Synthisised or not?

[sunthaiboy]

Q, When can a receiver be considered to be "synthesized" or "synthesized" if you are from across the pond?

If the receiver claims to have PLL technology (for the VCO) and not have direct digital frequency read-out (analogue dials etc), is it said to be "synthesised"? I was looking to buy a more modern SSB communications receiver (more modern than my RA17) and am confused by the terms used by the sellers and the manufactures, especially for the up market VHF/AM tuners in modern Hi-Fi systems .

I assumed "synthesised" referred to the demodulated (received) audio signal being derived from the wanted carrier frequency, mixed with a LO derived from a "synthesized" frequency source. The LO (mixing signal) in turn is derived from an accurate master frequency source (phase locked) from which a combination of mathematical and logic functions generates the LO (mixing carrier).

Please educate me on this matter?

[G6Tanuki]

I always assume that it means the frequency of the LO/VFO is arrived-at by some form of mixing process.

You can have analog synthesis; I'd call the "mixer-VFO' approach [where you have a really stable VFO designed to tune a single fixed frequency-span, and this is then mixed with other frequencies derived from crystals to arrive at the frequency needed for the required 'local oscillator" - the Wadley Loop used in the likes of the RA17 is a rather sophisticated example of this.

It gives 'continuous' tuning.

Another analog synthesis approach uses banks of crystals - say a series of ten of them at 1KHz increments, then another ten of them at 10KHz increments, another bank at 100KHz increments and so on: you then use rotary switches to select the right ones, tens and hundreds of KHz which are then mixed together to generate the required LO. This approach was used in a number of military and commercial aviation radios.

The rotary-switches can simply be calibrated 0-9 so you can 'dial up' the frequency you want. This means the radio tunes in, say 1KHz or 100Hz steps so for SSB use you need a 'clarifier' or similar control. It's not so easy (but can be done) to associate this kind of synthesis with memories.



*DIGITAL* synthesis is where counters/timers and dividers - usually locked to a crystal frequency-standard - are used to 'lock' a free-running VFO to the desired frequency.

*DIRECT DIGITAL* synthesis is where the frequency desired is generated by sending digital representations of the rise and fall of the waveform at the required frequency to a digital-to-analog converter which converts this digital representation to an analog [well, actually a rising and falling digital 'staircase' of voltage] signal at the required frequency.

Both forms of digital synthesis are easy to associate with memories for storing frequently-used frequencies, and being able to scan through them. Like the crystal-bank synthesizer they tune in fixed steps so you need a clarifier or similar for SSB (or a *very* small step-size).

[Radio Wrangler]

Synthesis means making one thing from one or more different things, so in the case of the local oscillator in a superhet receiver, if the tuning control varies the LO frequency directly with no other sources involved, then it is not synthesised.

The common crystal + VFO into mixer LO generator is synthesised under the strict meaning of the word, but most people would say it wasn't. Even when a PLL is involved in the combining of the two frequencies (rather than the plain old mixer and bandpass filter approach) many people would say that was not a synthesiser.

The Wadley system really does create an effective LO frequency out of a VFO and a crystal-controlled harmonic series. The RA17 can be considered to be one of the earliest synthesised radios. There are some high resolution signal sources which amount to a nested family of Wadley converters, and those are generally accepted as synthesisers.

Most people think of phase locked loops with programmable dividers in them used to create a steppable, stable frequency which can be digitally controlled when the synthesiser word is seen. Tanuki's mentioned the 'Direct synthesiser' which works by selecting one of a group of frequencies from a bank of crystal oscillators, Stages of this can be nested allowing resolution to be arbitrarily increased.

I wrote an intro to the different types of synthesiser and their evolution for the oscillators and synthesisers chapter of the ARRL handbook. You can find it in any edition from 1995 onwards, and I think it's on-line in a few places.

However, exercise caution. Many years ago the presence of a synthesiser was considered a big deal and made a receiver very sexy, but many sets have been made with very poor synths. If you want something just because it has a synthesiser, you want a good one. You need to read up on 'phase noise' and 'reciprocal mixing'.

Synthesisers in amateur radio gear were great for the manufacturers and were far cheaper to make than precision gearboxes and tuning scales, but it took a long time before they offered a net improvement in radio performance. Most often, freedom from drift was traded for a wide variety of nasty tendencies.

David