Half wave & full wave AM detectors.

[colly0410]

Remember reading somewhere (I think) that some posh radios had full wave AM detectors, but can't remember what (if any) advantage they had over common or garden half wave types. Was it just a gimmick so they could charge more for them?

[Craig]

If two diodes are used as a voltage doubler, you get twice the output (audio) voltage. At first sight that can look like a two diode bridge rectifier.

Apparently a technique originally used in crystal sets.

[pwdrive]

With full wave detection the ripple will be twice the IF so easier to filter.

Lawrence.

[Synchrodyne]

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

[Geordie McBoyne]

Full wave demodulators were used in many pre-war superhet TV receivers that employed a very low vision IF. Such receivers were the Murphy A42V and the GEC BT8161. In these sets the VIF was 4Mhz or thereabouts. Rediffusion employed a full wave demodulator in cable TV receivers, again the reason for the FW demodulator was because the lowest cable system frequency was only 4.95Mhz for 405 line BBC TV.
In 1938 Murphy introduced the 9" CRT A58V TV receiver, this set employed the same low VIF as the A42V but employed a half wave vision demodulator. Murphy cited the case for the simple demodulator was because the CRT was so small any 3.5Mz vision-sound beat pattern would be hardly noticeable at normal viewing distances.

Geordie McBoyne.