Sony XDR-F1HD

The XDR-F1HD is Sony's first home HD Radio tuner. It receives all AM and FM HD Radio modes, including multicasts, as well as analog AM and FM.

At 7⅛″ × 6⅜″ × 2⅜″, the tuner is much smaller than standard stereo components. It weighs less than 2½ lbs. The FM antenna input is a 75Ω F-connector, while spring-loaded AM terminals accept wires. The tuner comes with an AM loop and an FM dipole. RCA jacks provide analog audio. The captive two-wire line cord has a polarized plug.

The front panel has an LCD and a power button. On top of the cabinet near the front are ten control buttons. The rear panel has a recessed reset button. The tuner includes an infrared remote control. It requires two AAA batteries, not supplied.

The cabinet is made of rigid plastic. Perforations cover much of the bottom and vent the upper rear panel. Louvered vents span the top surface at the rear. The tuner gets quite warm. Rated power consumption is 13 watts.

Under the Hood

Five screws retain the top cover, which easily comes off. Inside is a power board, main motherboard, display board, and pushbutton board. The power board delivers unregulated 5.2 V and 10.5 V. Its five rectifiers are bypassed, which suppresses interference on AM. Directly above the rectifiers are two green electrolytics rated at 105° C. In parallel on the motherboard below are two blue 85° electrolytics with about half the capacitance. The power transformer was still too hot to touch ten minutes after removing the cover. The silkscreen identifies both pins of the transformer's internal fusible link. Should the link ever fail due to a temporary fault, you can install an external fuse. The motherboard has surface-mount parts on the underside, including six voltage regulators. The system controller is on the display board. All boards are well marked, with components, signals, voltages, and test points identified. No adjustments are visible.

Mounted vertically on the motherboard is a seventh voltage regulator on a heatsink and two shielded modules. The tuner module is next to the transformer. The other module is the HD Radio processor. Two snap-on shields are soldered to the tuner module at its upper corners. Unsolder the right-hand shield and inside you'll find the NXP TEF6730/SAF7730 chipset. The HD module contains an SAF3550.

Inside the top cover is a curious bare copper PC board attached both with screws and adhesive. It is electrically isolated and not likely a shield. The board is marked SHIKIRI PWB. Shikiri means partition, division, boundary, or compartment. It also is the ritual where sumo wrestlers down on their fists glare fiercely at each other before a match begins. Surely one of these definitions provides insight.

Features

The XDR-F1HD tunes in 100-kHz steps on FM and 10-kHz steps on AM with the TUNE + and TUNE − buttons. SCAN scans the band in 200-kHz steps on FM and 10-kHz steps on AM (up only). The tuner pauses for three seconds at each signal found. Pressing just about any button halts the scan. HD SCAN excludes analog signals.

The tuner provides 20 presets on FM and 20 on AM. PRESET + and PRESET − sequentially tune them. The remote control provides random preset access.

The TUNE buttons, also labeled SELECT, select an HD Radio multicast channel. A thoughtful feature is the small LCD arrow that tells whether another channel exists. No need to risk blowing HD lock checking for HD-3.

A signal-strength indicator shows zero to three bars. Successive bars appear at RF signal levels of 19, 29, and 38 dBf.

The tuner has a clock that resets if the unit is left unplugged for more than five minutes. The presets behave the same way.

MENU lets you set the clock, LCD contrast, and LCD brightness (the lowest setting is still too bright in a bedroom). DISPLAY switches between a screen that shows frequency and another that emphasizes time. For RDS or HD Radio you can select a third screen that fills with transmit text, which only scrolls across a small window in the other screens. The tuner does not display the callsign encoded in the RDS PI field. That would greatly benefit DXing, as it does in the Sangean HDT-1X.

Compared to the HDT-1X, the XDR-F1HD does not display carrier-to-noise ratio, HD Radio transmission mode, HD Radio station ID, firmware version, or the audio spectrum. It does not provide forced mono, forced analog, split-audio mode, direct frequency entry, or digital output. It does not have a stereo indicator and it does not receive C-QUAM AM stereo. The Sony has a sleep timer the Sangean lacks, and it exhibits far fewer anomalies, quirks, and bugs.

Operating instructions are here. A service manual, which includes PCB but not module schematics, is $8.06 + $3.50 shipping at 1-800-488-SONY. Order part number 988797201.

Analog FM

The XDR-F1HD uses advanced digital signal processing algorithms to dramatically improve reception of FM signals corrupted by noise and interference.

Threshold extension suppresses the impulse noise ordinary detectors generate for weak FM signals. The special character of this noise can make reception unpleasant even when the nonimpulsive background noise is adequately low. The technique, which mitigates detection-vector phase reversal due to additive noise, has been used in high-performance satellite and point-to-point terrestrial FM systems. It is not normally found in consumer equipment.

Adaptive noise reduction forms a filter that tracks the stereo audio spectrum. The filter suppresses noise between and beyond spectral peaks without restricting audio bandwidth. It also suppresses co-channel interference and multipath distortion, factors that can limit reception quality for stronger stereo signals. Adaptive noise reduction has been used to eliminate tape hiss when remastering older analog recordings. This may be its first appearance in consumer audio equipment.

Adaptive digital IF filtering eliminates adjacent-channel interference in nearly all cases. The filter has extremely steep skirts and automatically adjusts its bandwidth for interference. It is much more effective than conventional ceramic filters and does not have their unit-to-unit variation that necessitates filter selection or tuned compensation for optimal performance. A symmetrical finite impulse response eliminates the group delay error that causes audio distortion in ceramic filters.

To fully benefit from the processing, the XDR-F1HD does not switch or abruptly blend to mono at low RF signal levels. The Sony can deliver a clean stereo signal with wide channel separation at far weaker signal levels than can any other tuner. Only a Carver tuner with Asymmetrical Charge Coupled Detector (and perhaps a Pioneer F-93) is remotely comparable.

Sound quality for slightly impaired to deeply compromised signals is strikingly better than that from conventional tuners. The performance of the Sony XDR-F1HD on stereo FM is spectacular and unprecedented.

This shows the audio spectrum for a 1-kHz right-channel tone at an RF level of 22 dBf. The horizontal scale is 200 Hz/div and vertical is 10 dB/div. The shape of the adaptive filter is evident from the noise hump.

This is the left-channel response. The noise has a pronounced narrowband sound quality. The tone frequency determines the position and bandwidth of the noise hump. Multiple tones cause multiple humps, and a complex signal spectrum acquires a filter adapted to its specific shape. Exploration with a sine wave reveals that the noise reduction algorithm uses a discrete frequency-domain filter bank rather than a continuous time-domain technique. I counted 17 filter passbands.

The adaptive noise reduction affects only the noisy L−R stereo subchannel. The algorithm attenuates each filter bank output according to critical-band auditory masking criteria. U.S. patent 7,110,549 describes the method, while 7,292,694 refines it. The quadrature L−R signal provides a robust background-noise estimator.

The noise reduction algorithm is remarkably well behaved. I've noticed just two artifacts. One may occur for monophonic sound on very noisy stereo signals, signals that otherwise would be unlistenable. For some of these signals, low-level L−R noise may emerge from time to time from a quiet background. I noticed the second artifact when a station lost one stereo channel. With my balance control set all the way to the unmodulated channel and the volume turned up, I was able to hear high-frequency sounds reminiscent of aliasing at very low levels. Acoustic image displacement occurs only for lower-level sounds in noisy signals. For strong signals I cannot distinguish the sound of the XDR-F1HD from that of a conventional tuner, except that the Sony occasionally suppresses a slight sibilant splash due to a touch of multipath.

In addition to the adaptive noise reduction on L−R, the XDR-F1HD gradually applies a high-cut noise filter to L and R when the RF level drops below 26.5 dBf. The filter progressively affects lower frequencies as the signal level falls, as shown above.

While the high-cut filter is still dropping, a soft-muting function begins to attenuate the entire spectrum. With no signal the residual noise is down 30 dB.

For the following measurements I used IEEE 185-1975, updated as described here. I used the test equipment listed here. The figures are for an unmodified, factory-aligned tuner.

50-dB quieting sensitivity, mono    13.5 dBf
50-dB quieting sensitivity, stereo  13.5 dBf
THD, 1 kHz, mono                    0.07%
THD, 1 kHz, stereo                  0.055%
Stereo separation, 1 kHz            54 dB
S/N, 65 dBf, mono                   70 dB
S/N, 65 dBf, stereo                 68 dB
Capture ratio, 30 dB                1.1 dB
Capture ratio, 50 dB                8.4 dB
Capture ratio, stereo, 30 dB        1.3 dB
Capture ratio, stereo, 50 dB        13.5 dB
AM suppression ratio                80 dB
Adjacent-channel selectivity        82 dB     (noise limited)
RF intermod                         89 dBf    (97.7 + 98.5 -> 96.9)
RF spur                             96 dBf    (96.24 -> 96.9)
RF image                            87.5 dBf  (118.3 -> 96.9)
RF AGC threshold                    87 dBf
RF mismatch loss                    0.7–2.1 dB
Modulation acceptance, 1 kHz        200%
Modulation acceptance, 20 Hz        76%
Minimum stereo pilot injection      3.5%
Deemphasis error, mono              +0.0/-2.2 dB
Deemphasis error, stereo            +0.0/-1.3 dB
Bass response, -3 dB                10.5 Hz
Output level                        0.7 V
Output impedance                    2.2kΩ
Audio latency                       27 ms
Power-on delay                      6 sec
Power consumption, operating        11 W
Power consumption, standby          2 W

The XDR-F1HD has the best sensitivity figures I've ever measured. In mono this is due partly to the threshold extension and partly to the high-cut filter, not to a low noise figure. The input signal drives a transient suppressor, PIN diode attenuator, single tuned circuit, and finally the TEF6730 mixer, whose rated noise figure is 3 dB typical, 4.5 dB maximum. There is no RF amplifier.

The stereo sensitivity figure is not a typo. Noise is 50 dB down for an unmodulated 13.5-dBf stereo signal. This figure is at least 20 dB better than that of conventional tuners. Channel separation is still 26 dB at this signal level. Background noise near the tone frequency does rise with modulation, as shown in the previous images, and a single tone does not entirely mask it. A standard stereo sensitivity test really isn't appropriate for a tuner with adaptive noise reduction. Still, stereo reception of weak broadcast signals is an order of magnitude better than with a conventional tuner.

Treble loss, not background noise, will limit weak-signal audio quality for some listeners. The high-cut filter causes 10 kHz to drop 1 dB at a signal level of 25 dBf and 3 dB at 22 dBf. Level variation due to soft muting during signal fades will be the limiting factor for others. 1 kHz falls 1 dB at 20 dBf and 3 dB at 18 dBf (see the curve below). An RF preamp will lower these signal levels by an amount approximately equal to its gain.

The S/N figures are 5–6 dB worse than those of the HDT-1X. Neither tuner comes close to the best conventional designs with S/N in the high 80s or low 90s. Nevertheless, I have been unable to hear any tuner noise, even on quiet program material. In fact, what's striking is how utterly quiet are stereo signals that on conventional tuners have unlistenable levels of background noise or grunge. Evidently 68 dB of ultimate S/N is enough at the volume levels I use.

Capture ratio is how far below an unmodulated 65-dBf monophonic signal a 100%-modulated monophonic signal must be to obtain the specified quieting. With both signals in stereo, the XDR-F1HD can suppress a co-channel signal 19 dB stronger than one the HDT-1X can suppress. At my location this profoundly improves reception quality for many signals, setting the Sony apart from any other tuner.

The astronomical selectivity figure is real. The XDR-F1HD is noticeably more selective than the HDT-1X, sometimes retrieving listenable signals that are buried beneath adjacent-channel splatter or completely inaudible in the Sangean. The Sony's selectivity is more than 30 dB better than that of the best conventional tuner I've ever tested, a Kenwood L-07TII modified to cascade one 150-kHz and two 110-kHz Murata ceramic filters in narrow-IF mode.

RF intermod, RF spur, and RF image are the 50-dB quieting levels for a third-order intermodulation product, an untuned signal, and a mixer image. The Sony's RF intermod figure is 5 dB better than that of the HDT-1X. RF spur is 9 dB better. Although the XDR-F1HD has a single tuned circuit in the RF signal path while the HDT-1X has two, the Sony's image rejection (RF image minus 50-dB quieting) is 15 dB better due to its image-cancelling mixer. I made these RF measurements in a way that sidesteps tuner and signal generator phase noise.

RF AGC threshold is the signal level where the front-end PIN diode attenuator begins to operate. Untuned signals in the RF passband above this level may cause a weak tuned signal to become noisier. Although the tuner uses keyed AGC to minimize the problem, the XDR-F1HD may be more susceptible to strong-signal desensing than other tuners because its RF passband is quite broad.

Modulation acceptance is the modulation level for 1% THD. The XDR-F1HD figure at 1 kHz may seem like overkill since FCC rules limit stations to 100% modulation. But for years one local NPR station deviated 140%. Another station just across the border in Mexico sometimes exceeds 250%. The XDR-F1HD had no problem with the first signal and cleanly demodulates the second most of the time. But see the graph below for low-frequency modulation acceptance, which is much lower. Modulation acceptance for the Sangean HDT-1X is 150% at all frequencies. The highest I've seen is 270% for a Pioneer F-90.

Compare the XDR-F1HD latency of 27 ms to 118 ms for the HDT-1X. The Sony's audio delay is short enough to let you simultaneously play the same station in another room from a conventional low-latency tuner.

This is the left-channel deemphasis error for an IEEE load (100kΩ || 1000 pF). I don't know why the curves differ or why they are wavy. To flatten the droop, see Treble Correction.

This is the bass response normalized to 100 Hz. I used 44% modulation to avoid the problem described next.

I observed these glitches for a 100%-modulated, 20-Hz, monophonic test signal. I suspect the tuner may have narrowed its IF filter bandwidth too much, perhaps due to a deviation detector with inadequate low-frequency response. At 50% modulation the glitches did not occur for any modulation frequency.

This shows the maximum glitch-free modulation level at low frequencies. I have heard no glitches on broadcast signals.

This compares 1-kHz stereo separation for the XDR-F1HD and HDT-1X as a function of signal level. The Sangean protects the listener from noise by rapidly blending the channels when S/N drops below 56 dB. The Sony's adaptive noise reduction takes care of the problem for a further 20+ dB drop in signal level without degrading channel separation.

This compares monophonic quieting curves. S/N is the ratio of audio output levels for 100% 1-kHz modulation present and absent. The sudden change in slope of the HDT-1X curve just below 17 dBf marks its FM threshold. Here additive noise is large enough to begin to cause IF-signal phase reversal, which the FM detector renders as high-amplitude spikes. Spike occurrence greatly increases as the signal level drops. The XDR-F1HD S/N curve shows a gradual change in slope with no threshold. During A/B tests with the Sangean, very weak signals were markedly more readable on the Sony. (The XDR-F1HD curve is for a second tuner with 50-dB monophonic quieting of 14.0 dBf, not the 13.5 dBf of the first tuner measured earlier.)

This compares audio presentation strategies at low signal levels. The HDT-1X presents a nearly constant signal level while the XDR-F1HD maintains a nearly constant noise level. The resulting XDR-F1HD soft muting is very effective at suppressing noise bursts during brief signal fades. It also seems just right for monitoring a clear channel for a band opening. Set the background noise near the threshold of audibility and a readable signal will pop up to alert you. The downside is that the loudness of a fading signal in the soft-muting region will vary. An RF preamp can mitigate this annoyance.

The SAF7730 provides 16 IF filters of various bandwidths. It automatically selects one based on interference and modulation level. The 64-tap FIR filters use 16-bit coefficients. This graph shows the response of four of the stock filters, from the narrowest to the widest. Also shown is the response of a 180-kHz Murata ceramic, which the XDR-F1HD uses as an IF roofing filter.

With an I²C bus controller it is possible to alter the SAF7730 filter coefficients. This single-sided plot compares FIR filter #5 with an optimized filter.

This shows RF return loss from 88 to 108 MHz with the XDR-F1HD tuned to 98 MHz. The dip is 1 MHz high. Its frequency does not monotonically increase with tuned frequency, backstepping at 90.2, 92.8, 95.4, 98.0, 101.4, 104.7, and 108.0 MHz. The backstep is as large as 800 kHz and never drops below the tuned frequency. All this suggests a misaligned piecewise-linear approximation. The backstep frequencies differ among tuner samples, which suggests that Sony uses automatic alignment during manufacture.

This is tuned-frequency return loss and the resulting mismatch loss. Swamping the loss by adding a 10-dB gain RF preamp increased sensitivity 2.5 dB at 96.9 MHz. See Alignment for another way to overcome the mismatch loss.

This is the distortion spectrum for 1-kHz, single-channel, stereo modulation deviated 75 kHz with 9% pilot.

For a strong unmodulated test signal, I could hear a faint whine in the background noise with the volume turned way up. This image shows the audio spectrum to 20 kHz using a 30-Hz analysis-filter bandwidth and postdetection smoothing. I think I was hearing the pip just above 3 kHz. Close examination reveals it to be at 3125 Hz and 78 dB below 100%, 1-kHz modulation. I have yet to hear the whine in a broadcast signal. (The thicket between 13 and 14 kHz was absent in a second tuner.)

At 63″, the FM dipole supplied with the XDR-F1HD is rather long. Mounted in the clear about 6′ above the floor, resonance occurred below the FM band at 85 MHz. Reducing the effective length with a piece of string as shown optimizes the response for 88–92 MHz. Tie the string so that the horizontal wires are 3″ above the mounting hole. This configuration reduces mismatch loss 0.3 dB at 88 MHz, 1.4 dB at 90 MHz, and 2.0 dB at 92 MHz. To cover 88–108 MHz, use a folded dipole instead. Tilt the antenna to maximize signal strength.

Analog AM

The AM antenna is a 4″ × 5″ rotatable loop with eight single-layer turns. Its inductance is 21.3 µH with a Q of 83 at 1000 kHz. The loop exhibits two nulls in opposite directions at all frequencies, handling it does not increase signal strength, neither AM antenna terminal is marked as ground, and each terminal has a resistance of 1Ω to ground. All this suggests that the tuner provides a balanced, differential antenna input circuit, which can reduce local noise pickup. The usual unbalanced, single-ended input causes a loop and its feedline to respond to the electric-field component of the electromagnetic wave as well as to the magnetic-field component the loop is designed for. The electric component is much stronger than the magnetic for many local noise sources.

As an experiment, I turned on a noisy lamp dimmer at the far end of the house. Across the AM band the noise was much lower for the differential connection than when I reconnected one feed wire to tuner ground. The schematic shows a single-ended input circuit, but a red jumper wire on the motherboard and another inside the tuner module implement the differential input.

For two loops I examined, the insulation on the wire ends was cut but not stripped. I wonder if this explains the occasional report of no AM reception. The stripped wire seemed rather fragile when inserted into the spring-loaded antenna terminals, which bent and separated the tiny strands. I tinned them to add strength.

The operating instructions warn not to place the loop near the tuner as it may pick up noise. I noticed some low-level interference at the low end of the band, but it was easy to minimize by repositioning and reorienting the loop.

I connected a signal generator terminated in 50Ω between one antenna terminal and the shell of an RCA audio jack. Sensitivity was the same for both terminals, confirming the input balance. At 1500 kHz, −93 dBm yielded 30 dB S/N for a 90%-modulated, 1-kHz tone. 400-Hz THD at 30% modulation was 0.08%. It reached 0.14% only at −10 dBm, and the audio stayed clean to 0 dBm. This is a much higher RF level than the Sangean HDT-1X tolerated without noticeable audio distortion. Both the Sony and Sangean have an AM RF amplifier.

This compares the frequency response of the XDR-F1HD and HDT-1X with the NRSC-1-A AM deemphasis standard. Ideally the red and blue curves should coincide with the green curve. I wondered whether the Sony's bass roll-off might be intended to comply with the old tonal balance rule for AM radios, which states that the product of the low- and high-frequency limits should be about 500,000. The bass response actually extends twice as far as the rule allows.

This is the audio output spectrum for a test signal consisting of preemphasized tones spaced 250 Hz. The horizontal scale is 1 kHz/div and vertical is 10 dB/div. The plot confirms my listening impression that the XDR-F1HD severely rolls off the AM high-end. The response is down 24 dB at 4 kHz.

I tried flattening the treble response with an octave equalizer, but I wasn't able to make a worthwhile improvement. It should be possible to equalize the response to 4 kHz with a custom circuit, perhaps one with a high-Q treble pole and a low-Q bass pole. Although the unequalized bandwidth is less than that of a good telephone circuit and the treble roll-off attenuates sibilants and certain vowel formants, I had no trouble understanding speech.

The benefit of the bandwidth limiting, which is done at IF, is immunity to adjacent-channel interference. It is easy to receive a weak skywave signal next to a strong local. Only the occasional sibilant splat from a wideband adjacent may intrude. Unlike the HDT-1X, the XDR-F1HD has neither variable IF bandwidth nor synchronous detection. Illustrating the latter was distortion on some skywave signals during selective fades.

The XDR-F1HD has an automatic noise blanker. This shows it blanking lamp dimmer pulses in sinewave modulation (2 ms/div). Blanked audio sounds somewhat rough, with 1.2-ms waveform segments erased every 8.3 ms (for 120-Hz pulses). Still, with strong pulse interference the audio sounds much better blanked than not.

HD Radio

Except for one occasion when the XDR-F1HD locked to an FM signal and the HDT-1X did not, the tuners performed the same on HD Radio on both AM and FM. I noticed just two operational differences. First, the Sony will flash its HD indicator when tuned 300 kHz above or below an FM HD signal. Second, when manually tuning a weak station running service mode MP3, occasionally only HD-3 appears. Once locked, the analog signal and the other digital channels become tunable. This is the one firmware bug I've found in the XDR-F1HD.

An aligned XDR-F1HD reliably locked to a 29-dBf FM HD Radio signal with −20-dBc digital sidebands. An aligned HDT-1X did the same.

For an FM station transmitting silence on HD, I measured the residual noise as 84 dB below the RMS level of a 1-kHz sine wave with 1.5-V peak amplitude, a typical HD Radio waveform level. I used a 200–15,000 Hz bandpass filter per IEEE 185-1975. For the same reference level, an HDT-1X yielded 85 dB S/N during HD mute.

Alignment

Following Peter Körner, I unsoldered the right-hand shield from the tuner module. The FM antenna coil is near the upper-right corner. Just as Peter found for two tuners, rotating the slug a quarter turn counterclockwise increased the audio level 2 dB for a modulated signal in the soft-muting region. I peaked the coil at 96.8 MHz, near the center of a varactor tuning segment, and replaced the shield. After the module warmed up but without the cabinet, 50-dB quieting sensitivity at 96.8 MHz had improved 1.4 dB to a remarkable 12.6 dBf. It also was 12.6 dBf at 95.3 and 95.4 MHz, endpoints of adjacent tuning segments. Equal sensitivity at adjacent endpoints should be optimal. (Immediately after replacing the shield, I measured 12.0, 12.2, and 12.8 dBf. I was just lucky that the sensitivities equalized after the module warmed up. A shield adjustment hole would let you align the tracking at a higher temperature, one closer to that with the cabinet in place.)

Near the lower center is the IF coil. In one tuner Peter was able to increase the weak-signal audio level 1 dB by peaking it. The coil was already peaked in his second tuner and in mine. This adjustment does not affect stereo distortion.

In the lower-right corner is the AM input transformer. It is not varactor tuned and I did not adjust it. The red jumper wire next to the transformer, along with another on the motherboard, are factory changes that provide differential RF input.

Disabling the Backlight

I modified my XDR-F1HD to turn off the LCD backlight in standby. This requires cutting a trace on the controller board and installing a transistor and resistor.

The transistor switches the backlight ground return. The controller power-on signal drives the base through the resistor. Total backlight current is 40 mA at the brightest setting. I used a high-gain Zetex ZTX1051A and a 10kΩ resistor. Any NPN transistor will work given enough base drive. The power-on signal minus VBE is about 2.4 V. If you use a transistor with a saturated current gain of 50, for example, use a base drive of 40 ⁄ 50 = 0.8 mA and a resistance of 2.4 ⁄ 0.0008 = 3kΩ. This value should work for a 2N2222A. Limit the drive current to 4.5 mA in all cases.

Remove three screws from the pushbutton board and two from the controller board. Pull the boards and lay them to the right of the tuner. Cut the horizontal ground trace under the T in RESET just to the right of R479 in the lower-left corner of the board. Solder the transistor emitter to the ground jumper pad to the right of MUTE. Solder the collector to the lower right lead of Q404. Solder the resistor between the base and the left lead of R468. The body of the resistor must clear the controller chip.

Extending Memory Retention

Although the XDR-F1HD has two EEPROMs for nonvolatile storage, neither retains the tuned frequency or station presets. This information remains valid in controller RAM only for a few minutes after power is lost. This is long enough to ride through a brief power interruption, but too short to let you switch off the tuner overnight with another audio component. (You'd still have to press the power button to turn it back on since the tuner always powers up in standby.)

C926, a 4700-µF motherboard electrolytic, provides VCC backup for the controller. Paralleling a common 0.047-F memory backup capacitor extended my tuner's backup time to one hour. This lets it accomodate longer power outages. A 1.5-F capacitor that costs a few dollars should extend the time to about 29 hours. Any capacitor should be rated for at least 3.5 V. When installing a value greater than 0.047 F, you may need to add a series resistance to limit the charging current. 47Ω ¼W should suffice. C926 should be paralleled as shown, not replaced. Proper controller shutdown requires a fairly stiff supply voltage.

For two battery backup schemes with much longer retention time, see Other Reviews.

Extending Audio Headroom

The audio amplifiers in my XDR-F1HD clipped on digital signals with abnormally high audio levels. This image shows one such clip. The lower waveform is output at the RCA jack. The upper waveform is audio from the tuner module, inverted and scaled to match the lower trace. The baselines are at the top of the image and two divisions from the bottom; the waveform segments are entirely negative. The horizontal scale is 5 ms/div and vertical is 1 V/div. One division from the left the lower waveform clips at −1.8 V for several ms. The upper waveform goes to −2.2 V.

I never observed clipping for analog signals or for the great majority of digital signals with reasonable levels. Averaged over several seconds, the RMS level of the digital signal in the image was 3.7 dB higher than the station's analog signal. The peak digital amplitude was 2.5 times as great as the peak analog amplitude. This digital signal was hot.

In my tuner the 8.5 V that powers the audio amplifiers was somewhat low at 8.27 V. To provide a bit more headroom, I added 30kΩ across R904 to raise the voltage at tuner module pin 5 to 8.5 V. A single resistor from collector to base biases the audio gain stages. This simple method is beta-dependent. The high beta of the transistors installed in my tuner yielded 4.5 V at the collectors, well below the nominal 5.6 V and too low to prevent clipping on extreme negative peaks. Adding 12kΩ across collector loads R104 and R204 raised the voltage to 4.9 V and reduced the signal amplitude. Both increase headroom. The output level dropped 1.5 dB to 0.6 V.

In addition to clipping, the bipolar amplifiers in my tuner degraded second-harmonic distortion 10 dB. To replace them, see Treble Correction.

Clipping is so shallow and infrequent that I believe it is entirely inaudible. But I wanted to make accurate peak/RMS measurements with the tuner. And while the distortion degradation is rather alarming, the stock distortion level is below what I can perceive for an isolated sinewave, a very sensitive test.

Forcing Monophonic Reception

You may want to force monophonic reception when receiving very weak signals. Mono eliminates any L−R noise that may slip past the adaptive noise reduction. If the tuner drives equipment with no mono function, you can wire an outboard SPST switch across the audio output terminals. An emitter follower drives each output through a 2.2kΩ resistor. Interconnecting the outputs will not stress any component. A simple alternative for mono DXing is to parallel the outputs with a Y-cable. Always force monophonic reception when making a single-channel recording.

When driving my XDR-F1HD with a monophonic signal, the unloaded output amplitudes differ by 0.6%. Assuming 1%, and allowing for the 5% tolerance of the 2.2kΩ output resistors, interconnecting the outputs should suppress L−R at least 24 dB.

Forcing Analog Reception

Occasionally you may wish you could force analog reception. You may not care for the transmit processing a station uses for its digital signal, the HD-1 bit rate for a multicast signal may be low enough to cause coding artifacts, the tuner may switch back and forth between analog and digital on a marginal signal, or a distant co-channel HD Radio signal may co-opt the analog signal you're trying to receive. And then there is AM HD, which always sounds funny to me. This modification will let you keep the tuner in analog mode.

This photo from Peter Körner shows the underside of the motherboard. The tiny yellow mark indicates where to cut a trace. The control signal originates at pin 18 of the HD module at the top and passes through ferrite bead FB6 to pin 23 of the tuner module on the left. When high, the signal tells the tuner module to switch to the HD Radio bitstream. Cut the trace, wire an SPST switch across the cut, and mount the switch anywhere convenient. No pulldown resistor is necessary. The display will indicate HD reception for both switch positions, but you'll be listening to analog audio when the switch is open.

One way to add a forced-analog indicator is to mount a DPDT push-on/push-off switch with internal LED so that it protrudes through a hole in the top cover. Wire the LED so that it illuminates when forcing analog reception. The unregulated 5.2 V, identified on the motherboard, can provide LED current without increasing the power dissipation of any voltage regulator. Another option is to repurpose a seldom-used button, such as HD SCAN, to force analog. This requires a 3.3-V flip-flop and gate. Mount a bright red LED on the controller board at the edge of the LCD to alter the display color in forced-analog mode.

Treble Correction

Each D/A drives a ferrite bead and shunt capacitor within the tuner module. The module drives two-section RC lowpass filters on the motherboard. The filters remove low-level ultrasonic noise that extends to a few MHz, but they also cause the frequency response to droop within the audio passband. Disabling the filters by removing their capacitors flattens the response as shown above and causes no interference to AM or FM reception. The preceding photo identifies the filter capacitors as C11, C12, C21, and C22.

This shows the noise spectrum to 5 MHz at the unloaded tuner output with the RC-filter capacitors removed. The vertical scale is 10 dB/div and the resolution bandwidth is 10 kHz. The spike barely visible at the left edge is DC. The noise rises at a couple hundred kHz and then falls above 1 MHz at 12 dB/octave. A 100%-modulated 1-kHz tone is 60 dB above the −40 line. The wideband noise level measured −38.5 dB with no load and −53 dB with an IEEE load. The ultrasonic noise is due to the noise shaping the oversampled D/A uses to maximize S/N within the audio passband.

David Rich points out that a DSP A/V receiver with oversampled A/D but marginal antialias filters conceivably might fold some of the ultrasonic noise back into the audio passband. The noise also is visible on a scope, which offends the eye if not the ear. To eliminate it altogether, install active lowpass filters. The response ripple of the Chebyshev-inspired filter shown above compensates for the residual 0.5-dB roll-off. The red curve is with no tuner load, blue is 47kΩ || 470 pF, and green is 100kΩ || 1000 pF (IEEE). The circuit model includes the load and the components that cause the roll-off, but the schematic does not. Use 2%-tolerance capacitors or selected parts. 5% resistors are good enough, but 1% parts are cheap. Use a wideband, dual op-amp with rail-to-rail input/output rated for VCC = 8.5 V, such as a TLV2372.

The active filters have 6 dB gain so that they can replace the bipolar audio amplifiers as well as the RC filters. This eliminates clipping on digital signals with abnormally high levels and reduces harmonic distortion for all signals.

To install the filters, connect tuner module pin 4 to filter ground. Connect the positive terminal of C302 to the op-amp power pin. Bypass the power pin to the ground pin with a 0.1-µF ceramic. Cut jumper wires JW14 and JW15 and connect tuner module pins 26 and 27 to the filter inputs. With the PCB oriented as in the preceding photo, input to C102 is on the right and input to C202 is on top. Isolate each capacitor input by cutting a trace, or by remounting the capacitor vertically on its output pad, or by removing R106, R206, Q103, and Q203. Connect the op-amp outputs to the capacitor inputs, with the filter connected to pin 26 driving C102.

Disabling or replacing the RC filters also benefits HD Radio audio. AM HD response extends to 15 kHz. FM may go to 20 kHz, where the RC filters are down an additional dB.

Temperature

With a thermistor attached to C908 on the power board and the top cover in place, I measured 63° C (145° F) after one hour at 25° C ambient. I converted the 10.5-V supply from half- to full-wave rectification, intending to reduce transformer losses and lower the RMS ripple current in C908. After conversion the temperature rose more slowly, but it was only 1° F cooler after an hour.

Ken Wetzel added extra feet to enlarge the space under his tuner and improve air flow.

A tiny fan mounted inside the tuner should greatly lower its temperature. Some 12-V fans become inaudible when operated at a lower voltage. Try the unregulated 5.2 V, switching the fan and a back-biased diode with a transistor turned on by the 8.5 V.

Reducing temperature will prolong electrolytic capacitor life. The expected lifetime doubles for each drop of 10° C. A fan might well lower the temperature 20° C, quadrupling capacitor life. Both the tuner module and HD Radio module contain surface-mount electrolytics that would be difficult to replace.

Although it runs hot, the design seems safe. The transformer primary has an internal fusible link, and each secondary winding drives an external fuse. If a hot transformer or filter capacitor shorts, a fuse will blow. Each of the voltage regulators has thermal overload and short-circuit protection. If a regulator overheats or its load shorts, it will shut down.

Other Devices

Although I haven't verified their performance, the XDR-S3HD table radio and XT-100HD car adapter use the same DSP modules and algorithms as the XDR-F1HD.

Other Reviews

The Audio Critic reviews the XDR-F1HD here. CNET reviews it here. Ira Wilner gives a broadcast engineer's perspective here, as does Dan Houg here. DXers David Pierce and Mike Bugaj offer reviews here and here. Julian Hardstone describes several modifications here. Marty Duling augments the cabinet venting here. Hillel Hachlili adds external cooling and battery backup here.