Re: Replacement for Sony 5000F FM Stereo Tuner?

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 audio
output. The captive two-wire line cord has a polarized plug.

The front panel has an LCD and a small 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. Dan Houg measured the
power consumption as 11 watts operating and 2 watts in standby.

This compares the size of the XDR-F1HD with that of the Sangean
HDT-1X.
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 five power board rectifiers are bypassed, which suppresses
interference on AM. Directly above the rectifiers are two green-
sleeved electrolytics rated at 105° C. In parallel on the motherboard
below are two 85°-C electrolytics of 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, it
should be possible to substitute an external fuse. The power board
delivers unregulated 5.2 V and 10.5 V. 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. One module is the tuner and
the other 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 the 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. Only 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. This 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 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 is used to eliminate tape hiss when remastering older analog
recordings. This may be its first appearance in consumer audio
equipment.

A digital IF filter with extremely steep skirts eliminates adjacent-
channel interference in nearly all cases. The filter is much more
effective than conventional ceramic IF filters and has none of their
unit-to-unit variation, which necessitates filter selection or tuned
compensation for optimal performance. The digital filter does not
exhibit the passband group-delay variation of conventional filters,
avoiding the resulting audio distortion and multipath exaggeration.

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 from a conventional tuner. 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 21 filter
passbands.

The adaptive noise reduction affects only the noisy L−R stereo
subchannel. The algorithm attenuates individual bank-filter outputs
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. The
only image displacement I've noticed is for lower-level sounds in
certain noisy signals. For strong local 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 first-order, variable-cutoff, lowpass noise filter
on L+R as the RF level drops below 26 dBf. Attenuation begins at 15
kHz and affects progressively lower frequencies as the signal level
falls. The filter is similar to the one in the HDT-1X, which begins at
29 dBf and eventually levels off at 2.9 kHz at −3 dB. The Sony filter
stops falling at 3.2 kHz and the entire curve then smoothly drops with
signal level, providing a continuous soft mute. With no signal the
residual noise is down 30 dB.

This shows the lowpass filter response in mono at 73 dBf and 16 dBf.
The modulation is 15 preemphasized tones from 1 to 15 kHz. The level
has dropped 2 dB at 1 kHz and 13 dB at 15 kHz.

This is the response for left-channel stereo. 1 kHz has dropped 6 dB
and the overall response is flatter. In fact, above 20 dBf the curve
is almost perfectly flat, dropping uniformly with signal level from 26
dBf. The signal consists of equal parts L+R and L−R so the response
may be a combination of the lowpass filter operating on L+R and the
adaptive noise reduction on L−R.

The lowpass filter is active only for signals with zero or one bar.

For the following measurements I used IEEE 185-1975, updated as
described here. I used the test equipment listed here.

Mono 50-dB quieting sensitivity 13.5 dBf
Stereo 50-dB quieting sensitivity 13.5 dBf
Mono THD, 1 kHz 0.07%
Stereo THD, 1 kHz 0.055%
Stereo separation, 1 kHz 54 dB
Mono S/N, 65 dBf 70 dB
Stereo S/N, 65 dBf 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
Latency 27 ms

In mono or stereo, the XDR-F1HD is the most sensitive tuner I've ever
tested. Its mono sensitivity is due partly to the low-loss front-end,
partly to the noise reduction provided by the lowpass filter, and
partly to the threshold extension. (There may be other tricks
involved. The TEF6730 data*** indicates that sensitivity increases
5.2 dB when threshold extension and weak signal handling are enabled
in the SAF7730.) Most remarkable of all, the XDR-F1HD does not use an
RF amplifier (these days called an LNA, for low-noise amplifier). The
input signal drives a transient suppressor, PIN diode attenuator,
tuned circuit, and finally the TEF6730 mixer, whose rated noise figure
is 3 dB typical.

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 betters
that of ordinary tuners by an order of magnitude.

The stereo THD figure is 13 dB lower than that of the Sangean HDT-1X.

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 least 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 has a profound effect on reception quality for many
signals.

The astronomical selectivity figure is real. The XDR-F1HD is
noticeably more selective than the superb Sangean 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 approximate signal level at which the PIN
diode attenuator begins to operate. Untuned signals in the RF passband
above this level will raise the tuner's noise figure, causing a weak
tuned signal to become noisier.

Modulation acceptance at 1 kHz is the modulation level for 1% THD. The
XDR-F1HD figure may seem like overkill since FCC rules limit stations
to 100% modulation. But for years one local NPR station regularly
deviated 140%. Another station just across the border in Mexico hit at
least 170% on some peaks. These signals were impossible to receive
cleanly on tuners with narrow ceramic filters. The XDR-F1HD had no
problem with either signal. But see the graph below for low-frequency
modulation acceptance, which is much lower.

Compare the XDR-F1HD latency of 27 ms to 118 ms for the Sangean
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.

The XDR-F1HD appears to use a phase-locked loop FM detector that
unlocks at very low modulation frequencies. These glitches are for a
100%-modulated, 20-Hz, monophonic test signal.

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.

This shows the S/N components and 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.

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 adaptive
alignment during manufacture.

This is tuned-frequency return loss and the resulting mismatch loss.
Swamping the loss by adding a 10-dB 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, left-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% modulation. I have yet to hear it 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. Tilt the dipole 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.

For two loops I examined, the wire insulation was cut but not
stripped. I wonder if this explains the occasional report of no AM
reception. The conductors seemed rather fragile when inserted into the
spring-loaded antenna terminals, which bent and separated the 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 use an LNA ahead of the mixer.

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. I heard distortion on some skywave signals during selective
fades.

The XDR-F1HD has a 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 blanked every 8.3 ms
(for 120-Hz pulses). Still, with strong interference the audio sounds
much better blanked than not.

AM latency is 3 ms. Compare to 125 ms for the HDT-1X.
HD Radio

Except for one occasion when the XDR-F1HD locked to an FM signal and
the HDT-1X would 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 MP3
mode, occasionally only HD-3 appears. Once locked, the analog signal
and the other digital channels become tunable.

An aligned XDR-F1HD reliably locked to a 29-dBf FM HD Radio signal, as
did an aligned HDT-1X.

On 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 to the upper right of the
TEF6730. Just as Peter found for two tuners, adjusting the slug a
quarter turn counterclockwise increased the audio level 2 dB for a
modulated signal in the soft-muting region. Performance was the same
with the shield on or off so I did not bother to drill an adjustment
hole. 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 with the cabinet off), 50-dB quieting sensitivity at 96.8 MHz had
improved 1.4 dB to a remarkable 12.6 dBf. It was also 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, the sensitivities were 12.0, 12.2, and 12.8 dBf. I was
just lucky that they 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.)

Directly below the TEF6730 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 seems to function mainly as a broadband
differential input. I did not adjust it.
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, add a 47Ω ¼W resistor in series to limit the charging current. C926
should be paralleled as shown, not replaced. Its low internal series
resistance is necessary for proper shutdown if power is lost while the
tuner is operating.
Extending Audio Headroom

The XDR-F1HD audio amplifiers may clip on digital signals with very
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.

Clipping never occurs 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 shown was nearly 4 dB higher than
the station's analog signal level. 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
schematic's 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 to 0.6 V, which is standard for
most tuners.

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

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 signal trace. The 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. The display will indicate HD reception for
both switch positions, but you'll be listening to analog audio when
the switch is open.

To add an indicator, 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.
Install a bright red LED inside 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 cascaded RC filters on the motherboard. The
filters remove low-level (−47 dB) 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 filter capacitors C11,
C12, C21, and C22.

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 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 an inexpensive 1% 10kΩ resistor array will
provide high accuracy, accomodate both channels, and save space. Use a
wideband, dual op-amp with rail-to-rail input/output, such as a
TLV2372 (low cost) or LT1498 (high bandwidth), rated for 8.5-V VCC.
The unloaded model is down 0.1 dB at 20 kHz, 21 dB at 100 kHz, and 75
dB at 1 MHz.

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 high-level digital signals 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 input resistors. 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. Then connect the op-amp outputs to the capacitor
inputs, with the op-amp connected to pin 26 feeding the C102 circuit.

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 lower the RMS ripple current in C908 as
well as reduce transformer losses. 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, evidently 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.
DXers David Pierce and Mike Bugaj offer reviews here and here. Julian
Hardstone describes his experiments and modifications here. Ira Wilner
gives a broadcast engineer's perspective here.
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