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RESJi:iI.RCH DEI'AH'I'JIEENT . . -

TEST REPORT ON PElLCO (U. B.A. ) 48/482 Fl!![/A]'j~ )~ECl~Ii!JBl~ U'M roRTIO:;)_

Report No. G.045 Serial No. 1949/11

Research Department REPORT NO. G~045

S;;Ial No. 1949/11

.April, 1949.

Drawing Nos. G.045.1 to G.045.4

This report on the performance of the F.M, portion of the modGl 48/482 receiver, shows that although the discriminator used is excellent at suppressing slow variations of carrier amplitude, it is less efficient in suppressing amplitude modulation at audio frequencies. This results in the signal to noise ratio for both r~ndom fluctuation noise and impulsive interference being less than can be ottained with other discriminator circuHs.

The audio frequency response and distortion factor are also dealt with and a limited test of selectivity described.

2. Introduction

This receiver is of particular interest Oll account of its phase diicriminator circuit, a diagram of which is shown in Fig.4. It is claimedi!!: that this discriminator does not require a limi ter stage. The receiver covers the ~erican F.M.band of 88 - 108 Mcls and 1s intende'd for 75 kcls devi§tion.

~or the purpose of this report it has been decided that a minimum ratio of wanted progremme to unwanted background noise of ~ny form shall \e chosen and used as a criterion for objective tests .. This ratio 1s 40 db. measured on a square law meter preceded by the O.O.I.F. aural network, since this meter has been found to agree very well with subjective measurements es described in B.B.O. Research Department Report G.040.

3. E!~l'pjective Tests

The receiver is fitted with a short length of wire taped to thQ mains lead for use as an F.M. aerial. This aerial wes not sufficient at Kingswood \farren to receive a good signal from .A.lexandra Palace on 90.3 Mc/s. ~ half wave d11elo 0n the roof of Kingswood Warren was ther6lfore used insteAd of the fitted eer1nl wire •. The tuning

SE Electronics Engineering Manual - Single Stage F.M .. Detector, W1l1iam B,.Bradley.

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d'laracteristic was quite different from F.IvI. receivers previously tested. On each side of the correct tuning point a very large increase 0 r distorted outPI.~t occurs. .:Uthough this provides El

very definite indication for accurate tuning, the process of 80

doing is a little unpleasant. The 8udio qua.lity of th(J reproduction conforms with that of medium priced coIlime;ccial receivcr;~ "?roduced for medium wave a.mpli tude modulation reception. Thn a,' )ust:Lc frequency response does not appear quite so unbalanced as the objective measurements shown in Fig.l would indi.ca'te.

An aerial source resistance of 75,n was used in these tests and carrier'amplitudes quoted are those froIll a Tzero impedance' source feeding through the resistance in series wtth the reeei V0r input circui t.

4.1. S~nsitivity

A carrier input of lOO\.LV. frequency modulated ~ 30 kc/a (40 per-cen't modulation) at 400 c/G produced a 40 db. signal to random fluctuation noise ratio. With this input modulated 100 percent at 400 c/s and the audio gain sot to produCG an output of 250 mi1liwatts thG harmonic distortion was fl')und.to be 5 percent.

4.2. ImpulsivG Interference

The result of this test is shown in Fig.3. I+, will be 300n that the performanc8 is 8yrproximCttely 10 db. below that which can be obtained using two limitors followed by a Foster Sooley discriminator.

A cloviation of ::6 k.c/s at 400 c/s producod a signol to hum ratio of 40 db. This ropresents 8. re.tio of 54 clb. reletive to ,,=0 percont modulation.

4.4. Electrical Fidolity -. L

4.4.1. ~Gquoncy Resp~nse

Fig.l gi YOS t h0 measured' curve of frequency response,

4.4.2. Hcrmonic Distertion

Fig. 2 curve " sho~JS tho distortion percor.tcge plotted cgainst devietion 1:.'1 kC/Sa Curve B for which the ['udio output weB kept cC'Lftent shows that the distortion represented by curve A is due to the audio stages.

The R.F .. , I.F. and discriminator st'eges show no appreciable' distortion until the qeviation is !150 .k.o/s or 200 percent modulation. When the input carrier was modulated 40 percent at 400 c/s the receiver g~ve a standardqutpUt of 250 milliwatts with 3 percent distortion. .A.n audio output of 2 watts is obtainable for 10 percent distortion. ' ..

'4.5. Selectivity

These measurements were not comprehensive, due to l£,ck of sui table equipment.

4.5.1. CO-Channel Suppression

The receiver was tuned to a carrier of 1 millivolt, frequency modulated 40 percent, on a centre frequency of 90.3 Mc/s •. The. modulation frequency was 400 c/s. The audio output was adjusted to 250 milliwatts. ifuen the modulation was r.educeC!. to zero. amplitude an 'interfering' carrier on the same centre frequency modulated 40 percent at 400 cls produced an output 40 db. below 250 milliwatts when its amplitude wes 0.4 millivolts.

4.5.2. Adjacent Ohannel Suppression

Oontinuing from the previous measurement the 'interfering' carrier frequency was increased by 200 kcls to 90.5 Mcls and its amplitude wes increased to 2.1 millivolts, when it produced an audio output 40 db. below 250 milliwatts. The amplitude of the interfering carrier when tuned to 90.1 Mcls was 1,6 millivol'ts for this output.

4.6. Suppression of Ampli tl.:.de Modulation

The receiver was tuned to a oarrier\ of I mlllivolt at a frequency of 90.3 Mcls, modulated 40 percent et 400c/s. 'rhe audio output WBS set to 250 milliwatts. A high pass filter to rejeot frequencies below 250 cls was connected in the audio circuit before the lIleter. The frequency modulation was changed from 400 c/s to 60 c/s so that the meter ~ould ne longer measure the out~ut due to frequency

I modulation. The input carrier wes then ~mplitude modulated at 400 o/s to a depth of 30 percent and the audio output was found to be 28 db. below· 250 milliwatts. When the test was repeated with the carrier amplitude increased to 10 millivolts) the audio output due to amplitude modulation wes 27 db. below 250 milliwetts.

It w~s noted furing both these 1Jests thet reducing the frEietuency modu16t10n to zero deviation did not chatl.gethe eudio eutput, due to amplitude modulation. Furthermore there wes no criticsl tuning point et which the audio output due t.amplitude modulation was reduced to a minimUm, as exists. with the Foster Sealey discriminator.

5. Oonclusion

The audio output quality of this receiver is similar to that of most conunercial medium weve A.M. receivers using pentode.output stages without negative,feedback. It does no~ make full use of

. the opportunities offeredry V.H.F. broadcasting. '

The performance of thE? receiver as regards random. fluctuation noise and impulsive interference is .inferior to the'best that can be obtained in frequency modulation receivers. This is probably due to the discrimihator not sufficiently suppressing amplitude modulation.. . ,

Report written by G.F. New ell.

W.rk oerried out by G.F. Newell, J.D. RAmshsll.

(H .. L. Kirke)

This drawing / specification IS the property of the Br t"h Broadcasting CorpOratiOn and may not be reproduced or disclosed to a third party in any form without th(' written perrfllssloll of the Corporation.

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48/482 F.M./ AM. RECEIVER (F. M. PORTION,)

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