dongguan-x: XHDATA D-808: Non documented or overlooked behaviors

The XHDATA D-808 FM Stereo/LW/MW/SW-SSB AIR RDS Synthesized Receiver is supplied with a succinct Operation Guide that may not cover all the details of the radio operation behavior.

 

There are at least three versions of this D-808 model

• The original XHDATA D-808 released back in 2018, in grey color cabinet and display with white color backlight.

• In 2021 two variants were released and still in the market as of May-2022. The only advertised modifications were the Display with yellow color backlight and two choices of cabinet color:
  • The XHDATA D-808 in grey color cabinet.
  • The  SIHUADON D-808 in black color.

• PCB assembly versions
  • The model uses two PCB assemblies
    • XHDATA D-808 MCU/Display/KB PCB
      • Only one version known: KV02 2017-08-29
    • XHDATA D-808 Receiver PCB
      • Two versions are known:
        • MV02 2017-08-29
        • MV04 2018-12-24 
      • No information was found for the changes made. ​

 

Here are some of the behaviors detected by several owners and testers since this model was released:​

• Selectable RF bandwidths

  • The Operation Guide is misleading. Both old and new versions includes:

    • Seven (7) AM values: 6, 4, 3, 2.5, 2, 1.8, and 1 kHz

    • Six (6) SSB values: 4, 3, 2.2, 1.2, 1, and 0.5 kHz

 

• RF manual tuning steps

  • The Operation Guide omits this information.

    • FM: Fast mode 100 kHz; Slow mode/Fine Tuning 10 kHz

    • AM LW: Fast mode 3 kHz; Slow mode/Fine tuning 1 kHz

    • AM MW: Fast mode 9/10 kHz; Slow mode/Fine Tuning 1 kHz

    • AM SW: Fast mode 5 kHz; Slow mode/Fine Tuning 1 kHz

    • AM SSB: Fast mode 5 kHz, Slow mode 1 kHz; Fine Tuning +/- 990 Hz in 10 Hz steps

    • Air Band: Fast mode 25 kHz; Slow mode/Fine Tuning 1 kHz

• RF Memory Presets

  • It features 500 Memory Presets, 100 per RF band.

  • Each RF band 's 100 presets are divided into 10 Pages with 10 Memory numbers each. This arrangement allows to use the 10 numeric digits in the keypad to uniquely address each Memory Preset, by first selecting the Page number (0..9) and then selecting the Memory number (0..9).

  • Select the Page number using the PAGE key followed by a number 0 to 9. Select the Memory number by pressing a number 0 to 9.

  • To store a RF station, press and hold a number 0 to 9 for at least 2 seconds.

  • There is no way to scan through the stored RF stations, other than selecting the Page number (if needed)  followed by the Memory number..

• ATS 

  • Allegedly, the Auto Tuning and Store feature seems to only find the stronger RF signals, probably because the firmware was set to use a too high threshold detection level, and there is no user setup to change it. The same behavior happens for the Manual Scan feature, by presssing and holding the Up or Down keys for 2 seconds.

    • The above reasoning is supported by the fact that this model's sensitivity is on pair with similar radios of other brands, because doing a manual tuning or entering the RF frequency directly will acquire the RF signal without issues, while the ATS and the Auto Scan features fail to capture that RF signal.

    • In my 2021 unit, I can not confirm this behavior seen in the older original model of 2018. The radio is able to find signals with just 30dBu and 4dB S/N ratio. 30dBu may look like a high level signal but the radio is by the window inside the house using the telescopic antenna and the noise floor here is around 20 to 25 dBu.

  • The ATS frequencies are always stored starting in Page 0 Memory number 0 for the selected RF band, overwriting the existing values.

• Soft Muting

  • Doing manual tuning across RF bands results in muting/chuffing sounds when changing each RF step. This seems to be a design characteristic of DSP based radios. In this model the volume drop can be reduced by seleccting the Slow tuning mode.

• Battery Backup

  • It seems that there is no battery backup, as removing the battery will result in loss of clock time. Some owners report that several other setting are lost as well, but there is no consensus here. 

  • In my 2021 unit, this behavior is present, but the Memory Presets were not lost:

    • After removing the battery my unit keeps running the clock and showing the time for around 1 minute before switching off the display. A 2200uF (C98) capacitor is used to hold the MCU running for enough time to switch batteries when required.

    • After inserting the battery, the clock was reset at 12:00, as well as other settigs like the BEEP OFF, Sleep time ON, LW enabled, or RDS info mode settings. However, the Memory Presets were preserved.

• Charge Button

  • Despite being mentioned in the Operations Guide, there is no key for that. The D-808 was designed with automatic charging and management of the installed battery.

  • A DW01A (IC2) & a 82015A MOSFET (IC1) do overcharge protection, while a TP4056 (U2) charges the Li-Ion battery at 400mA until it reaches 4.2V.

• Battery indicator and consumption

  • Even when inserting a fully charged Li-Ion battery, the radio only shows 3 bars (corresponding to at least 4.1 Volt reading ) for a few minutes of usage. After that it will show 2 bars (around 3.7 Volt reading) for many hours of usage. Eventually it will show just 1 bar (low 3.4 Volt reading). 

    • My 2021 unit came with 3.81V in the battery (still a XHDATA 2000mAh device) and the battery level indicator had 2 bars.

    • After around 4 hours charging, the battery level indicator movement stopped and showed 3 static bars. The battery had then 4.15V.

    • The battery was charged using the radio integrated battery management and supplied microUSB cable conected to a 5V/2A smartphone power adapter.

    • After more around 12 hours of usage (with backlight off), the battery level indicator alternate between 3 and 2 bars. My guesss is that XHDATA have done corrections to the firmware on the new 2021 model, or else have modified the charging setup as the Receiver PCB waschanged from MV02 to MV04.

    • My unit came to a steady 2 bars when the battery voltage (under load) was at 3.95V. 

    • My unit start showing just 1 bar when the battery voltage was at 3.64V.

  • The current consumption values taken by different testers agree on 130uA when in Power OFF state, around 12mA for the LCD white backlight when lit, around 40mA when Powered ON with low volume and 120mA at maximum volume. Therefore the supplied 2000mAh battery would last for around 50 hours of continuous usage at low volume levels, or around 16 hours at maximum volume.

    • Some owners have reported a fast battery depletion even when the radio is powered off, but this seems to be a fault developed with usage or a manufacturing defect.

    • My 2021 unit consumes 140uA when powered off, just 9mA for the display yellow backlight (less than the 12mA of the bright white light as used on the 2018 model), and around 35mA on FM with moderate volume level at night time.

• Sensitivity loss

  • Some report loss of sensitivity after changing betwween RF bands.This seems to be a firmware bug, as it can be fixed by doing a power off/power on sequence, or remove and insert the battery, or doing a Reset (use a clip to insert in the Reset hole).

• Poor sensitivity on some RF bands

  • LW band seems to show poor sensitivity on DSP radios like this model category, due to a poor design of the RF analog passive components that favors the MW band.

    • Confirmed. Litz wire speccification used in the single coil around the magnetic ferrite rod antenna is optimized for MW frequency spectrum. An additional LW coil with adequate Litz wire would be required, as seen in the good old radios of yesterday.

  • SW band below 3 MHz have been repeatedly reported to show lower sensitivity, but above that it competes with other similar class radios. This seems to be a design issue in the RF analog input stage, not a DSP issue.

    • Confirmed empirically, by listening to some radio stations at night. This radio is kind of silent below 2.5MHz.

    • The testers/modders at radioscanner website have published some measurements showing a strong sensitivity decrease in the lower sub-band (1715 - 2999 kHz) of the RF Pre-Selector. Readings at 20dB S/N:

      • 1.4uV @1740kHz; 1.3uV @2MHz

      • 0.5uV @3MHz; 0.45uV @4MHz

      • 0.5uV @9MHz

      • 0.7uV @20MHz; 0.5uV @29.99MHz

  • Air Band alleged poor sensitivity - Not confirmed on my 2021 version unit.

    • ​I am currently located in the Northeast of Portugal, at an altitude of about 1000 meters, and I can receive radio transmission signals from the various passenger planes that fly over the area, using the telescopic antenna and being indoors at the window.

    • The squelch also works adequately in my opinion. Set it level 1 and it will not mute the audio due to the noise floor inside house (around 12dBu), but at level 2 it would mute it; then the radio is able to unmute the audio for signals as low as 25dBu. 

      • Some have reported that the squelch is not fast enough to unmute resulting in loss of the initial conversation. I can confirm this to be true, but it loses just a small fraction of a second, not really an issue.  

• Audio stage Background noise floor

  • Some owners and testers reports about a permanent background noise, audible when the volume knob is at minimum, but it becomes hidden when increasing the volume to 5 or 10% of the rotation knob.

  • This audio stage background noise is present no matter what RF band and settings are selected. It is present to the speaker or earphones having the analog volume potentiometer at the minimum position.

  • This audio stage noise is more audible thru the speaker than on earphones, what seems to make no sense. But when looking to the circuit we see the reason. The earphones are driven thru a series RC network with 100uF and 100 Ohm. Depending on the earphones impedance, the noise will be stronger for higher impedance units. However the common earphones would present 32 Ohm, a low value when compared with the series 100 Ohm resistor.

  • Initially I thought that this behavior was related to a defective component, but now that I have my unit (new version 2021), I am not so sure. As the volume is a pure analog potentiometer, the noise must be generated inside the CS4863 (LM4863 equivalent) audio power amplifier IC.

  • After analysing the LM4863 datasheet (the CS4863 datasheet is in Chinese but looks identical in content) and looking into the D-808 circuit design, I believe I found the root cause for the background noise.

    • ​As the audio output from the SiLabs Si4735-D60 DSP is around 80mVrms, a audio pre-amplifier should have been included in the D-808 design to rise the audio signal to around 2Vrms in order to be able to run the CS4863 audio power amp at unity gain where it shows the lowest and inaudible background noise.

    • The design choice used on the D-808 was to save costs on the audio pre-amp and simply configured the CS4863's OpAmp1A and OpAmp2A  with a very high bridged mode differential gain of 77 (!)

      • ​Calculation: AVD = 2 × (Rf / Ri)  where D-808 use ​Rf = 150K and Ri = 3.9K

      • A typical AVD would be between 1 and 5, maybe a mximum of 10, to contain the IC noise floor under inaudible levels. But doing so would result in very low volume levels because of the missing audio pre-amplifier stage.

• Allegedd quality control or design issues

  Verified owners have reported several recurrent issues:

  • Pop sounds when switching between RF bands. Also on MW band, crack sounds when selecting memory presets to fetch stored stations (strangely this only happens on MW band).

    • My 2021 unit present these transitory noises. I wonder if the muting system is not being enabled by the MCU firmware. Bug or hardware design fault?

    • The LM486/CS4863 audio power amp design can be the culprit after all.

      • The datasheet documents "Click and pop" noises related to the IC turn-on circuitry at the SHUTDOWN pin. The document goes deep into explain this behavior and gives solutions for adequate setup to reduce these noises.

      • At this time I do not have information on how the MCU is trying to kill these "pop and click/brrr" noises.

        • If the MCU firmware is shuting down the CS4863 to avoid these nasty noises during band switching and fetching memory preset stations, as it should, then probably would be possible to fix it by following the Texas Instruments LM4863 datasheet.

        • On the other hand, the AVD bridged mode differential gain configured on the D-808 is too high, generating lots of noise, and that may also be increasing these shudown/turn-on noises as well.

  • Earphone Jack issues - Loss of audio sound either on Speaker or on earphones.

  • Power supply issues - Power ON key fails to turn the radio ON, or else it power Off by itself, even with a fully charged battery, several causes for this, one being the use of inappropriate batteries (physically incompatible with the battery holder terminals). 

  • Also broken MicroUSB conector have been reported.

  • Telescopic antenna - Last section (with tip) detached from the rest. Also, the antenna mounting seems to be fragile, requiring great care when rotating it, as it can break the plastic support.

Comments and corrections are welcome.

 

The 2021 XHDATA D-808 version.

 

July-2023 UPDATE:

In late 2022 a new version was quietly released, despite the only visible difference to the previous versions being the use of a USB-C port instead of the older Micro-USB type. Otherwise the advertised features and performance remain the same.

The MCU/Display board is now version: XHDATA D-808 KV10 2022-11-28

XHDATA D-808 Receiver board  has no visible reference codes on the upper side; it would be required to remove the board to check underneath. 

Looking to the publicly available pictures, extensive changes were made in the circuit diagram and PCB layout of the Receiver board.

Gone is the RF front-End with the 4-Sub-Bands pre-scaler using four LC trimmers for proper alignment and better spurius strong signals rejection, being replaced by common LCR fixed values networks using SMD components.

The audio components section was rearranged and the DC-DC converter to supply voltage to the Pre-Scaler varicap is not visible anymore, so it seems it was simply eliminated.

The DC-DC converter was not properly shielded in the previous versions, being a source of RF noise. Other better radio brands, even ones using the same basic chassis, use shielded DC-DC converters.

The DSP section is now properly shielded with magnetic metal cans soldered to the PCB.

Other changes may exist, but I was not able to detect them as I do not have this newer version.

So the manufacturer have cut production costs by eliminating costly LC trimmers and active components of the Pre-Scaler and DC-DC converter, reduced the alignment requirements at the assembly line, and potentially have reduced some internal noise coming from the MCU/Display into the DSP, at the cost of offering a radio with lesser ability to reject strong spurius RF signals.

 

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