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Technical Description

Engineering direction dictated a tuner design

governed by insistence on great flexibility and ease
of use. This had to be done while designing an RF
section with great sensitivity and the world's best
selectivity in keeping with the needs of low distor-
tion. These values were achieved and they retain the

Mclntosh reputation of outstanding performance,

long life and reliability.

RF-IF SECTION

In the ANT SWITCH six RF PIN diodes are con-

nected as a single pole double throw switch. This

allows the use of two antennas or the use of an
antenna and a cable. Input to output isolation of this

switch is greater than 75dB.

Following the ANT SWITCH is an RF tuner of ex-

ceptional performance. This performance is achiev-
ed by the use of a low noise junction FET and a high

power bipolar transistor (a heatsink is used) con-
nected as a cascode amplifier. This connection

allows high gain, low noise, and high output levels

without the need for neutralization. Two additional

PIN diodes are used to insert a second tuned anten-
na Preselector circuit during strong signal recep-

tion. For lower intermodulation distortion the tuned

circuits are tuned by a series parallel connection of

four matched varactor diodes. This circuit and the
high tuning voltage (5-26 volts) eliminates the RF in-
termodulation distortion caused by diode non-

linearities.

A f t e r the RF amplifier two parallel tuned circuits

are used to provide the proper load impedance for

the bipolar transistor. The use of two tuned circuits

at this location greatly improves the image rejection

and overload performance of the tuner, as well as in-

creasing the RF selectivity.

The following balanced mixer is a matched dual

J-FET and bipolar transistor circuit. A low loss

toroidal phase splitting transformer is used as an

impedance matching network in the gate circuit of

the mixer. At the output a balanced double tuned

transformer is used as the drain load. This arrange-

ment gives great immunity to mixer overload in the

presence of a very strong signal. A bipolar transistor

is used as an oscillator buffer to prevent oscillator
pulling on strong signals, and as the constant cur-
rent source for the dual J-FET mixer.

The MR 80 has the narrowest IF bandwidth ever

used in a stereo tuner. It is the correct width to let

just one FM station through. The excellent selectivi-

ty of the MR 80 (210kHz wide at 60dB down) permits
tuning to stations that are impossible to receive on

ordinary tuners.

Constant delay design techniques are used in the

4-pole 4-zero crystal filter used in the SUPER NAR-

ROW position. However, due to the extreme selec-

tivity, delay distortion is very slightly increased over

the NARROW selectivity position.

Four differential amplifiers, coupled with PHASE-

LINEAR monolithic filters, comprise the NARROW

selectivity and signal strength sections of the IF
amplifier. One half of each differential amplifier out-

put drives a filter and the second output drives a  f u l l
wave rectifier circuit. The outputs of these four

rectifier circuits are summed at the inverting input of

an operational amplifier. A  f i f t h rectifier circuit that

is driven from the limiter is also summed at the input

to the operational amplifier. The output of the opera-

tional amplifier is a voltage that is proportional to

the log of the RF input level. Dynamic range is from
2

m

V to over 2V, that is, greater than 120dB. A solid

state signal strength meter is used as a front panel

indication of the incoming RF signal level. Twelve

LED's are used in place of the conventional meter

movement. The LED's have an almost unlimited life.

The fast response of the LED's plainly shows

multipath signals and antenna misorientation. To

better use the wide dynamic range of the five signal

strength detectors, an adjustment is provided to
reduce the full scale signal from 100,000

m

V to any

value below that. This meter can be set to give a full

scale indication on a signal as low as 2

m

V. This

allows setting the signal meter to indicate full scale

on the strongest station in the users area. Then the

signal strength meter indicates the RF level of all
stations received relative to the strongest in the

listener's area.

The signal strength voltage is also used to control

mono/stereo switching, stereo  f i l t e r insertion,
muting, and automatic scan stop. Because of it's

wide range and high repeatability this control
voltage is also used to adjust the stereo separation
at low RF signal levels. This produces the maximum

signal to noise ratio possible for all signal levels.

Following the selectivity section of the IF ampli-

fier is the LIMITER. A total gain of 80dB is used in

this circuit. The use of very high gain in the limiter
circuit produces hard limiting with very good im-

pulse noise rejection. Limiter bandwidth is greater

than 50MHz, producing excellent detector capture
characteristics.

A broadband Foster-Seeley discriminator is used

as the detector. This detector coupled with the

broadband limiter produces unmeasurable noise

and distortion. A high speed buffer amplifier follow-
ing the detector isolates the detector from all varia-

14

Содержание MR80

Страница 1: ...Reading Time 32 Minutes Price 2 00 THE MclNTOSH MR 80 DIGITAL FM TUNER...

Страница 2: ...the rear panel of the chassis Record that serial number here Serial Number The model serial number and purchase date are important to you for any future service Record the purchase date here Purchase...

Страница 3: ...RACT CER TIFICATE must be presented when the instrument is taken to the service agency 3 Always have service done by a Mclntosh authorized service agency If the instrument is modified or damaged as a...

Страница 4: ...tenna jack A flexible folded dipole antenna supplied with your MR 80 may be used in urban or in high signal strength areas The flexibility of the thin flat wire assembly permits it to be placed under...

Страница 5: ...R 2ND 75W ANTENNA LEAD IN FM ANTENNA OPTIONAL 75W ANTENNA LEAD IN REMOTE CONTROL PUSH BUTTON POWER AMPLIFIER PREAMPLIFIER LEFT SPEAKER RIGHT SPEAKER 5 MAXIMUM PERFORMANCE INDICATOR OR OSCILLOSCOPE TO...

Страница 6: ...4 These touch pads located to the left of the fre quency display select one of the four preset sta tions Just touch to operate A small rectangle will light above the pad indicating which preset has b...

Страница 7: ...ono reception In the STEREO position the tuner automatically switches from mono to stereo mode when a pre determined signal to noise ratio has been reached In the MONO position all stations will be re...

Страница 8: ...INPUT switch selects either an antenna or cable as the RF input source The CABLE input may also be used as a second antenna input Lock This switch will disable the lock circuits only when the main tun...

Страница 9: ...on 110dB Narrow 8dB 90dB Super Narrow 60dB 110dB Tuner IM RF 23dBm for 2 signals 1MHz apart Tuner Intercept Point 10dBm 75W ImageRejection 90dB Maximum Signal Input 8 volts across 75W 1W antenna termi...

Страница 10: ...Performance Charts SIGNALINPUT 10 SIGNAL INPUT...

Страница 11: ...11 FREQUENCY IN HERTZ FREQUENCY IN HERTZ FREQUENCY IN HERTZ...

Страница 12: ...12 FREQUENCY IN HERTZ FREQUENCY IN HERTZ FREQUENCY IN HERTZ...

Страница 13: ...13 FREQUENCY IN HERTZ FREQUENCY IN HERTZ FREQUENCY IN HERTZ...

Страница 14: ...on ordinary tuners Constant delay design techniques are used in the 4 pole 4 zero crystal filter used in the SUPER NAR ROW position However due to the extreme selec tivity delay distortion is very sli...

Страница 15: ...is for use with an external noise reduction adapter An electronically switched filter circuit im plemented with two J FETs of a quad J FET package is used to reduce out of phase noise when in the ste...

Страница 16: ...he scan circuitry on the tuning control logic board The logic circuitry will then select the auto scan circuit and start searching for the next acceptable station The direction of scan will be the las...

Страница 17: ...and in hibit the lock circuit under these conditions On the top cover control panel is a manual lock ON OFF switch that inhibits the lock circuit only if the tuner is in the main tuning knob mode This...

Страница 18: ...the IF offset by adding the negative of the IF frequency The gate signal is derived from a stable 200kHz crystal oscillator and a binary divider made from LS integrated circuits Two other signals are...

Страница 19: ...19...

Страница 20: ...Block Diagram 20 MclNTOSH MR 80...

Страница 21: ...DIGITAL FM TUNER 21...

Страница 22: ...MBERS ST BINGHAMTON N Y 13903 9990 607 723 3512 The continuous improvement of its products is the policy of Mclntosh Laboratory Incorporated who reserve the right to improve design and price without n...

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