The terms "1st and 2nd detector"

[ukcol]

I have always been puzzled by the terms 1st and 2nd detector as used when discussing superheterodyne receiver circuits.

In the dictionary definition of "detector" words like discover, expose, locate and uncover are used and in a second definition group other words like discern, hear, identify and perceive.

It is quite easy for me to apply the above to the stage in receiver which recovers the original modulating signal (the program audio frequencies) from the modulated carrier but I couldn't fit it to the frequency converter stage. There is no process of discovery going on in this latter stage but one of change or conversion.

Since these terms have fallen out of use to be replaced by "frequency changer" or "mixer" for the 1st detector and plain "detector" for the second, I thought the answer may lay in the history of the superhet' receiver.

This does indeed appear to be the case. In "The Superheterodyne Receiver" by A.T. Witts, the first chapter is given to the evolution of the superhet'. When attempting to solve the problem of receiving weak telegraphy signals by radio R.A. Fessenden in America hit upon the idea of transmitting on two different radio frequencies and beating the two together to produce an audio frequency in the receiver. By interrupting one or both of the radio frequencies information can be telegraphed. In a later development only one radio frequency was transmitted and the other generated in the receiver. This last was to evolve into the local oscillator used in modern superhets.

The word "detector" fits here since the heterodyne stage detects the original transmitted information. I now jump to the conclusion (I hope correctly) that the term "detector" continued to be used in this context until the more modern terminology came into general use.

I have posted this because I am sure I can't be the only one puzzled by these terms in the past.

[lesmw0sec]

One thing which often confuses my students are the terms BFO, CIO and VFO. In the case of BFO & CIO, it may well be the SAME oscillator, but given a different name according the function at the time!

[Aub]

I would have thought there would only ever be one detector. Certainly there could be a first and second IF.

Cheers

Aub

I think it is sane, the final detector converts RF into AF, the others convert RF into another RF. It's only a matter of frequency.

[jjl]

I always thought of the 1st and 2nd detector descriptions as American terms; rarely seen in British and European technical literature.

John

[Skywave]

Consider the verb "to detect". The use of that verb implies that 'something' is discovered and that 'something else' is being used to effect that discovery and a result of the ensuing interaction is produced.
Now apply that reasoning to a superhet radio. The thing that is being 'discovered' is the incoming R.F. signal - as transmitted from a distant transmitter. The 'device' that is being used to effect that 'discovery' is the local oscillator. And, finally, the 'result' is the signal at the I.F.
So on that basis, the frequency changer (or mixer + local oscillator) just about meets the description of a 'detector', but I'll readily agree that the use of 'detector' in the position of a freq. changer is stretching definitions a bit.

By the same token, the word 'mixer' (R.F. usage, not audio) and 'modulator' are the same thing.

Al. / March 10, '15 //

[Skywave]

Originally Posted by merlinmaxwell

I think it is sane, the final detector converts RF into AF, the others convert RF into another RF. It's only a matter of frequency.

Yes, yes, agreed - but that does not explain why the word 'detector is used.
Now if for the two phrases '1st, and 2nd. detector', the phrases '1st. and 2nd. freq. converter' were used, to my way of thinking, they would make more sense and be closely aligned with your analysis.

Al.

[Synchrodyne]

I was of the impression that the term “detector” goes right back to the early days when the coherer was used to detect the presence of radio waves, and thus was called a “detector”. As improved detection methods were developed, such as crystal diodes, the name “detector” was retained. This was still during the CW era, but the terminology was carried over when AM came along, and again for FM. Just how embedded was the term “detector” was illustrated by the fact that in the late 1940s, Avins and Seeley of RCA named their then-new FM demodulator circuit as the “ratio detector”.

The “split” as it were into 1st and 2nd detectors when superheterodyne receivers arrived seems to be harder to rationalize. What kinds of devices were used as mixers in the earliest superhet receivers? Possibly crystal diodes were used to some extent. Given that an original goal for the superhet was to get the RF to a low enough frequency where it could be amplified effectively with the triodes of the time, the same triodes might not have worked too well as mixers at incoming signal frequency. If crystal diodes did see early use as superhet mixers, then one could see that because until then they were used primarily as and probably frequently referred to as detectors, that name was carried over to the new function.

But what terminology applied to multiple conversion receivers? It would appear that the final conversion to baseband was always the 2nd detector. So in the case of a double-conversion receiver, were the 1st and 2nd mixers referred to as the 1st 1st detector and 2nd 1st detector respectively?

Cheers,

[G6Tanuki]

It does get confusing: one thing to remember is that at one time at the birth of radio there were receivers where a locally-generated signal a KHz or so off the desired reception-frequency was fed to a non-regenerative 'detector' tuned to signal-frequency - to produce a beat-note. Essentially a "direct conversion" receiver of sorts. Such things are shown in the pre-WWII "Admiralty Handbook of Wireless Telegrapgy". In such a case referring to this as a detector is quite sane. Later designs pushed the beat-note into the supersonic range and needed a second mixing process to generate an audible component [i.e. the superhet]. You still see some instances of IF amplifiers being called "Note magnifiers".

I generally refer to the "First mixer", "First conversion oscillator", "Second mixer", "second conversion-oscillator" etc - and will even refer to a product-detector as a 'mixer' since that's essentially what it is - mixing down to audio- or digital-frequencies.

These days you don't find many 'classic'-style final detectors in use: today the output is more likely to be I and Q components.

[wireful3]

I don't have any evidence for an explanation but I can rationalise the situation. In the very early days the first stage in receiving was converting the RF into something that could be used. It would be reasonable at the time to call it a detector. A coherer was one of the early detectors. When crystals were used I gather at some stage the carrier was modulated by some means (noisy arc, commutator or other) in any case the carrier could be heard in phones. It would be reasonable to call this the detector also.

With valves and cleaner carriers, the modulated carrier was demodulated by this first stage so presumably it was still called the detector and by letting it oscillate it produced a beat so CW could be received and heard. A superhet took this a bit further and used a supersonic beat frequency that was amplified before being detected again by demodulating it in more or less the same way as the simple radio. It is not surprising that this would be called a second detector.

At some stage the first detector must have been given the more satisfactory name of frequency changer. This was probably when TRF and superhets were both in common use but it is easy to understand why old hands would not bother to use this new term. It is like having to call condensers capacitors and cycles per second Hertz. They all knew what they meant. With multiple conversion superhets then frequency changer is more useful leaving the term detector for the final demodulator.

It might just be my imagination running wild but it helps me to think about why the terms are used. Perhaps looking through the Admiralty Handbook of Wireless Telegraphy might throw some light.

[Leon Crampin]

My understanding of the term "detector" in radio usage is a device or circuit which converts a signal carrying information in a non-readable form to one in which information can be derived. The obvious example is a diode detector which rectifies the modulated carrier, enabling the filtered AF to be extracted.

Rectification, I think is the key here. In a superhet, the local oscillator biases the mixer to cutoff at each half-cycle and increases its gain on the alternate half-cycles. This action is essential to enable a filtered IF to be derived from the mixer. Of course, in order to achieve this necessary action, the magnitude of the oscillator output is several volts - many orders higher than the incoming signal amplitude.

So, rectification takes place in the mixer - just as it does later on in the signal chain to derive the audio from the modulated IF. It is therefore entirely reasonable and understandable that the mixer, driven (hard) by its local oscillator be described as a "first detector".

Leon.

[ukcol]

Yes a rectifier or some other non-linear device like the coherer as mentioned by wireful3. This has become a really interesting discussion, thank you gents.

[kalee20]

Originally Posted by Skywave

Yes, yes, agreed - but that does not explain why the word 'detector is used.

No, it doesn't - I agree.

One way of looking at the situation is: It's really stupid, we can't ask the people who used the terms first detector and ask them, because they are all dead. And as we don't understand, we'll dump the term first detactor and call it a mixer or frequency changer; and we'll dump the term second detector and just call it a detector or demodulator.

And that is what we have done.

But to try to get some insight, it's worth remembering that the first superhets applied the incoming RF (possibly after some RF amplification) to either a diode, or a grid-leak circuit, but with the local oscillator in series. It's necessary to have non-linearity in an additive circuit for any beat frequencies to be produced.

And as the diode or leaky-grid circuit is recognisable as a detector circuit (just without the oscillator injection), it my well have got called a first detector.

When multigrid, multiplacative mixers appeared, of course, it's not obvious at all so could well be why the term was dropped.