[Synchrodyne]

I have since been able to deduce some information on the TDA1071 IC after finding a couple of the schematics for some Roberts models that used it, namely the RM30 and RM33. I still have not found a datasheet for it, but I have discovered that it was referred to in Mullard Technical Communications 14, No. 134, 1977. Mullard Technical Communications appear to be unobtainium, but this reference does suggest that it was a Philips/Mullard device. Thus it is surprising that it was not mentioned in any of the Ambit catalogues 1 through 3, which otherwise covered the Philips/Mullard RF and AF ICs of the era, including the TDA1005A, TDA1028/9, TDA1072 and TDA1074. Given Ambit’s apparent enthusiasm for the Plessey SL624C, one imagines that the TDA1071 would have appealed, and that interesting uses would have been found for it beyond those delineated in the application notes.

Essentially the TDA1071 appears to have been a consumer IC, intended to handle many of the FM-AM radio receiver functions. In that sense it was not unusual. On the FM side, it provided IF amplification, limiting and quadrature demodulation. On the AM side it included frequency changing (with separate mixer and oscillator), IF amplification with agc, and quasi-synchronous demodulation. The last-mentioned was its point of departure, and it was probably rare amongst consumer application ICs in specifically catering for AM quasi-synchronous demodulation. The Plessey SL624C for example was primarily a professional applications IC. And other FM-AM radio ICs, such as the TDA1220, tended to use more conventional forms of AM demodulator, even when they incorporated quadrature-type FM demodulators.

In the TDA1071 the same multiplier was used for both FM and AM demodulation. The feed from the limiter to the multiplier was internal. The second feed to the multiplier was external, and by external switching could either from the limiter output via a π/2 phase shift and tank circuit for FM, or from the IF amplifier output direct for AM.

In the Eddystone 1570/1590 case, the AM front end was discrete, using dual-gate mosfet RF and mixer stages, with a dedicated agc loop for the RF amplifier. So the frequency changing capability of the TDA1071 was not needed. Instead, the mixer was used as an additional gain controlled amplifier stage, ahead of the main selectivity, which was obtained by switching amongst various bandwidth filters between the mixer and the IF amplifier sections of the IC. The IF from the front end went into the mixer RF port, and it looks as if the two local oscillator ports were used for gain control, one for agc and the other for mgc. The agc voltage was obtained from the IC itself, possibly from the IF amplifier agc decoupling pin. I should guess that the TDA1071 mixer was of the “transistor-tree” multiplier form, as in the MC1496, and that reminded me that way back when I had seen the schematic (attached) for a 10.7 MHz IF strip that used a couple of MC1496s, with variable DC on the “carrier” inputs, to provide agc. Also, the first stage of the Motorola MC1350 TV IF amplifier IC was a transistor-tree with agc DC applied to the upper section, thus acting as a variable gain differential cascode amplifier with input impedance essentially unaffected by the gain setting.

Eddystone used a second TDA1071 for AM AFC on the 1570/1590, configured with a quadrature demodulator. This was fed from the IF amplifier input of the main TDA1071, and as in that case, the “mixer” section was used to provide gain, although in this case without agc. The FM IF section of the 1570 used yet another TDA1071, with the “mixer” section used as an IF pre-stage ahead of the main selectivity filter, and fed directly from the UM1181 tuner module. (I think it was a Mullard module; anyway, it was described in Ambit Catalogue #3.)

One wonders whether, given the apparently unusual nature of the TDA1071 in respect of AM demodulation, there was some reluctance to use the quasi-synchronous technique for AM (LF, MF and HF) broadcast reception, whereas once executable in IC form, it had adopted quickly for TV vision demodulation. I seem to recall some criticism as compared with diode demodulation voiced in a Practical Wireless magazine of the late 1990s. The Australian manufacturer Allen Wright included both precision rectifier and quasi-synchronous demodulators in Wideband AM Tuner 2 (reviewed in Electronics Australia August, 1980 edition.) The rationale for the quasi-synchronous demodulator (which was based upon an MC1330 IC) was given as: "While this detector gives a little more distortion, it can be useful at night to reduce 'monkey chatter' from adjacent signals."

On the other hand, parts of the NAB Engineering Handbook are available on line at Google Books and the section on C-QUAM decoding (page 709) includes the comment: “The envelope detector demodulates the monophonic, L+R information. It may be a simple diode detector; however, most stereo demodulator integrated circuits utilize a limiter/multiplier approach that offers superior performance. Distortion measurements in the 0.1% to 0.3% region are commonly found at 99% negative modulation when this technique is used”. Thus it would seem that any distortion problems, if in fact they exist, relate to execution and not the concept itself.

If the TDA1071 demodulator section was like that in the SL624C, then the limited reference would have been fed into the bottom part of the transistor-tree, with the AM signal into the top half. This was the reverse of what might be considered normal, as found in TV vision quasi-synchronous demodulators and for the MC1496 when configured for AM demodulation. But this “reverse” arrangement does seem to have been normal for FM quadrature demodulator ICs, and so was also used when such ICs were employed for AM demodulation.

Cheers,