08-08-2017, 01:20 AM
Easier IF filtering may have been the main raison for using full-wave AM detection.
Two valve-era examples come to mind. One is the Murphy TA160 export receiver of 1951. About this Murphy said:
“The secondary winding of the second i.f.t. is centre tapped to reduce the damping effect of the diode load and maintain the high "Q" of the transformer. Full wave detection takes place in V5, and is used because less i.f. filtering is required. Since the i.f. filter is untuned, any increase in i.f. rejection would entail a loss of the higher audio frequencies.”
The other is the Heathkit AFM-1 FM-AM tuner of 1962, for which the rationale was:
“A push-pull detector circuit maintains the symmetrical loading of the driver stage ; it uses two germanium diodes connected in such a way that one conducts for each half of the R.F. waveform. This reduces both R.F. and audio distortion and, since the R.F. ripple frequency is doubled from 470 to 940 Kc s. filtering of the I.F. signal is more effective.”
Possibly a well-balanced full-wave push-pull detector was more linear than a conventional diode detector, but getting the balance exactly right may not have been easy. It does not seem to have been much used in the valve era, and adequately low distortion (for hi-fi purposes) was obtainable with conventional diode detectors when due consideration was given to their AC and DC loading. (The Quad AMII tuner was a good example).
I think that there has been some use of full-wave detection in solid-state equipment – the AVI FM-AM tuner of the 1990s comes to mind as an example. Also in some solid-state equipment is found the precision rectifier, sometimes full-wave. This mitigated against diode non-linearities, although it did not avoid the fundamental problems inherent with rectifying-type detection.
Cheers,
Steve
Two valve-era examples come to mind. One is the Murphy TA160 export receiver of 1951. About this Murphy said:
“The secondary winding of the second i.f.t. is centre tapped to reduce the damping effect of the diode load and maintain the high "Q" of the transformer. Full wave detection takes place in V5, and is used because less i.f. filtering is required. Since the i.f. filter is untuned, any increase in i.f. rejection would entail a loss of the higher audio frequencies.”
The other is the Heathkit AFM-1 FM-AM tuner of 1962, for which the rationale was:
“A push-pull detector circuit maintains the symmetrical loading of the driver stage ; it uses two germanium diodes connected in such a way that one conducts for each half of the R.F. waveform. This reduces both R.F. and audio distortion and, since the R.F. ripple frequency is doubled from 470 to 940 Kc s. filtering of the I.F. signal is more effective.”
Possibly a well-balanced full-wave push-pull detector was more linear than a conventional diode detector, but getting the balance exactly right may not have been easy. It does not seem to have been much used in the valve era, and adequately low distortion (for hi-fi purposes) was obtainable with conventional diode detectors when due consideration was given to their AC and DC loading. (The Quad AMII tuner was a good example).
I think that there has been some use of full-wave detection in solid-state equipment – the AVI FM-AM tuner of the 1990s comes to mind as an example. Also in some solid-state equipment is found the precision rectifier, sometimes full-wave. This mitigated against diode non-linearities, although it did not avoid the fundamental problems inherent with rectifying-type detection.
Cheers,
Steve
