DIODE RING MIXERS
Double Balanced Mixer {DBM}
A passive mixer where F1 is introduced at one port whilst F2 is introduced at the second port.
These frequencies are then mixed with the result that {F1 + F2}
and {F2 - F1} appear at the output port.
We select the one we are looking for and discard the other.
The important feature of the mixer, because of the balance of the hot carrier diodes
and transmission line transformers means even harmonics are cancelled at both the R.F. input (port 1) and L.O. frequencies (port 2) and provides isolation among ports.
In this application the diodes operate as high speed switches where the incoming F1 signal is "chopped" at the rate of the F2 signal.
It can be mathematically proven that "sum" and "difference" signals result

ADVANTAGES OF DOUBLE BALANCED MIXERS

Reasonable conversion loss on signal F1. This is usually about 7dB.
You will see "sum" and "difference" signals only.
No F1 or F2 signals should appear at the output.
Similarly because of this balance the F1 and F2 signals do not appear at each others port.
Consumes no power except for the losses incurred in conversion.
Broadband in nature and therefore suited to multiband designs.
High intercept points.

DISADVANTAGES OF DOUBLE BALANCED MIXERS

A relatively high noise figure, about the same as the conversion loss.
Fairly high local oscillator drive requirements.
Typical values are +7~13dBm
Some mixers may require even higher levels.
Each port is highly sensitive to reactive terminations.
A pure 50 ohms would be ideal. Proper termination is absolutely critical,
particularly the "sum" and "difference" or I.F. port.
High quality, high speed diodes which will take the necessary saturating current and large reverse voltages across the non conducting diodes are used.
Diodes need to be well "matched".
Somewhat prone to harmonic mixing. Diodes make ideal harmonic generators.

OVERCOMING DISADVANTAGES OF DOUBLE BALANCED MIXERS

1. Relative high noise figure
in some applications you can use a singly balanced mixer where a noise figure of about 5dB might be expected.
Noise figure is rarely an issue at H.F. freqs.
At V.H.F. and beyond it is likely you would employ a low noise r.f. preamp anyway.

2. High local oscillator drive
The local oscillator should be well "buffered" and filtered.
A low pass filter followed with a 6dB attenuator is suggested.

3. Reactive ports
The most critical port is the I.F. port which MUST see 50 ohms at all frequencies.
The simple solution here is to place a diplexer at this port together with a post mixer amplifier.
The R.F. input port should have some filtering ahead of it leading to the 50 ohm mixer load.
Buy a commercially manufactured mixer. Minicircuits ADE-1 would be an example.

The ADE series of mixers, from MiniCircuits,
are a very economical double balanced mixer(DBM)
The ADE-1 is a LEVEL 7 mixer, meaning it is designed (and the specifications based upon)
an LO drive of +7dBm, which is 0.5V rms into 50 ohms and 5 milliwatt dissipation.
Some designers shy away from using these passive double balanced mixers
(DBM) because of the fairly high LO drive required.
However, they can be used successfully with lower drive levels.
The efffect will be the conversion loss and port-to-port isolation
(defined below) will get worse, but since we are using these devices
at the very bottom of their 1GHz range, the effects are minimal.
LEVEL 7 mixers can be used by driving the LO port
with a single transistor crystal oscillator, delivering about
+3dBm which is 0.32V rms into 50 ohms (2mW ) with good results.
LOW POWER ADE mixers, the ADE-1L and ADEX-10L.
are also worth consideration
Type Nr RF/LO FREQ IF FREQ LO DRIVE CONVERSION
LOSS
ISOLATION
ADE-1 0.5-500 MHz DC-500MHz +7dBm 5.0dB 66dB
+4dBm 5.2dB 55dB
ADE-1L 2-500 MHz DC-500MHz +7dBm 4.9dB 69dB
+3dBm 5.3dB 57dB
ADEX-10L 10-1000 MHz DC-800MHz +7dBm 7.3dB 83dB
+4dBm 7.3dB 58dB
SBL-1 (ref) 1-500 MHz DC-500MHz +7dBm 5.6dB 62dB
+4dBm 5.9dB 62dB


Definitions:
RF/LO FREQ: Range of the RF and LO frequencies for stated specs.
Mixers can be operated below lowest frequency cited
with some degradation of conversion loss and port-port isolation values.

IF FREQ: Output range of the IF frequencies.
All DBM's will allow an IF down to DC, as they are also used as phase detectors
(a dc voltage representing a very small difference between the LO and RF ports).
This allows them to be used as a product detector as well (audio frequencies).

LO DRIVE: Recommended LO drive level, and the level for which the other specifications are based.
Note that on the LEVEL 7 mixers, the slight degradation of specs
when operated at a lower LO drive.
On the otherhand, if you drive a LEVEL 3 mixer with =+7dBm LO,
the conversion loss and isolation improves,
so it won't hurt to use a lower drive device in a circuit
originally designed for LEVEL 7's.

CONVERSION LOSS is the loss in dB from the RF input signal to the IF output.
As a general rule of thumb, most DBM's have a conversion loss in the 5-7dB range.
This is easily made up by a post-mixer IF amplifier.
Note that the conversion loss is listed for both Level 7 and Level 4 mixers,
to show the slight difference in driving a Level 7 mixer with only +3 or +4dB of drive.
Figures are based on the data sheet values for 5-10MHz.
In the case of the Level 3 or 4 mixers, driving them at Level 7 will improve the loss a bit.

ISOLATION lists the port-to-port isolation of the device.

RF-LO: The attenuation of power from one port to the other.
Values shown are for the intended LO drive for that device.
For example, with +7dBm LO drive on the ADE-1, there is -66dB of isolation to the RF port,
or -59dBm of LO power.
However, in a direct- conversion receiver, this -59dBm of LO power
appearing on the RF port equates to a fairly strong signal still!
In a superhet receiver, it is normally far enough removed
from the RF frequency to be of little concern.
LO-IF: the attenuation of the LO port power to the IF port,
and vice verse. On the ADE-1 with +7dBm LO drive,
and -55dB isolation, -48dBm LO power could appear on the IF port.
For a receive or transmit mixer, not a concern,
but if used as a product detector or direct conversion receiver,
this could present a problem with reciprocal mixing products.



In the "ideal mixer," there would be absolutely no RF or LO appearing on the IF output, or absolutely no LO power appearing on the RF port.
In the real world, however, it's not quite this clean.
Thus, the isolation specs are important for identifying the "real world" leakage in the mixer.
HOWEVER ... no matter how poor the isolation specs may be on a DBM,
it's still tens of dB better than an active mixer such as the NE612!!!
Now notice the specs again for the new ADEX-10L.
It is designed for only +4dBm of LO drive, which is 355mVrms or about 1Vpp on a scope.
It can be driven with a single transistor crystal oscillator,
feeding the LO port via a 47 ohm resistor and .1uF cap
for an approximate 50 ohm match.
This scheme delivers about 1Vpp to the ADE-1L LO port,
or at the design spec.
This prevents having to add a buffer driver transistor.
The ADE-1 and ADE-1L provide a nice clean output at 10.1 MHz,
but there is a little 1.9MHz amplitude modulation on it ... due to the VFO
(LO) port power leaking into both the LO (12MHz) and IF (10.1MHz) ports.
This 1.9MHz modulation shows up at the antenna terminals,
following the class C PA on a spectrum analyzer, which is unwanted power.
It is knocked way down on the class E version of the PA,
since the 10.1MHz from the mixer is amplified and turned into a square wave by a high speed comparator,
effectively removing the 1.9MHz element.
This port-to-port leakage is such that on an o-scope,
you can see 12MHz on the 1.9MHz VFO output,
and 1.9MHz right at the 12MHz crystal oscillator.
What is needed, is a bit more port-to-port isolation in the mixer.
(As stated above, this is nothing compared to the poor port-to-port isolation offered by the NE612).
But look at the specs of the new ADEX-10L.
It has an LO-RF port isolation of 80dB!
That's 15dB or so better than most passive mixers.
Even though they are LEVEL 7 mixers, driving them from a single transistor oscillator
at only .8-1Vpp causes little degraded performance
in the HF part of their frequency range.
And while a DBM will LOSE power in the mixing process (conversion loss),
the much higher port-to-port isolation does much to prevent chirping,
pulling of the VFO and other effects experienced by the active mixers.
For new designs, I would consider one of the Level 3 or 4 mixers
now being offered, such as the ADE-1L and ADEX-10L listed above.
For more information on MiniCircuit DBM mixers,
visit the website for data sheets and application notes.