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Reuter RDR55D


The Reuter company is a household name among shortwave listeners and beyond  and is well known for its high-quality products. Above all, the receivers of the RDR-series were the benchmark as regards workmanship and receiving quality.   The receiver technology is digital as with all other SDRs, but signal processing is different.  Most SDRs, e.g.,  Perseus SDR or ELAD FDM-S2  operate  time-based. The receivers of the RDR series on the other hand make use of frequency-based signal processing. Information about frequency-based signal processing can be found in Dipl.-Ing.  Ralph Menn's lecture in paragraph 2.1.

Time and frequency domain
Modular design from RDR55D

Now I have the latest receiver RDR55D with the software version V500 in front of me.  There is a little story behind it.  The combination of the RDR 54, which has measure instrument qualities and its smaller brother RDR 50, which has a big and user-friendly touch screen is the result of many a customer's request.  This shows how this company respects  customers' wishes and ideas.

Like the RDR54, the RDR55D is modular-structured. By adding further modules on the  rear panel, the radio can be enlarged. It can also be made into a fully-fledged ham radio QRP rig. The case is made of milled and anodized aluminum, the tuning knob is made of high-grade steel.  The RDR55D can be expanded with various modules and thus customized according to the user's whishes.

This RDR55D has the following modules built in:

RAD18EF (preselector, pre-amplifier, attenuator, 4x16bit ADC and FM module)
RDA31B (High-End Stereo Audio-DAC with High Power headphone amplifier)
RDR35B (GPS and WLAN module)
RPS25G (high-quality power supply)
Frequency range with module RAD18EF:
1KHz-30MHz (separate antenna jack ANT1)
50MHZ-71MHz (separate antenna jack ANT2)
87,5MHZ-108MHZ (separate antenna jack ANT3
136MHz-154MHz (separate antenna jack ANT4)
Band width filters: 10Hz-20KHz, freely selectable in 40 Hz steps
Modes frequency-based: Sync-DSB-LSB-USB-EUSB-SBCW-CW
Tuning steps: freely selectable from 0.5Hz-999.999.5KHz
199 memories
Passband tuning
AGC:  rise and decay freely selectable
GPS module for frequency calibration
WLAN connection for cable-free firmware update and screenshot functions
Built-in preselection
very good large signal immunity
7" (178mm) WVGA-color display and touch screen
Ham radio upgrades  via modules are available


Except for the On/Off switch, the tuning knob made of high-quality steel is the only mechanical element for operating the radio. Practically all parameters are controlled by this knob.  The selection of the functions is done by using the touch screen, e.g., if you want to tune the band width filter, you touch the band width indicator with your finger and then select the bandwidth with the tuning knob.  Then you touch the next function or the action can be set in such a way that the active function goes back automatically to the frequency indicator. The length of time on the active function is also user settable. Except for the adjustments in the menu, all functions can be controlled in this manner.

The eye catcher, of course, is the spectrum in real time.  That means, whatever you see, and you can listen to without any time delay.  The spectrum has a max. Range of 164 KHZ and can be zoomed to 1.6 KHz.  A waterfall diagram is also possible. Basically operation of this radio is foolproof, but you have to get used to the very small touch field and if you have small fingers, you are at an advantage, but you can also use a plastic pen for the PDAs.

For frequency calibration, you have a GPS receiver at your disposal, which comes with the RDR35B module.  If you connect a standard GPS antenna, some satellites will be received after a few minutes.  With the help of these satellites, the internal oscillator will be calibrated exactly to 1 Hz. If you select the GPS function, the following data are shown:

-- Number of satellites which can be received
-- Satellite identification
-- Signal/noise ratio of the satellite signal
-- Correction of the internal oscillator
-- Geographical length and latitude of present position
-- height above sea level.
-- current time (UTC)

Via the WLAN, which is also contained in the RDR35B module, a wireless connection to a PC is possible.  At the moment, this connection is used for firmware updates and screenshots and for sending them to the PC.  Soon it will be possible to operate the RDR55D with suitable software via this interface.  The advantage of the WLAN connection is that you don't need any cables anymore.  It is a well known fact that a PC can cause heavy interferences in the long wave, medium wave and shortwave range.  Of course, the RDR35B module also has a standard USB2.0 interface.  There is also a S/PDIF interface for digital audio signals.

The RDA31B module offers a high-end 24bit headphones amplifier. The audio signal can also be sent to a stereo system via a cinch plug.  This module only processes the time-based modes AM-E, FM, FM-W, DSB/SSB-Q and DIGI.  The audio quality of FM-signals via this module is of outstanding quality.

In the RAD18E module, the signals from the antenna pass through the preselector before they are passed on to the 4x16bit ADCs. Here, the filtered analogue signals are digitalized and redirected for demodulation.  When using this module, only the antennas have to be connected to the correct jack.  


I had my first experiences with time-based operation in 2011 with the RDR54. Later, there was the RDR50. Ever since, due to the development of ADCs (Analogue Digital Converter)  and FPGAs ( Field Programmable Gate Array) big improvements of reception have become possible.  The earlier RDR models, which had already been low-noise, were further improved.  The RD55D is the receiver with the least noise that I could ever review.  If you work with this radio for a longer period of time and then use a standard receiver, you will be surprised at the amount of noise that these radios have.

Well, I compared the RDR55D with the ICOM-R9500 over some weeks. For antennas, I used the Fenu-CrossLoop/RLA4B and the NTi ML200 with the 8m loop.

The sensitivity of the RDR55D is linear from the lowest to the highest frequency. Signals on VLF, LW and MW were received best and most pleasant.  AM-signals are best received in the "Synch" mode, where the signals are automatically got rid of noise and fading. In addition, some local interferences were filtered out , e.g., PLC-interference from  the neighborhood.  On shortwave, the differences were smaller.  The IC-R9500 was able to render some signals better which was due to its complete setup. Getting rid of interferences worked better with the IC-R9500.  When listening became difficult, the RDR has its surround function, which works very well even in case of weak stations.  SSB reception worked very well, as was expected.  Because of the sharp filters of the RDR55D, the IC-R9500 was at a disadvantage on the ham radio bands. You will only notice the extreme selectivity when you slowly reduce the bandwidth.  You will already notice a reduction of 40 Hz.

Because of the frequency-based operation of the RDR55D, the audio is somewhat "tinny" But you won't notice that after some time and you get used to it. But you do notice the lack of intelligibility of weak signals.  In these instances, the IC-R95090 was mostly better, but there was more noise.

If you operate the RDR55D in a noise-free environment, you can use the time-based mode "AM-E" when listening to an AM station.  This mode simulates an envelope detector and the audio sounds as if it is coming from a radio with tubes, warm and full. Unfortunately, this mode has only a few fixed bandwidths. All other functions, e.g., band pass tuning, notch filter etc. are not available in this mode. That is really too bad and a lot of potential is not made use of. All time-based modes suffer from this lack of functions.

A real treat is the reception of FM signals. Here, the RDR55D operates time-based, i.e. in the usual way. Because of the high-quality parts of this module, reception becomes a high-end experience.  Reception in stereo and mono is possible, of course.  To even enhance this experience,  the stereo width is user selectable. RDS is included as well.  FM-DXers are well taken care of. The FM- module is designed for highest sensitivity and predestined for DX.  For difficult situations, the RDR55D has a special attenuator, which can attenuate the signals up to 48dB in 1dB steps.

Currently, the following band widths are available for FM:
--38 KHz S
--50 KHz S
--60 KHz HQ
--80 KHz HQ
--120KHz S
--120 KHz HQ
--300 KHz HQ
"S" means "sharp" > steep filtering slope
"HQ" stands for "High Quality" > round, less steep filtering slope. Sounds more pleasant, fuller.

To start with, FM reception of the RDR55D surpasses the IC-R9500 by far regarding sound, selectivity, sensitivity and functionality.

Large signal handling of the FM module is beyond criticism. Reception via a cable network was without any problems.  Here, the RDR55D gets really going and can show its FM qualities.  The audio quality with a 300 KHz HQ filter was simply superb.  Noise, which is often noticeable with such wide filters, could not be detected when using the cable network. However, signals had to be attenuated by -20dB, because the ADCs suffered from overload.  But that is really not too bad considering such high sensitivity.


Reviewing the RDR55D was not a difficult task, i.e., absolute top class receiver! Starting with the workmanship and the material of the case up to the reception quality on long-, medium- and shortwave and FM, the RDR55D was without serious flaws.   However, there were two aspects, which I did not like: The lack of intelligibility in case of frequency-based weak signals on shortwave and the unsatisfactory function of the noise blanker and the noise reduction.  These small shortcomings can probably be corrected by a correction of the software.  Except for this, the RDR55 is a dream of a receiver.  Unfortunately, this luxury receiver with the qualities of a measuring tool costs ca. 5000 in this setup.

posted at 5.11.2015


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