Gilbert cells are very useful in many "compromise" circuits. If you have a
fixed power budget on the very low end, Gilbert cells may be the best
choice because they make a balanced mixer stage that has substantial gain
with miniscule LO drive requirements.

Contrast that with a DBM diode mixer where they take substantial LO drive
and operate some loss.

In the no-compromise time-nuts context, there may be a good place for a
Gilbert cell. Maybe in a circuit where the Gilbert cell is getting hit with
a well-defined signal with low dynamic range that dwarfs the noise
contribution. The low LO requirement of a Gilbert cell also makes it easier
to isolate the LO from the rest of the circuitry and that can reduce
shielding needs. But it's not gonna go where you need both low noise and
high dynamic range.

Tim N3QE

On Wed, Jan 6, 2016 at 9:30 PM, Richard (Rick) Karlquist <
richard@karlquist.com> wrote:

Hi

If your intention is to run a mixer with saturated inputs …. just run
an X-OR gate. It will handle the high level signals much better than
an over-driven analog part.

Yes somebody should check out a board built that way …. I’ll
let you know when I do.

Bob

On 1/7/2016 3:11 PM, Bob Camp wrote:

If you look at the schematic of an XOR gate IC and compare it
to the schematic of, for example, an MC1496 mixer, you will
see a lot of similarity.  If the gate is of the ECL type,
it will have the addition of emitter followers, but that
it a minor detail of implementation.  I'm not sure there
is a huge difference.  ECL is a great logic family in
general (self-confessed ECL-phile here :-) but it is
probably the worst for phase noise, compared to the
saturating logic types.

Rick Karlquist N6RK

Hi

The board I have uses high speed CMOS single gate XOR’s. They have a pretty good
phase noise floor (-170’s) so they should be pretty reasonable.

Bob

Hi Bob,
In some article, I see people use a D-flipflop to sample the input
signal with reference clock. When you want implement a mixer what's the
difference between D-flipflop and XOR gate? Acorrding to my understanding,
to multiply 1bit with another, I should use an AND gate, right?
When you refer high speed CMOS XOR gate, do you mean 74LVC1G86?

Thanks

BI7LNQ

2016-01-09 6:42 GMT+08:00 Bob Camp kb8tq@n1k.org:

Hi

You can use a D flip flop to sample (down convert) a signal. You may or
may not get into metastability problems when you do.

If you treat the gate inputs as -1 and +1 rather than 0 and 1, the XOR is a
multiplier. If you put two signals into the gate and look at the output on a
spectrum analyzer, you get the expected multiplier output. The why of
the -1 and +1 stuff is something I will leave to others. It’s a bit involved.

The 74LVC is a good series to use. The NC7SZ series is also a good one.
In both cases, you will get a better noise floor at 5.5 V than at 3 V.

Bob

On Sat, 9 Jan 2016 10:19:05 +0800
Li Ang lllaaa@gmail.com wrote:

A D-Flipflop is a rather weird mixer. I have not done the calculation,
but i'm pretty sure that the output is not exactly what you'd expect
it from a normal mixer (namely having half the energy at the frequeny
difference and half at the sum).

An XOR gate on the other hand, produces a very nice spectrum, given
you input two clean square wave signals.

If you think of the signals as digital in the computational sense,
with "high" representing "1" and "low" representing "0" then yes.
But in signal theory, it's more appropriate to think of signals
as "high" representing "+1" and low representing "-1".
In the latter case, the XOR is the multiplicative element, and not
the AND gate.

Generally speaking: Faster CMOS better than slower CMOS in terms of phase noise.
(Though, I have yet to see actual measurements of this)

Single gate chips better than multi gate chips.
(no interference through the power supply of the different sub-parts)

			Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Attila,

On 01/09/2016 09:25 PM, Attila Kinali wrote:

It's not that wierd. It's a sampler, and thus it acts like a mixer as if
the signal is spikes, which is just another interpretation of the
Nyquist frequency aliasing. Meta-stability however creates an
"interesting" aspect.

Indeed.

An interesting variant of the XOR gate as being used as a mixer is when
you build a rubidium. One synthesis approach being used is to divide the
5 MHz OCXO signal with 16 to get 312,5 kHz. Then XORing it with 5 MHz
produces as one of it's mirror signals 5,3125 MHz which is then fed with
a step-up signal of 60 MHz or 90 MHz into the SDR diode in the cavity.

A third digital phase-detector is the SR flip-flop. It avoids the 180
degree phase property (really a triangle wave signal) of the XOR, but
give a 360 degree phase sawtooth. This can be helpful in certain lock-up
conditions.

The phase-frequency detector of the 4046 and the like has additional
flip-flops to remember slipped cycles and forcing the frequency to
regain that. Those provide a strong frequency lock mechanism with a
phase detector in one.

Well, you should wire the other parts into passive mode.

Cheers,
Magnus

In message 20160109212523.39180e2b7a788fe1ee2d7a3a@kinali.ch, Attila Kinali
writes:

Would paralleing multiple gates in the same chip make things
better or worse ?

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi

The gotcha is that each family gets measured as the parts come out. Thus the data
is spread out over about a 40+ year period. Most of it was taken down in log books
off of a screen ….

Bob