[Synchrodyne]

I think that the transistor era for TV and FM front ends is easier to categorize than was the valve era.

With VHF TV front ends, I don’t think that the typical three-transistor arrangement had much advantage over valves when it came to noise. But it was a lot worse in respect of signal handling and spurious responses, such as cross-modulation. That did not seem to much inhibit its adoption in Europe, but it was of major concern in the USA. Some makers held on to valve VHF tuners, at least for higher performance models, until mosfet-based types became available.

Given that domestic-application jfets were available before corresponding mosfets, some work was done, e.g. by TI, on jfet-based VHF TV tuners. Although good performance could be obtained, there were stumbling blocks. A cascode jfet pair was generally fine as RF amplifier, except that it did not have an adequate AGC range. A single jfet was an excellent mixer, but had little gain, necessitating an additional device for a supplementary gain stage. Thus relative economics were against the use of jfets. On the other hand, dual-gate mosfets did provide the answer. Initial use (by RCA c.1968) was for the RF stage, soon extending to the mixer stage. Some makers waited until protected-gate mosfets became available. With dual-gate mosfets, it was possible to match or better valve VHF tuner performance in respect of noise, gain, signal handling and spurious response. But I don’t think that there was a quantum leap in terms of noise performance. The Zenith paper mentioned above covers some of this ground. The subject solid-state tuner had a slightly lower noise factor than a good valve tuner at Band III, but slightly higher at Band I. Anyway, mosfet-based tuners provided American TV receiver makers with the opportunity to convert to solid-state without material performance penalty.

UHF was a different case, and in Europe two-transistor UHF tuners were adopted quickly once they became available, having much better noise performance than their valve counterparts, although signal-handling was more limited. In the USA, the transistor quickly replaced the valve as oscillator in their essentially passive UHF tuners.

The existing European bipolar VHF and UHF TV tuner layouts were more-or-less amenable to the use of varactor tuning, albeit with the possibility of some reduction in signal-handling capability. See WW 1976 January p.56 for a comparison of the Mullard ECL1042 (mechanical tuning) and ECL1043 (varicap tuning) in that regard.

But the advent of varicap tuning in the USA indicated that some changes be made. Firstly, it was no longer convenient to feed the UHF tuner output into the VHF tuner RF input. Probably this was because the VHF tuning range was not easily extended downwards to encompass the IF channel. With switched tuning of some form, the UHF IF input was essentially just another channel. This meant that additional gain had to be provided for the UHF channel. Typically this was a common base bipolar stage just after the mixer, and in the UHF tuner box, although sometimes in the VHF tuner box. I think the latter tended to apply where a conventional UHF tuner was used in conjunction with a varactor VHF tuner. An alternative was to use an active mixer, per European practice, and I think that Sylvania did this. The addition of varactor tuning to the UHF tuner itself necessitated the addition of an RF amplifier in order to maintain gain and Q of the input circuits. So UHF tuners would then have a transistor RF amplifier, a transistor oscillator, a diode mixer and a transistor IF pre-stage.

Dual-gate mosfets suitable for UHF RF amplifier use arrived c.1975. RCA was an early user, in its KRK226 tuner, described in the 1976 IEEE paper mentioned above. Somewhat oddly in retrospect, this retained the bipolar IF preamplifier of its all-bipolar predecessor, even though it fed the mixer dual-gate mosfet of the corresponding KRK228 VHF tuner. The KRK226 UHF tuner was said to have a better noise factor (7 dB at channel 83) than its bipolar predecessor, but comparative numbers were not given.

I think it was not until the 1980s, or even later, before the dual-gate mosfet found widespread use in European TV front-end practice, although I have a vague notion that there were exceptions, such as Tandberg in the 1970s on the VHF side. At least here in NZ, the difference in signal-handling at VHF was fairly obvious. With just two channels, one Band I and one Band III, a major make with a biopolar tuner showed serious cross-modulation with a signal level that a valve monochrome receiver handled easily, and in fact the bipolar unit required 18 dB of attenuation to clear the problem. That was in Auckland in the 1970s; I had a repeat performance about a decade later in Wellington with a later model from the same make. In the early solid-state days, only one maker in NZ (Pye) used a mosfet-based tuner. (I don’t know if it sourced from RCA or a Japanese maker.)

I suspect that in the VHF case, looking at a wide range of noise factor numbers might show that there is more scatter between makes and models, than amongst valve (cascode & later), bipolar and mosfet circuits. On the other hand, in the UHF case, bipolar generally should show a step improvement as compared with valves. Mosfets might have show a consistent but perhaps not as large further improvement, but that is uncertain. Quite where the American UHF tuners with passive signal paths and transistor oscillators would fit I don’t know. But that American practice did not change until it was forced to by the advent of varactor tuning suggests that they could be made quite enough, such that a change to the 1960s European bipolar style was not considered worthwhile.


The FM case was somewhat different, more to follow.


Cheers,