CONTROLS and
FUNCTIONS
Creek have designed the 3140 to have the
performance of laboratory equipment without the penalty of laboratory looks or
complexity of use. Our
philosophy is to design products with facilities which benefit the user rather
than the reviewer, and which are different from those of other manufacturers
because we try to think our designs through from the beginning, rather than
trying to copy all the unnecessary functions which are normally provided to
fill up space on front panels. We are sure
that the functions we have incorporated in this tuner will make it very easy to
use, so that you will constantly obtain the high performance of which it is
capable.
Switch-on
Stabilisation Delay
When the
3140 is switched on, a delay circuit waits for the internal control functions
to stabilise before operation will start. This normally takes between 15 and 20
seconds. A further 2 seconds delay then occurs before the AFC system is
enabled, ensuring that the tuner will return to the last station tuned before
switching off.
-
Muting,
Mono/Stereo, Blend and Tuning Indicator
Tuning
accuracy is very important for high fidelity reception of FM broadcasts. The
3140 uses specially developed integrated circuits to allow exceptional tuning
accuracy with great simplicity for the user. The same circuits also control the
operation of the entire tuner to ensure that the best possible quality of audio
is obtained at all times, automatically.
The Tuning
Indicator, Muting, Blend and Stereo/Mono functions are operated by control
signals derived from ICs from three parameters of the received signal –
Detuning, signal/Noise Ration and Signal Strength. The Stereo/Mono and Muting
functions are not simple on/off switching functions, but are progressive,
proportional and carefully balanced against the three control parameters to
ensure optimum listening performance but with extreme ease of use.
The control
signals are monitored continuously, and are derived and perform control as
follows:
Signal-to-Noise
information is obtained by extracting a DC signal proportional to the non-audio
content of the received and demodulated signal. The signals present above 75kHz
are a function of the noise content of the demodulator output, while those below
10Hz are a function of inter-modulation and cross modulation products between
adjacent channels. The subsonic content is also used to trigger the blend
circuits in the presence of aircraft flutter, momentarily forcing the tuner to
mono to reduce the irritation of this interference.
Signal
Strength is extracted from the IF limiter, and is linearised to give control
signal proportional to the RF at the aerial socket over a range greater than
80dB. This control signal is also used to operate the Automatic Gain Control
(AGC) in the RF tuner head to prevent overloading in the presence of very
strong input signals, and a further control signal is derived to ‘gate’ (or
switch) the AGC operation, described below. The brightness of the frequency
display is controlled by a derivative of the signal Strength monitor to give a
clear indication of relative signal strength to the user.
Detuning is
a measure of the amount that the tuner is off-station. It is derived from the
Automatic Frequency Control (AFC) signals, with further processing to obtain a
linear control voltage proportional to the magnitude of the difference between
the required station and the actual tuning point of the tuner. The Detuning
monitor is also used to disable the AFC action between stations, described in
more detail below.
These
various control signals are then combined to provide progressive control of the
muting circuits and the stereo decoder. The operation of the system as a whole
is best understood by examining in detail what happens as the tuner is tuned
from an empty part of the band onto a strong station…
When the
tuner is completely off-tune, and receiving no signals at all, then the Signal
Strength monitor is at zero, the detuning monitor is at zero (cannot be
de-tuned from nothing) and the signal/noise monitor shows zero (no signal – all
noise). Combining thse signals on a ‘voting’ basis gives two out of the three
voting for no signal worthy of any attention, and the audio output is held
firmly muted. The stereo decoder is also forced to mono, but as there is no
pilot tone signal for it to lock onto it holds itself in mono, anyway.
As we tune
towards a station the Signal Strength monitor starts to rise, as does the
Signal/Noise monitor (since the supersonic content of the recovered audio falls
rapidly as a station is approached). However the subsonic content of the audio
is now very high (due to the amplitude modulation products of the FM signal
operating in the non-linear ‘skirt’ region of the IF filters) and the Detuning
monitor shows that we are strongly off-tune, and so the audio is still held
firmly muted by a two-against-one vote as before.
As we now
tune more into the range of the IF filters, within 150kHz of the centre of the
desired station (75kHz in Narrow mode) the Signal Strength monitor reaches its
maximum, and (if the signal is strong enough) operates the AGC in the
front-end. At this point the Stereo Decoder is released from mono, to allow it
to lock onto a pilot tone if one is present, thereby lighting the Stereo
Indicator. The proportional Stereo/Mono Blend is set by the (now stable) Signal
Strength monitor to give the ‘amount’ of stereo available once the mute is
finally released.
This is
described in ore detail below.
If AFC is
switched On it is also now released to take control of tuning, accelerating the
tuner circuits rapidly into the centre of the station frequency. At the exact
centre of the desired station the Detuning monitor returns to zero, and this
un-mutes the audio output fully, and lights the Tuning indicator.
If the AFC
is Off, then the muting is progressively released as the station centre is
approached, lighting the Tuning indicator at the exact centre frequency when
the mute is fully open. The monitor circuits continue to operate, so that if a
burst of noise or interference is detected the mute will close briefly to
prevent these unwanted signals reaching the amplifier.
The effect
of the progressive muting function is to make the tuning knob seem to operate
as a ‘clickless’ rotary switch. As the knob is turned through each station the
tuner un-mutes at the point where the station is perfectly tuned, lighting the
Tuning Indicator. This ‘switch’ effect is much more pronounced with the AFC
switched Off. With AFC operating the T40 tunes rather like an AM radio – the
station is apparently loudest at the point of perfect tuning, and gets quieter
as you tune away to either side of this point. This makes it easy for those
users unfamiliar with FM tuners to achieve the perfect tuning essential to
obtain full FM performance. It is quite possible to tune a station entirely by
ear, although the Tuning Indicator lights exactly at the centre point so it may
be a little silly not to use it!
Frequency
Display
The green,
digital frequency display indicates the frequency to which the tuner is
actually tuned and is calibrated in MHz to a
resolution of 100kHz.
Although the
display cannot therefore show the correct frequency for one or two European
broadcast stations which do not transmit on exact multiples of 100kHz (e.g.
95.125MHz), the tuning system in your 3140 will ensure that the tuner locks
accurately to the centre of any channel to which it is tuned.
More about
Mono/Stereo and Blend
As mentioned
earlier, the effect of going from mono to stereo is to require 20dB more RF
signal to achieve the same audio signal-to-noise ratio. The 3140 incorporates a
progressive changeover stereo decoder, which is controlled by the Signal
Strength and Signal/Noise monitor signals. The decoder operates by mixing or
blending the Left and Right stereo signals to give partial mono over a 20dB
range of signal strength. The effect of this is to maintain an almost constant
audio signal/noise ratio during the transition from full Mono to full Stereo.
Even partial Mono is still very pleasing, certainly much more listenable than
full, but 20dB noisier, Stereo. When the decoder is in the fully Mono condition
the Stereo Indicator is extinguished, although the pilot tone oscillator is
left running to maintain stability. This ‘Monoing’ can happen very quickly and
briefly during flutter conditions, effectively masking this irritating
interference.
The Blend
Function is also controlled by the Signal/Noise monitor, so that in the
presence of Adjacent Channel Interference (birdies) a degree of blend is
introduced regardless of signal strength, reducing the audibility of this
interference considerably.
The
decoder’s oscillator operates at 76kHz, and is divided down to 38kHz to obtain
the stereo sub-carrier, phaselocked to the 19kHz pilot tone. This feature makes
the 3140 immune to birdie-like interference problems which result from other
transmitted signals such s for the German VWF/ARI traffic information system,
which operates on a 56kHz sub-carrier and is used in many European countries.
More about
AGC and AFC
The 3140
incorporates two additional features using its other signal monitors to enhance
performance – Gated AGC and Detuning-limited AFC.
In some
areas there are one or two very powerful local transmitters which can prevent
reception of weaker stations adjacent to them in the FM band by dominating the
front-end AGC. Normally the AGC signals are derived from the first IF
amplifiers, which look at a10.7MHz wide ‘window’ onto the band. Obviously, one
very strong station within this wide window will cause the AGC to reduce the
sensitivity of the front end, preventing it from detecting any weaker signals
in the same window. In the T40 an extra monitor signal is obtained from the
second IF circuits, which look through a window only 300kHz wide (150kHz in
Narrow mode).
If this
second signal indicates that the AGC is reducing front-end gain due to a strong
station outside the second IF window, and therefore not of interest, the AGC
circuits are turned off, or ‘gates’. This allows reception of any weaker
transmitters being masked by the presence of one dominant station elsewhere in
the band.
Normal AFC
control can also sometimes be self-defeating, since the AFC system must be a
compromise between holding on firmly to the tuned station, and allowing the
tuner to be deliberately moved to another one when wished. The two extremes of
this trade-off either allow the tuner to drift away from the desired station
too easily, or alternatively hang on to it permanently, preventing selection of
a different station without first disabling the (‘grim death’) AFC.
In the 3140
the AFC is itself controlled by the detuning monitor, so that we can achieve
very firm AFC once the desired station is tuned, but while actually tuning
along the band there is no AFC action at all. The prevents the tuner from
‘flipping’ from one strong station to the next as it is tuned, missing out any
weaker signals in between. This Detuning-limited AFC allows the 3140 to
eliminate the compromise and use both extremes when appropriate, ensuring both
easy tuning and then very stable locking to the required station.
Specifictions:
Tuning
Range: 88 - 108 MHz
Input Signal (for -3dB limiting): 0.7 µV
Mono:
(S+N)/N=26
dB: 0,9 µV
(S+N)/N=46
dB: 2,2 µV
Stereo
(channel separation 32 dB):
(S+N)/N=46
dB: 20 µV
Input Signal
to Light Stereo LED: 5 µV
For 40 dB
Separation: 20 µV
Stereo
Channel Separation (for Vin better than100 µV): 46 dB
AM
Suppression (at Vin = 1mV): better than
50 dB
Total
Harmonic Distortion (with input signal ± 75kHz): less than 0,2%
Audio
Bandwidth (-1dB): 25 Hz – 15 kHz
Pilot Tone
Suppression: more than 64 dB
Aerial
Input: 300 Ω balanced and 75 Ω unbalanced
Dimensions
(W x H x D): 420 x 165 x 64 mm (16 ½ x 6 ½ x 2 ½”)
Weight: 3,3 kg
(7 lbs)
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