PRIVATE AND CONFIDENTIAL

RESEARCH DEPARTMENT

BUSH A.Ma/F.M. RECEIVER, MODEL VHF 6ij

Report No. G-071

( 1958/19)

THE BRITISH BROADCASTING CORPORATIO-N

ENGINEERING DIVISION

RESEARCH DEPARTMENT

BUSH A.Me/F.M. RECEIVER, MODEL VHF 6~

J.A.S.Fox, B.Sc.

Report No. G-071 ( 1958/19)

(w. Proctor Wilson)

This Report is the property ot the British Bro-adeasting Co"rporation and lIlay not be reproduced or disolosed to a third party in any torlll without the written permission ot the Corporation.

Report No. G-071

BITSH AoMo/F.Mo RECEIVER~ MODEL VHF 64

Section Title Page

1

2

3

4

SUMMARY 0 •

DESCRIPTION OF RECEIVER

1.1. General Description 1.2. Description of Circuit

TEST RESULTS

2.1. Sensitivity

o , Cl , .. .. .. .. .. ., .. .. .. .. .. .. ..

2.1.1. Absolute Sensitivity 2.1.2. Maximum Deviation Sensitivity for 10%

Total Harmonic Distortion . • . • • . . • 2.1.3. Sensitivity for Standard Signal-to-Noise Ratio 2.1.4. Signal-to-Hum Ratio, .•••.•••

2.2. Fidelity 2.2.1. Variation of Harmonic Distortion with

1

1

1

2

4

4 5

5 5

5

5

Modulation Depth • • • • • • . • . • • • • • . • .• 5 2.2.2. Maximum Output Power for 10% Total

Harmonic Distortion • 0 • • • • • • 2.2.3. Modulation-Frequency Characteristic

2.3. Selecti vi ty . . . • • . • , . • . . . • • •

2.4. Local-oscillator Performance •. . • • • • • 2.4.1. Local-oscillator Frequency Drift 2.4.2. Dependence of Local-Oscillator Frequency

on Mains Voltage 2.4.3. Local-oscillator Radiation

2.5. Co-Channel Suppression Ratio

2.6. Suppression of Amplitude Modulation.

2.7. Dependence of Output on Carrier Level •

2.8. Impulsive Interference Performance

2.9. Subjective Measurements of Selectivity and Co-Channel Suppression Ratio . . . , 0 , •

CONCLUSIONS

REFERENCE

5 5

6

6 6

6

7

7

7

8

8

8

10

10

July 1958

SUMMARY

Report No. G-071

( 1958/19)

Private and Confidential

BUSH A.M./F.M. RECEIVER, MODEL VHF 64

This report gives the results of tests on the Bush VHF 64 receiver on the v.h.f. f.m. range.

The performance generally is good, although the audio-frequency section does not have the high performance expected from a receiver of its price class.

1. DESCRIPTION OF RECEIVER

1.1. General Description

The receiver, a photograph of which is shown in Fig. 1, is a table model housed in a well-finished wooden cabinet; it covers the long-, medium- and short-wave a.m. ranges in addition to the v.h.f. f.m. band.

Dimensions

22°5 in. x 14°5 in. x 9°25 in. (57 cm x 37 cm x 24 cm)

Controls

(a) Tuning control Cb) Volume control (c) Treble tone control (d) Bass tone control {e) Piaricrkey switches for mains "off", long, medium, short and v.h.f.

bands and gramophone

Wavebands

Longg 1050-1935 m Medium~ 187-560 m Short; 16-50 m V.H.F. 874 5-100 Mc/s

POVTer Supply

200/250 V 40/60 c/s a.c.

External Connections

External loudspeaker - low impedance Tape recorder input from 20 dB attenuating network connected across

Gramophone pickup A.M. aerial input

F.M. aerial input

the external speaker terminals high impedance to augment pickup of ferrite rod aerial incorp-orated in the receiver 2 pin balanced socket, 75 ohms. An internal di-pole can be connected to this socket

2

Fig. J 1.2. Description of Circuit

The receiver, the circuit of which is shown in Fig. 2, employs eight valves, including a tuning indicator and rectifier. The types and functions of the valves are given in Table 1.

The receiver can be switched to either of two circuit arrangements, one for a,m. reception in the long-, medium- and short-wave ranges, and the other for f.m. reception in the v.h.f. band. The intermediate frequency is 420 kc/s for a.m. reception and 10'7 Mc/s for f.m. reception.

Permeability tuning is employed in the v.h.f. section which is followed by three i.f. amplifier stages; the final stage provides a useful degree of limiting by virtue of the short time constant of the grid circuit. A.G.C. in the f.m. condition is provided in the final i.f. stage by controlling the voltage of the suppressor grid from the reservoir condenser of the ratio detector.

r-----------------, I I I I

, \. _____________________ J

Fig. 2 ~ Circuit diagram

WITH THE REefIVE!!. ON HO-VOLT TAP AND IlO_VOLl INPUT. VOlTA

4

Table 1

Valve Type Section Function on a. m. Function on f.m.

V1:ECC85 twin triode triode 1 not used r.f. amplifier triode 2 not used self-oscillating

mixer

V2:ECH81 triode-heptode triode local oscillator not used heptode mixer 1st i. f. amplifier

V3:EF89 r. f. pentode 1st i.f. amplifier 2nd Lf. amplifier

V4:EF89 r. f. pentode 2nd i. f. amplifier 3rd i. f. amplifier

V5:EABC80 triple-diode- diode 1 a.m. detector not used triode diodes 2 and 3 not used ratio detector

triode a. f. amplifier a.f. ~plifier

V6:EM81 magic eye tuning indicator

V7:EL84 output pentode a. f. output stage

V8:EZ81 rectifier h.t. rectifier

Negative feedback is used in the audio-frequency stages, but with certain settings of the tone controls the amount of feedback is not very great.

Three loudspeakers are provided, two being permanent magnet moving-coil units, whilst the third is an electrostatic high-frequency unit polarized from the high tension supply and coupled to the output valve anode by a small capacity.

2. TEST RESULTS

The test procedure adopted was that described in Research Department Technical Memorandum No. G-l003 (issue 2) with some additional tests; in particular, it should be noted that whenever ratios of signal to noise, hum or interference are quoted, they were measured with a mean-square meter preceded by an aural sensitivity weighting network based on the C.C.I.F. (1934) cur~e for broadcast relay circuits. 1

Unless otherwise stated, all signal levels refer to the open-circuit voltage from a 75-ohm source.

2.1. Sensitivity

The sensitivity of the receiver is defined as the m1n~mum amplitude of carrier input which satisfied simultaneously the following tests, 2.1.1, 2.1.2 and 2.1.3.

The measured value was 28 ~V.

5

2.1.1. Absolute Sensitivity

This is the minimum input carrier amplitude, deviated ± 35 kc/s* at a frequency of 2000 cls, which will produce an output of 50 mW with the receiver gain control at maximum.

The measured value was 28 }-LV.

2.1.2. Maximum Deviation Sensitivity for 1O~ Total Harmonic Distortion

This is the minimum input carrier amplitude, deviated ±75 kcls at a frequency of 400 cls, which will produce a total harmonic distortion of 1~.

In this receiver this figure is less than the absolute sensitivity and was not recorded; the distortion at 28 }-LV under the stated conditions was fO'lmd tobe 5·5~.

2.1.3. Sensitivity for Standard Signal-to-Noise Ratio

This is the minimum input carrier amplitude, modulated ± 35 kcls at a frequency of 2000 cls, which will produce an output signal-to-noise ratio of 40 dB.

The measured value was 8 }-LV.

2.1.4. Signal-to-Hum Ratio

With an input carrier level of 10 mV modulated I35 kcls at 2000 cls the signal-to-hum ratio was greater than 75 dB.

2.2. Fidelity

2.2.1. Variation of Harmonic Distortion with Modula-tion Depth

Fig. 3 shows the total harmonic distortion as a function of modulation depth with the receiver gain control set for 50 mW output with ± ro kcls modulation at a frequency of 400 c/s.

6'" 5'10

J:

.~ 1;4'10

'" on :

6

20

18

16

r- 14 ID

~ 12

'" -u 10 g or \I .. Cl .. ~ .. 4 !l .. .~

~ 0 ~

-2 .. ~-4 .. ..

C> -6

-8

I

-10 20c/s

I

::s r

~+20 u .¥ V ... ... ii

+10

o

First reod"lng token 30 secs" otter switchi n9 on

F'.. -0--

10 20 30 40 50 60 70 Time (minute 5)

Fig. 5 - Local-oscillator frequency drift

2.4.3. Local-oscillator Radiation

7

80 90 10 o

The voltage at the input terminals of the receiver due to the local oscillator was measured, the input terminals being terminated in 75 ohms.

The measured value was 0'9 mY.

2.5. Co-Channel Suppression Ratio

As for test 2.3, but with the interfering signal frequency differing from the wanted signal frequency by less than 1 kc/so

The measured value when the receiver was tuned for maKimum audio output was - 10 dB. The measured value when the receiver wa.s tuned for maximum co-channel sup-pression was - 9 dB,

2.6. Suppression of Amplitude Modulation

The a.m. suppression ratio is the ratio between the output due to a carrier which is frequency modulated ± 35 kc/s at 2000 c/s to that due to a carrier which is simultaneously amplitude modulated to a depth of 40~ at 2000 c/s and frequency modul-ated ±30kc/s at 100 c/s, the 100 c/s output being rejected by a high-pass filter.

For this test the audio output used as standard was taken as 12'5 mW, not 50 mW as recommended in Research Department Technical Memorandum No. 8-1003 (issue 2). This was because, with the gain control setting which gave 50 mWoutput with an input signal modulated ± 35 kc/s at 2000 c/s, sufficient harmonic distortion of the 100 c/s output was produced to affect the measurement of high values of amplitude-modulation suppression.

The measured ratios are given in Table 3.

Table 3 "-

Carrier level Amplitude-modulation suppression ratio

10 mV 44'5 dB 1 mV 46 dB

300 }J-V 38 dB 100 }J- V 32 dB

8

In the above tests the receiver was tuned for maKimum audio output and no change of suppression ratio was found when tuning to the point indicated by the tuning indicator; the bandwidth over which the ratios remained constant was found to be about 80 kc/so

2.7. Dependence of Output on Carrier Level

For this test the input carrier was frequency modulated at 2000 c/s with ± 35 kc/s deviation. The receiver was tuned with an input carrier level of 1 mV and the gain control adjusted so that overloading did not occur at any input level up to 100 mY. The carrier level was then varied in 10 dB steps; the variation of audio output with carrier level is shown in Fig. 6

The test was repeated, tuning the receiver for each carrier level, but no significant difference was observed.

i-'"' ,...

~ .... 1-"

/ /

V /

/

... -15 :::0 Q. ... :::0 0

.2 -20 .., :::0 ...

10 V SO}lv I 0 V ImV 5mV IOmV 50mV 10 'jJ. OmV -25

Input carrier level (open circuit volts tram 75 A source)

Fig. 6 - Variation of a.f. output with carrier level

2.8. Impulsive Interference Performance

Fig. 7 shows the output due to impulsive interference relative to the output due to ± 35 kc/s deviation at 2000 c/s for various input impulse amplitudes between - 5 and +30 dB relative to 1 j.J-V peak per kilocycle of bandwidth.

The measurement was made in the presence of an input carrier ata level of 0'5 mV, first unmodulated and then frequency modulated ±SOkc/s at 12 kc/so The impulse repetition frequency was 2'5 kc/s in both cases.

2.9. Subjective Measurements of Selectivity and Co-Channel Suppression Ratio

The receiver was fed with two carriers, a wanted carrier of 1 mV and an interfering carrier of controllable amplitude which was set in turn to a frequency within 1 kc/s of the wanted carrier and then spaced ± 200 kc/s and ± 400 kc/s from the wanted carrier.

Both carriers were frequency modulated with programme in accordance with standard B.B.C. transmitter practice, the wanted programme being chiefly speech and the interfering programme piano music which gave aconsistently high level of modulation.

-20

-25

-30

."

~-40 ... c 2 ...

.. > '0; E ~ '" -55 ~ ... " Q. .. " o

-60

-65

0

/ / p /

./ /

/ R.F. ccrr ier IQVol 0·5 mV

/' ./ Impulse p.t.f. 2·5 kc/s /'

I "'; . ...,-

V"

V I I / 9 I

j / i r

I

/ I I f ~

........ ~I / ""'With f.m. (.:t. 30 kc/s deviction) No f.~."""------ I I I

9 I ! r I

/1 /~

V ~ -C Y

-5 -t5 +10 +15 +20 + 25 +30 Input (dB relctive to I)lV peck per kc/s of bandwidth)

Fig, 7 - Input/output characteristic for impulsive interference

9

The amplitude of the interfering signal was adjusted to give the following subjective grades of interference:

JP The interference was just perceptible in quiet passages of the wanted programme.

P The interference was perceptible in quiet passages of the wanted pro-gramme without careful listening.

SD The interference was slightly disturbing when listening to the wanted programme.

D The interference was disturbing.

10

The results given in Table 4 below are the averages for three observers, the receiver having been tuned in each case in the absence of interference, using the tuning indicator.

Table 4

Frequency of interfering -1 .-

Amplitude of int erfering

V +26 +2 -31 -7 +9

signals relative to wanted +28 +6 -27 -5'5 +11 signal in dB for subjec- SD +30 +8 -22 -4'5 +12 tive grade of interference D +33 +9 -17 -3 +14

3. CONCLUSIONS

Apart from the audio-frequency section, the performance of the receiver on the v.h.f. range is good, particularly in respect of local-oscillator frequency stability and amplitude-modulation suppression. In a separate listening test, slight distortion was noticed on piano music at certain settings of the tone controls.

4. REFERENCE

1. Maurice, R.D.A., Newell, G.F. and Spencer, J.G., "Electrical Noise", Wireless Engineer, Vol. 27, No. 316, p. 2, January 1950.

MV~ __________________________________________________________ ___

Printed by B.B.C. Research Department, Kingswood Warren, Tadworth# Surrey