EMRFD Message Archive 1367

Message Date From Subject
1367 2008-01-24 13:49:09 Rick Diode Ring Mixers
Hi All,

I enjoyed the reading the discussion on diode ring mixers, which I have used in about half
of my RF designs for the last 3 decades. A common question I hear is:

"Why did you use a diode ring when everyone knows a _______ is better?"

The answer to that is fairly simple: I use a diode ring when I don't want the mixer to be
one of the unknowns in the experiment. I know how they work, I know that I can buy one
or build a good one with microwave Schottky diodes, ordinary switching diodes, or even
blue LEDs, and most importantly, I've spent a good many years doing experiments, making
measurements, and thinking about the surrounding circuitry.

With a diode ring, I know what to expect in terms of performance, so when I get strange
results, I know to look for a wiring error first. We have all had the experience that Bob
nicely illustrated--questioning everything from semiconductor physics to the validity of
arcane papers to try and explain something that can be fixed by simply removing the short
circuit hidden under the transformer.

"But what about your other projects, the ones that didn't use a diode ring? Did you need
higher performance?"

The answer to that question is much more complicated. It is often possible to pick one
spec and design a better mixer, but that will sacrifice performance, ease of use, expense,
bandwidth and/or convenience in other areas. When the mixer is the focus of my
experiments, I usually employ passive shunt FET designs, but that involves a completely
different design, including all of the surrounding passive components, for every
application. I use passive FET mixers when I plan to devote at least a few months to the
design and experiments with the mixer and all of the surrounding passive circuitry.

An example of a project where I didn't use a diode ring is the simple direct conversion
receiver on page 8.5 in EMRFD. That circuit was optimized to minimize parts count, but
there are a few subtle details that may escape a casual glance. The mixer is not just two
diodes and a trifilar transformer: it includes the secondary winding on the double-tuned
RF input circuit. That is necessary both as a DC path, and as a short circuit at audio. The
47nF capacitor from the point between the two diodes to ground is also part of the mixer,
and provides an AC ground at both the RF and LO frequencies. I have found that I can
obtain good performance from a two-diode single-balanced-mixer if I also design all of
the surrounding circuitry at the same time. If I want to experiment with different RF input
circuits, I wouldn't use a mixer that includes a link on a toroid in the RF stage or requires a
bypass capacitor to ground from the IF port.

A topic that needs some good experimental work is diode ring mixers for low noise HF
direct conversion receivers. I would encourage someone in the group to do some careful
experimental work and publish the results on 1/f noise with different diodes. I usually use
commercial packaged diode ring mixers in my projects because they are small, relatively
inexpensive and easy to use. However, in the early work I published in QST in August
1992, I observed higher than expected noise. I have occasionally encountered packaged
mixers with unacceptably high noise in direct conversion applications. The few examples
of mixers I've built using hand-wound toroids and silicon switching diodes were all very
quiet, and I've listened to Roy Lewallen's original Optimized QRP Transceiver and found it
to be very quiet as well.

I've reviewed the scientific and technical literature on the subject and found some of it to
be useful, but clearly the topic of noise in HF mixers for direct conversion applications
could benefit from further experiments. The appropriate test fixture should probably be a
complete receiver and not just the mixer, as misconceptions about noise figure,
conversion loss, and how to deal with the opposite sideband noise contribution are
common in the literature. Then substitution measurement may be made.

Best Regards,

Rick kk7b
1400 2008-02-04 13:17:05 Larry Jones Re: Diode Ring Mixers

Put me down for 10 of the ADE-1 mixers.

Larry <>< N5OSG
Garland Tx.

Looking for last minute shopping deals? Find them fast with Yahoo! Search.

[Non-text portions of this message have been removed]
1402 2008-02-04 14:25:08 Pat Bunn Re: Diode Ring Mixers
I just received the order acceptance from Mini Circuits.

Total cost for 100 mixers including shipping is $206. Adding 3 % to cover Paypal or Credit card fees brings the price to $2.12

The mixers will be $2.12 each plus a cost of $5.00 which includes first class shipping and insurance for up to $50 plus a delivery confirmation. If you just want to pay first class shipping and not worry about insurance or delivery confirmation then $2 covers postage but do so at your risk.

I have ordered 100 mixers and want to keep at least 15.

You can Paypal me for the total to pbunn@matrixei.com or go to my website and make a lump sum credit card payment using the form on the front page.

I will ship the mixers as soon as they arrive.

Pat Bunn

----- Original Message -----
From: Larry Jones
Sent: Monday, February 04, 2008 10:29 AM
Subject: [emrfd] Re: Diode Ring Mixers


Put me down for 10 of the ADE-1 mixers.

Larry <>< N5OSG
Garland Tx.

Looking for last minute shopping deals? Find them fast with Yahoo! Search.

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]
1405 2008-02-04 19:40:36 Robert Cerreto Re: Diode Ring Mixers

Can you save me ten? I cannot use paypal from my location on a business trip. I will be home in four days. Then I can send you a paypal.

73 es tnks,


Never miss a thing. Make Yahoo your homepage.

[Non-text portions of this message have been removed]
1415 2008-02-05 22:26:11 Pat Bunn Re: Diode Ring Mixers
Yes - If I run out, I may order some more as there seems to be a lot of requests.

----- Original Message -----
1422 2008-02-07 06:55:03 Robert Cerreto Re: Diode Ring Mixers
Thanks Pat. I'll catch up with you when I get home. I really appreciate the help. It's tough to maintain the hobby on business trips!!!!.....and the hobby is more important. After all, the company store does is pay for the hobby <>.!!!!


Pat Bunn <pbunn@patbunn.com> wrote:
Yes - If I run out, I may order some more as there seems to be a lot of requests.

----- Original Message -----
1430 2008-02-16 08:20:08 Nic Hamilton Re: Diode Ring Mixers
As a result of KK7B's thought-provoking email, I have been back
through some of my old note-books. This is what I find...

Direct demodulation (the Radio Moscow effect) is one of the major
problems with Direct Conversion receivers (DC RX). The RF input is
envelope detected to dc. This is all well known.

However, direct demodulation happens on the Local Oscillator (LO)
side of the mixer as well, giving rise to a dc offset at the output
of the mixer. The dc offset is small in voltage terms, but measured
in signal terms (dBm), it is huge. As the offset is an envelope
detected representation of the LO signal, the AM noise on the LO
results in a slight noise ripple on the dc offset, and this results
in an apparently noisy mixer.

I have researched this effect using an HP 8640B sig gen with 1kHz AM
modulation at a depth of about 0.5% to an SRA-1 type mixer. This
makes the receiver howl. By introducing a dc bias to the mixer, the
howl can be minimised. Where the howl is minimised, so too is
the `mixer noise'. At frequencies below 10 MHz, this occurs when the
bias counteracts the dc offset, so the mixer output voltage is very
close to 0V. So a good way of minimising the noise is to use an op-
amp to ensure that the mixer's output is a virtual earth at
frequencies below 50/60Hz. The trouble is that this only works well
for an LO of up to 10MHz.

Viewed in RF terms, applying a dc bias to the mixer results in
cancellation of the mixer's LO leakage by introducing an anti-phase
LO leakage. Above 10MHz the mixer leakage has a significant
quadrature element, and this cannot be cancelled by simple dc bias.
Although it is possible to introduce I/Q leakage, the required
circuitry gets unrealistically complex.

So why the 10MHz limit? At first sight, there is nothing too strange
going on in the mixer at this frequency. However, the DC RX is
sensitive to odd harmonic frequencies of the LO. Although the RC RX
harmonic sensitivity falls off as the LO harmonic number increases,
the mixer balance gets rapidly worse at the higher frequencies, so
the contribution to the dc offset and LO noise due to the harmonics
does not fall off as quickly as one might expect.

I have not investigated the effects of the dc mixer bias on the RF
direct demodulation. I would expect it to be improved.

If anyone wants to investigate `pre-packaged' mixers in DC RXs
further, I speculate that there is improvement to be had by using
phase detector type mixers designed or selected for low dc offset.

This technical stuff is all very interesting, but is it useful?
Frankly, on the low HF bands I have not found receiver noise to be
much of a problem. A half-wave dipole gives a huge output
1486 2008-03-01 10:02:13 Paul - VE1DY Re: Diode Ring Mixers
Hi Pat,

Just to let you know, the ADE-1's arrived safe and sound.

Thank you kindly.

Paul - ve1dy-

> Yes - If I run out, I may order some more as there seems to be a lot of
> requests.
> Pat