About the RMDR
Investigation and innovative qualities for clear transmit signals, receive audio, high RMDR (Reciprocal Mixing Dynamic Range), and the OVF indication, to fully understand their true performance. This third article will focus on RMDR.
The meaning of the title may be difficult to believe that quickly. However, recently more focus has been put on RMDR than was previously put on IP3. To get more DX entities, to pick up a weak signal in a heavy pile up, to win a contest, a high power and a high-gain antenna are an advantage. But when you cannot hear a signal, there is no chance to make a QSO, even if you are running 1kW or more. An example is when a very strong station’s signal overpowers a weak rare DX station’s signal. Such QSO chances are unconsciously being missed. So, it is the RMDR we inspected this time that has become the problem.
What is RMDR?
RMDR stands for Reciprocal Mixing Dynamic Range and is a valuable measurement used to characterize the receiver’s performance. A high RMDR means that the transceiver can receive a target signal easily (Quoted from the Icom brochure), even in the presence of interference, and the receiver can reject adjacent interference.
The graph shows the RMDR values for the IC-7851, IC-7610, IC-7300, IC-7600, A company’s mid-class transceiver, and B company’s flagship transceiver.
At a 2 kHz offset, the RMDR value of the IC-7610 reaches 110 dB, which is close to that of one of the highest class transceivers, the IC-7851. Even the IC-7300, which is a good cost performance transceiver, has a value of just over 100 dB. That is extraordinary RMDR, and quite a surprise.
Refer to the graph shown above and you can see that the IC-7851 has an ideal RMDR characteristic. It is amazing that even the IC-7610, a mid-class transceiver, and the IC-7300, an entry class transceiver, achieve the RMDR that approaches the higher class transceiver’s characteristics.
When the reasons are investigated, we find that an extremely low phase noise clock oscillator is a key factor. The extreme low phase noise clock was adopted as a master clock in the IC-7300 and the IC-7610, and a very low noise device is also incorporated in the clock’s power supply circuit. As we have introduced the direct sampling method in the receive circuit, a high RMDR has been achieved, even in the mid-class and entry-class transceivers.
Examination of RMDR
You will be able to realize that the RMDR of the IC-7300 and the IC-7610 is excellent. The examples are just values, and it is hard to understand the difference in actual operation. So, the IC-7600, IC- 7610 and the IC-7300 were put through a bench test, as shown below.
・A standard signal generator (SSG) to produce a strong station signal (Interference signal), and a transmitter to produce a target DX station signal are used. Apply both signals, through a combiner, to each transceiver as Equipment Under Test (EUT).
・Set the transmitter output power for no S-meter indication on the EUT S-meter, but at a level where you can hear the CW signal.
・Set the SSG level for S9 + 40dB on the S-meter of the EUT.
Before talking about the RMDR comparison, let’s look at the differences of the receiver performance under different levels of phase noise.
<Video data explanations>
1. When you click the video data icon to start the video, at first you can hear the CW sound as a weak DX station signal.
2. Next, you can suddenly observe the interference signal (S9 + 40dB) on the screen. The signal overlaps the same frequency that the DX station is operating on. In this case, you cannot hear the DX station signal at all.
3. Next, the interference signal gradually moves above the frequency that the DX station is operating on. The test shows how far the interference signal moves above the frequency that the DX station is operating on, and when we can again hear the DX station’s signal.
RMDR comparison between the previous IC-7600 model and the IC-7300
Set the IC-7600 and IC-7300 as the EUT on 14.200 MHz/CW. Apply a CW keying signal to each EUT from the transmitter, and apply the strong station signal (the interference signal) to the EUT from the SSG, above the receive frequency, in 100 Hz steps.
The situation in the video is this…..While receiving the DX station signals (CW keying signals) with the IC-7600, suddenly the strong station calls the DX station on the same frequency. The signal is extremely strong. The DX station signal is overlapped by the strong station’s signal, and you cannot receive the DX station’s signal at all.
Then, the strong station moves 100 Hz above and calls the DX station again. You still cannot receive the DX station. When the strong station again moves 200 Hz above the frequency, you can barely hear the DX station’s signal if you listen carefully, but the signal is still not strong enough for a QSO.
However, when you change the EUT (IC-7600) to the IC-7300, you can now solidly copy the DX station’s signal when the strong station moves up only 200 Hz. And, when the strong station moves to 300 Hz above, you can clearly copy the DX signal.
RMDR comparison between the previous IC-7600 model and the IC-7610 (1)
You can clearly hear and copy the DX station’s signal, even if the strong local station appears just 200 Hz above on the IC-7610, as you did with the IC-7300. In addition, you can hear the signal much clearer than with the IC-7600 because there is almost no LO phase noise in the IC-7610 and IC-7300.
RMDR comparison between the previous IC-7600 model and the IC-7610 (2)
We made another test with an interference signal 10 dB stronger, and compared the performance between them.
As you can see in the YouTube video above, the IC-7600 cannot receive the DX station’s signal, even if the strong station appears 1 kHz higher. However, the IC-7610 can receive it completely, even if the strong station appears only 200 Hz above it.
Technology evolves with time, but the performance evolution from the IC-7600 to the IC-7300, or the IC-7610 is drastic. It is amazing. When the IC-7600 was sold, its performance and functions were held in high esteem around the world, and was a long-time best-selling radio. Now we understand that the performance of the IC-7300 and the IC-7610 are much better than that of the IC-7600. We found that a signal can easily be picked out by a transceiver that has great RMDR characteristics. It can also be said that a higher RMDR number gives you a better chance of making a QSO.
The following is heard a lot from the users of not only the IC-7851, but also users of the IC-7610 and IC-7300, too.
“A DX station was operating in split mode, and the signal was quite weak. When a local station called the DX station 1 to 2 kHz up, the weak DX station’s signal was often blocked by the strong local station’s signal when I used an old previous model. Now, I can hear the DX signal.”
“When blocked by an extremely strong signal of a local station, I could not hear a call back from the DX station because of the strong station signal. However, now I can hear the call back to my station.”
“The chance of hearing a signal of a rare entity, which is close to a strong station signal, has increased.”
These are examples of what a difference a high RMDR can make.