Opamp frequency mixer

[Tauno Voipio]

On 4.7.15 23:30, bitrex wrote:

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two
tubes.

I'm thinking about going solid state for the mixers in the first
iteration at least...since the RF is so low I could probably just use a
dual opamp with sufficient GBW. There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

Forget tubes or opamps. The semiconductor solution is called a
Gilbert cell (Google for it). With unsorted transistors from
the same batch, you will get a far better mixer than with tubes.
Do not forget the low-pass filter after mixing.

In the Theremin case, you do not even need fight with the dynamic
range in the same way as radio receiver designers do.

<nag>
You should eject the idea of using tubes at all. A couple of FETs
will get a far better thing. If you need the dull glow of heaters
and associated show, use one or some tubes with only heaters.
</nag>

--

-TV

 

[Tauno Voipio]

On 4.7.15 23:30, bitrex wrote:

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two
tubes.

I'm thinking about going solid state for the mixers in the first
iteration at least...since the RF is so low I could probably just use a
dual opamp with sufficient GBW. There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

If you insist on tubes, look for 7360 beam-deflection tubes for mixers.
They were the best of their kind at their time. Also, you'll a hefty
sensitivity for magnetic disturbances in addition of the other tube quirks.

--

-TV

 

On Saturday, July 4, 2015 at 4:30:29 PM UTC-4, bitrex wrote:

... There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Links are not broken, click of "Fig. 1" and not "opamps".

 

On Sat, 04 Jul 2015 16:30:24 -0400, bitrex
<bitrex@de.lete.earthlink.net> wrote:

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two tubes.

If you just need the difference between the fixed and variable
frequency oscillator, any envelope detection (AM detection) circuit
should work. Feed the detector with both frequencies to the rectifier.
There is a beat (amplitude variations) between the two frequencies.
Using a half wave rectifier will create a DC biased waveform with the
beat (the difference frequency) riding on top of it and some high
frequency noise above that. Remove the DC bias with a capacitor and
the original frequency and their sum frequency with some low pass
filtering.

This should be perfectly sufficient when there are only exactly two
frequencies sufficiently higher (say 50 kHz) above the desired audio
difference frequency.

 

[Simon S Aysdie]

On Saturday, July 4, 2015 at 1:30:29 PM UTC-7, bitrex wrote:

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two tubes.

I'm thinking about going solid state for the mixers in the first
iteration at least...since the RF is so low I could probably just use a
dual opamp with sufficient GBW. There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

The transconductance OA has been mentioned already---and if I was insisting on OAs, I'd probably go with that. The Gilbert (or Jones) cell method was mentioned, and is very effective, but it is not an OA.

One comment on your link "complained" about the need for a LPF on the output. I don't fully get the complaint. After all, LPF is always needed unless "image reject" double balanced and linear mixing is done. But even that is too hard to do for obtaining very high rejection.

If considered non-OA methods, a simple analog switch in chopping mode will do it. That would be "singly balanced." IIRC, one can differentially drive four analog switches, that are also differentially chopped, and obtain double balance.


I/O
+ -
O O
| |
+---+ +---+
| | | |
o o o o
\...|...|...\.....O LO
o o o o SW/Relay control
| | | |
| \ / |
| X |
| / \ |
| | | |
+---+ +---+
| |
O O
+ -
I/O

The "X" is simply a wire cross for the wires drawn at 45 degrees.

 

[Joerg]

On 2015-07-04 4:37 PM, bitrex wrote:

On 7/4/2015 5:40 PM, Joerg wrote:
On 2015-07-04 2:24 PM, Tim Wescott wrote:

[...]

You could make a 180 degree shifter and summer by making an appropriate
transformer (probably at least mildly resonant at good toob impedances)
from signal A, with the secondary center-tapped and fed with signal B.
Feed a pair of triodes with the transformer outputs, and poof! you
have a
single balanced mixer.


For 180 degree shifting all you need is one triode as a follower. Make
the anode resistor the same as the cathode resistor. Then the anode
carries the 180 degree signal.

Only problem then is that the two outputs have wildly differing output
impedances, which could be a problem...

In that application it's not such a problem because the anode resistor
is very low and it's value is the impedance of that node now. The tube
doesn't amplify the signal voltage, it can't.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Clifford Heath]

On 05/07/15 07:06, Jim Thompson wrote:

On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com
wrote:
On 2015-07-04 1:30 PM, bitrex wrote:
Can anyone suggest an opamp mixer topology for the low 100s of kHz?
If it absolutely has to be semiconductors I'd use the MC1496.
I'd go with the MC1494... if you can get it. It has the level
shifting built-in... quite adequate for audio, and immensely eases the
pain of doing your own level-shifting.

If you can't get MC1494 but can arrange one bipolar drive signal, you
can make the rest of a Gilbert cell very easily using the matched
transistors in a LM3046, which are readily available, cheap and
sufficiently retro. You even get a spare transistor you could use for
the phase splitter.

 

[Clifford Heath]

On 06/07/15 02:22, Clifford Heath wrote:

On 05/07/15 07:06, Jim Thompson wrote:
On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com
wrote:
On 2015-07-04 1:30 PM, bitrex wrote:
Can anyone suggest an opamp mixer topology for the low 100s of kHz?
If it absolutely has to be semiconductors I'd use the MC1496.
I'd go with the MC1494... if you can get it. It has the level
shifting built-in... quite adequate for audio, and immensely eases the
pain of doing your own level-shifting.

If you can't get MC1494 but can arrange one bipolar drive signal, you
can make the rest of a Gilbert cell very easily using the matched
transistors in a LM3046, which are readily available, cheap and
sufficiently retro. You even get a spare transistor you could use for
the phase splitter.

Forgot to mention, the Ft of the LM3046 is circa 350MHz, so no problem
operating at HF. And there's always the rather faster and more expensive
HFA3101 if you need to operate up to 1GHz . But the LM3046 is one of
my jelly-beans for discrete stuff up to 50MHz. It can go higher, but
it's less trouble to design using faster transistors.

 

[Phil Hobbs]

Which design was that? I can't imagine modulating filament power at low
audio frequencies could be very great for it...

No kidding. Running a tube in the cathode-emission-limited condition is for a good time, not a long time. (Rectifiers are an exception.)

Cheers

Phil Hobbs

 

[whit3rd]

On Sunday, July 5, 2015 at 11:59:27 AM UTC-7, bitrex wrote:

There's a very simple circuit for a four-quadrant multiplier in the
LM13700 application notes. I'm not sure if it will work at a few
hundred kHz, however.

Two-quadrant multiplication with an OTA is more than enough, and it'll work
fine at megahertz frequencies, because you don't care about
power gain. Feed your low-level signal into the diff inputs,
and modulate the bias current with the LO.

The LM13700 is a very useful chip and works at low supply voltages as
well. It's unfortunate they don't make better OTAs that would be
suitable for much higher frequency applications.

They do: TI's OPA861, for example. They just have such opaque
datasheets and descriptions, that it's hard to recognize the
face under the greasepaint and red nose.

<http://www.ti.com/lit/ds/sbos338g/sbos338g.pdf>

 

[bitrex]

On 7/5/2015 2:03 PM, Simon S Aysdie wrote:

On Saturday, July 4, 2015 at 1:30:29 PM UTC-7, bitrex wrote:
So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two tubes.

I'm thinking about going solid state for the mixers in the first
iteration at least...since the RF is so low I could probably just use a
dual opamp with sufficient GBW. There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

The transconductance OA has been mentioned already---and if I was insisting on OAs, I'd probably go with that. The Gilbert (or Jones) cell method was mentioned, and is very effective, but it is not an OA.

One comment on your link "complained" about the need for a LPF on the output. I don't fully get the complaint. After all, LPF is always needed unless "image reject" double balanced and linear mixing is done. But even that is too hard to do for obtaining very high rejection.

If considered non-OA methods, a simple analog switch in chopping mode will do it. That would be "singly balanced." IIRC, one can differentially drive four analog switches, that are also differentially chopped, and obtain double balance.


I/O
+ -
O O
| |
+---+ +---+
| | | |
o o o o
\...|...|...\.....O LO
o o o o SW/Relay control
| | | |
| \ / |
| X |
| / \ |
| | | |
+---+ +---+
| |
O O
+ -
I/O

The "X" is simply a wire cross for the wires drawn at 45 degrees.

There's a very simple circuit for a four-quadrant multiplier in the
LM13700 application notes. I'm not sure if it will work at a few
hundred kHz, however.

The LM13700 is a very useful chip and works at low supply voltages as
well. It's unfortunate they don't make better OTAs that would be
suitable for much higher frequency applications.

 

[piglet]

On 05/07/2015 19:59, bitrex wrote:

The LM13700 is a very useful chip and works at low supply voltages as
well. It's unfortunate they don't make better OTAs that would be
suitable for much higher frequency applications.

You might find the LM13700 especially useful in your Theremin project
for the volume control circuitry. That VCA stage can be a problem area
in Theremin design. I find the old RCA design which modulates tube
filament power to vary the volume delightfully zany!

piglet

 

[John Larkin]

On Sat, 04 Jul 2015 19:27:27 -0400, bitrex
<bitrex@de.lete.earthlink.net> wrote:

On 7/4/2015 6:23 PM, Phil Hobbs wrote:
On 7/4/2015 5:53 PM, John Larkin wrote:
On Sat, 04 Jul 2015 16:30:24 -0400, bitrex
bitrex@de.lete.earthlink.net> wrote:

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap
on eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two tubes.

I'm thinking about going solid state for the mixers in the first
iteration at least...since the RF is so low I could probably just use a
dual opamp with sufficient GBW. There was this article but of course
all the links to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

Any tube will mix: diode, triode, pentode, CRT, PMT. Armstrong didn't
need any fancy-pants megagrid tubes when he invented the superhet:

https://www.google.com/patents/US1342885?dq=armstrong+1920&hl=en&sa=X&ei=LlSYVfrkLNPcoATEo47YDw&ved=0CE8Q6AEwBw

Armstrong was an amazing guy--he invented the oscillator!

Also the superhet, the superregen, FM radio, and many other things.

One of my technical heroes.

Chees

Phil Hobbs



I've read about the story of Armstrong and it's very sad. I often found
myself wondering why, at some point, someone didn't urge him to cut his
losses, come to some kind of settlement with Sarnoff and RCA, and then
just walk away.

It would have been far better for his health in the long run. What good
is pride if the end result is a 12 story swan dive off a hotel balcony?

A lot of creative inventor types are bipolar and compulsive and a
little schitzoid. Not survival traits. Business types and sociopaths
and evil VCs often take advantage of them.

 

[Tim Williams]

Like I said, you can use a pair, if you don't mind the count of two.
They're cheap enough to throw in anywhere, so who cares?

I don't think they made a heptode, at least in the CKxxxx series for
example. I see some battery op types (1AE5, 2G21/22), but those might be
incongrouous with your system (or an alternate direction to go in, for the
whole thing, if you buy a case of filament-type pentodes instead).

Or you can "cheat" the submini theme by tossing in a regular "miniature"
socket and use a 6BE6 or whatever. I don't see that as a sacrifice; it'd be
like using a SOT-223 LDO (instead of a DFN or CSP variety) in a circuit
that's mostly TSSOP and QFN. It's not like it's terrifically different, and
if it works, it works. I've got a 6AL5 in my submini SW radio, since I
don't have any submini diodes handy.

If you're going to give in and throw SS at it, you might as well do the
whole thing in SS, in which case I'd recommend making/using SBMs like the
CA3028, or DBMs like the MC1496, etc. Compare with my volume mixer:
http://seventransistorlabs.com/Theremin/Images/Mixer1.png

BTW, all of your timbre (tone quality) comes from the oscillators and mixer:
their distortion/purity, locking behavior (defines how rough the low notes
are), and distortion/IMD in the product.

So if you're looking for a "particular sound", that's where you need to do
it. SS vs. toob will have some play there.

You can always distort a sine wave more, so you can instead make as clean an
audio output as possible, then distort it with add-ons. You can literally
use guitar pedals (and I would recommend it), with one caveat: try to get a
fixed amplitude audio signal, then distort it, then volume-control it. So
you'll be putting the pedal on a "patch" loop on the unit, not the output.
Putting the final (variable amplitude) output into a guitar pedal (or some
circuit like that) may have undesirable results (attack/decay, change in
timbre in the process), depending on the circuit. But that might also be a
good thing, so see what works for you (when you get there).

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"bitrex" <bitrex@de.lete.earthlink.net> wrote in message
news:55984260$0$17453$4c5ecfc7@frugalusenet.com...

So as a hobby project I'm thinking of building a tiny little theremin
using some of those Soviet subminiature pentodes that are really cheap on
eBay. Unfortunately, it seems that the most complicated part of the
project would be the frequency mixers, as I don't think they really made
subminiature heptodes and a "proper" mixer would probably require two
tubes.

I'm thinking about going solid state for the mixers in the first iteration
at least...since the RF is so low I could probably just use a dual opamp
with sufficient GBW. There was this article but of course all the links
to the schematics are broken:

http://electronicdesign.com/analog/make-frequency-mixer-op-amps

Can anyone suggest an opamp mixer topology for the low 100s of kHz?

 

[bitrex]

On 7/5/2015 3:18 PM, piglet wrote:

On 05/07/2015 19:59, bitrex wrote:

The LM13700 is a very useful chip and works at low supply voltages as
well. It's unfortunate they don't make better OTAs that would be
suitable for much higher frequency applications.

You might find the LM13700 especially useful in your Theremin project
for the volume control circuitry. That VCA stage can be a problem area
in Theremin design. I find the old RCA design which modulates tube
filament power to vary the volume delightfully zany!

piglet

Which design was that? I can't imagine modulating filament power at low
audio frequencies could be very great for it...

 

[Spehro Pefhany]

On Sat, 04 Jul 2015 15:36:16 -0700, the renowned John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

I'm just finishing this one

http://www.amazon.com/Much-ADO-about-Almost-Nothing/dp/0615139957/ref=sr_1_1?ie=UTF8&qid=1436049038&sr=8-1&keywords=much+ado+almost+nothing

This is his too.. I've not seen it.

http://www.amazon.com/Circumstantial-Evidence-John-Penter/dp/B002JSC26G



--
Best regards,
Spehro Pefhany
Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

 

[Phil Hobbs]

On 7/5/2015 6:26 PM, whit3rd wrote:

On Sunday, July 5, 2015 at 11:59:27 AM UTC-7, bitrex wrote:

There's a very simple circuit for a four-quadrant multiplier in the
LM13700 application notes. I'm not sure if it will work at a few
hundred kHz, however.

Two-quadrant multiplication with an OTA is more than enough, and it'll work
fine at megahertz frequencies, because you don't care about
power gain. Feed your low-level signal into the diff inputs,
and modulate the bias current with the LO.

OTAs are just an input diff pair and four current mirrors: one to turn
the bias current input (I_ABC in the LM13700 datasheet) into the tail
current for the diff pair, and three more to take the diff pair's
collector currents and mirror them from the supply rails. (There's a
hard-to-avoid asymmetry, because the diff pair collectors always sink
current, so the + current source only needs one mirror, whereas the -
current source needs two in cascade.)

The LM13700 is a very useful chip and works at low supply voltages as
well. It's unfortunate they don't make better OTAs that would be
suitable for much higher frequency applications.

I still have a couple of dozen VTC VA713 and VA2713s from the early
'90s. They're a lot like the LM13700 except a hundred or so times
faster. Long gone, of course.

They do: TI's OPA861, for example. They just have such opaque
datasheets and descriptions, that it's hard to recognize the
face under the greasepaint and red nose.

http://www.ti.com/lit/ds/sbos338g/sbos338g.pdf

The OPA861 isn't an OTA, despite the name--it's intended as an improved
BJT. It lacks differential inputs, for a start. TI rented the name for
a completely different device, drat them.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

Digikey shows 6000-odd of the MC1496 in stock. http://www.digikey.com/product-detail/en/MC1496DR2G/MC1496DR2GOSCT-ND/1139685?WT.z_cid=ref_octopart_dkc_buynow&site=us

Cheers

Phil Hobbs

 

[piglet]

On 05/07/2015 22:58, Phil Hobbs wrote:

Which design was that? I can't imagine modulating filament power at low
audio frequencies could be very great for it...

No kidding. Running a tube in the cathode-emission-limited condition is for a good time, not a long time. (Rectifiers are an exception.)

Cheers

Phil Hobbs

No cathode. Directly heated, the thermal TC of a cathode would respond
far too slowly to volume hand movement.

piglet

 

[piglet]

On 05/07/2015 22:45, bitrex wrote:

Which design was that? I can't imagine modulating filament power at low
audio frequencies could be very great for it...

The filament runs at the volume oscillator frequency (ca 400khz in the
original 1929 RCA) via tuned circuit like a slope detector so that
varying the volume oscillator frequency by hand movement varies filament
emission and varies current drive to one of the AF stages. Its 90 year
old VCA. Zany.

Schematics and descriptions here:

http://www.pavekmuseum.org/theremin/theop.html

http://www.theremin.us/RCA/rca_theremin.html

piglet