov 2011 10:27:44 -0600, Tim Wescott wrote:
On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday.  In
an attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file.  (To be
attached later to thread.)  This worked fine.  I then replaced the
two transformers with opposite polarity voltage sources at one half
the amplitude, and kept the series inductance.  This doesn’t work
nearly as well!  The transformers are doing something that I don’t
understand. (I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong.  The transformers are arranged as below
(note the dots), which leads to the noted voltage source arrangement..

                             .----------
                             |
      .---------            /+\
      ( o                  (   )
   -. (                     \-/
    )|(                      | )||---------    < == >   o----------
    )|( o                    |
   -' (                     /+\
      (                    (   ) '---------            \-/
                             |
                             '----------
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

Oh -- I'm not sure about your voltage levels, but your input and output
impedances are way high for the usual diode DBM.  And some source
impedance should be assumed in the LO: you're really interested in
driving the correct current through the diodes; drive a fixed voltage
through them and the current (which goes exponentially to voltage) will
likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms.  That's something like
1.4V p-p (or maybe 2.8 -- it's been a while.  Do the math).  But you
need that source impedance!  Put resistors in series with your
inductors on the drive side, at least.

--www.wescottdesign.com-Hide quoted text -

- Show quoted text -

Yeah, I wanted to make one that works below 100kHz.  So I ordered some
Audio transformers.  600 ohm impedance.. so the 1 k ohm is not too far
off.

The audio transformers only claim to work up to 15kHz.  But here's a
'scope shot of the DBM at 100kHz LO and 1 kHz RF

http://imageshack.us/photo/my-images/850/tek0032.png/Things get a bit
'squirrelly' above 300kHz.

The same drives but 10kHz on the LO.

http://imageshack.us/photo/my-images/838/tek0033.png/

It's great fun, I've got this SRS spectrum analyzer so I can play with
the drive levels and watch all the harmonics move around.

Have you checked the MiniCircuits catalog?  I seem to recall that they
have mixers that work down to 50kHz or even lower.  They're on the web,
at the obvious address.
Click to expand...
Yup not only the catalog but I've got an SBL-3+ from minicircuits
lashed up in front of me.
Good for 25 kHz to 200 MHz. (well my some what sloppy soldering job
may not get to 200 Meg.)
Things will work at your 600 ohm impedance level, but the distortion-free
power levels will suck.  If I recall what you're trying to do that
shouldn't be a big deal -- just don't try to use them for receiving LORAN..

Also check the amateur radio literature.  Winding a pair of balanced
transformers for 100kHz service shouldn't be too tedious if you start
with the right core material -- audio transformers need all the windings
so they'll work down at 300Hz (or 30Hz, or whatever); those windings (and
core losses) just get in the way at higher frequencies.
Click to expand...
Well, for like $3 I bought a few from digikey. I'l have to measure
it's freqeucny response.

On most of those mixers there's at least one port that works all the way
down to DC, and if there's no bells and whistles there's nothing to keep
the mixer from operating in "reverse".  So you do need to inject your
high-level drive into the LO at the rated frequency, and you do need to
deal with signals above the rated frequency at the "RF" port, but there's
nothing to say that the "IF" port (the one that goes down to DC, usually)
has to be an outie -- you can inject your low-level low frequency signal
there and extract your high-frequency signal at the "RF" port.
Click to expand...
Yeah I saw that on some audio site. (they call 'em diode ring
modulators) Putting the one signal into the 'IF' port and taking the
output from the RF. I'll try that with the mini-circuits DBM. It
would be nice if I could run the LO at ~50kHz and then modulate at
~1kHz.


Well it works... but the output is not so clean. I'll play around
with spice at home tonight.

George H.
--www.wescottdesign.com- Hide quoted text -

- Show quoted text -
Click to expand...
 
George Herold wrote:
On Nov 8, 11:31 am, Tim Wescott <t...@seemywebsite.com> wrote:

On Tue, 08 Nov 2011 10:27:44 -0600, Tim Wescott wrote:

On Tue, 08 Nov 2011 07:05:38 -0800, George Herold wrote:

I was playing around with a double balanced mixer the yesterday. In an
attempt to understand how the thing worked, (I could find no good
explanations on the web.) I made a little spice file. (To be attached
later to thread.) This worked fine. I then replaced the two
transformers with opposite polarity voltage sources at one half the
amplitude, and kept the series inductance. This doesn’t work nearly as
well! The transformers are doing something that I don’t understand.
(I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

(LTSpice file snipped)

The polarities are wrong. The transformers are arranged as below (note
the dots), which leads to the noted voltage source arrangement.

.----------
|
.--------- /+\
( o ( )
-. ( \-/
)|( |
)||--------- < == > o----------
)|( o |
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(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

Oh -- I'm not sure about your voltage levels, but your input and output
impedances are way high for the usual diode DBM. And some source
impedance should be assumed in the LO: you're really interested in
driving the correct current through the diodes; drive a fixed voltage
through them and the current (which goes exponentially to voltage) will
likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms. That's something like
1.4V p-p (or maybe 2.8 -- it's been a while. Do the math). But you need
that source impedance! Put resistors in series with your inductors on
the drive side, at least.

--www.wescottdesign.com- Hide quoted text -

- Show quoted text -


Yeah, I wanted to make one that works below 100kHz. So I ordered some
Audio transformers. 600 ohm impedance.. so the 1 k ohm is not too far
off.

The audio transformers only claim to work up to 15kHz. But here's a
'scope shot of the DBM at 100kHz LO and 1 kHz RF

http://imageshack.us/photo/my-images/850/tek0032.png/
Things get a bit 'squirrelly' above 300kHz.

The same drives but 10kHz on the LO.

http://imageshack.us/photo/my-images/838/tek0033.png/

It's great fun, I've got this SRS spectrum analyzer so I can play with
the drive levels and watch all the harmonics move around.

George H.
Look up LM1496
Click to expand...

Jamie
 
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