Reciprocal Mixing Testing
|Sep 1st 2011, 19:50|
Total Topics: 0
Total Posts: 0
Reciprocal Mixing Testing; What Is It?
I’d like to inform those who read this forum on an explanation of reciprocal mixing, a test we first reported with the Yaesu FT-450 in December, 2007 QST. Some of our member may have overlooked the reciprocal mixing figures since they haven’t stood out from the rest of the data tables you see in the monthly QST Product Review. Along with Blocking Gain Dynamic Range and Two-Tone Third Order Dynamic Range,
Reciprocal Mixing must be seriously considered while evaluating the overall performance of a receiver. In fact, it is probably the most significant figure in receiver performance! For this reason, at the earliest opportunity, ARRL will change the listing for reciprocal mixing to show "reciprocal mixing dynamic range" to ensure that less technically astute people realize that the reporting of reciprocal mixing is indeed part of a complete evaluation of receiver performance.
In one sense, one could say that a receiver is only as good as its weakest dynamic range measurement, but in reality, each type of "dynamic range" can have a different effect on the performance of a receiver, depending on how it is used. The effect of reciprocal mixing will be worse at low levels of desired signal, while it would have little effect on signal that are S9, for example.
Reciprocal mixing is noise that is generated in a super-heterodyne receiver when noise from the local oscillator mixes with strong, adjacent signals. All local oscillators have some noise on each sideband, some more than others. This sideband noise mixes with the strong adjacent off channel signal and noise is generated at the output of the mixer. This noise can degrade the sensitivity of the receiver and is most notable when just outside the IF passband. Note that the 2 kHz spacing reciprocal mixing is always the worst figure on our Product Review Data Tables.
We perform the Reciprocal Mixing Test at 14.025 MHz, using a very low noise Wenzel test oscillator with a measured output of +14 dBm. It’s low noise because the test oscillator’s sideband noise is considerable lower than the reciprocal mixing we’re measuring. The output of the oscillator is fed to a step attenuator, which is adjusted until a 3 dB increase of background noise is indicated on an audio meter. The output level at which the 3 dB increase is measure is noted.
If the receiver MDS is –133 dBm and a strong station, 2 kHz away, causes a 3 dB increase of noise at a level of -53 dBm into the receiver’s antenna jack, the reciprocal mixing figure is MDS minus (3 dB increase figure), or –133 dBm – (–53 dBm)= –80 dBm. We report as –80 dBc
In the real world example above, if your noise floor (MDS) is –133 dBm, a signal 2 kHz away at 20 dB over S9 will cause the noise in your receiver to increase by 3 dB and your MDS ability of your receiver is now –130 dBm.
A stronger signal will create more noise, but our benchmark for testing is a 3 dB increase of noise.
Very good reciprocal mixing: –120/105/–90 dBc for spacing of 20/5/2 kHz.
Not so great reciprocal mixing: –85/–65/–60 dBc.
The more manufacturers spend on developing low noise mixers, the better off we all are.
Bob Allison, WB1GCM
ARRL Test Engineer