Creek 3140 FM Tuner

 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.

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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)