Varicap tuning

[stevehertz]

I just worked on a Roberts RM33 small mains powered table radio. The stations were off at each end of (and throughout) the VHF scale. So much so that I could not tune to BBC R2 on 88.3MHz. Acquiring the alignment info - and expecting to re-align by inputting 'frequencies', be it a sig gen or using stations at each end - I was taken back at having to set up voltages! - Varicap tuning. Although I'm familiar with the phrase, I have never worked on a set with it before. It set up quite nicely actually. Interesting technology. Thoughts? Is it better or not as good as 'normal' tuning? cheaper? more stable? (this one wasn't!)

[G6Tanuki]

Varicap tuning is cheaper to implement: tuning capacitors replaced by a few semiconductors; no need for "string" drives so the assembly process becomes simpler. The interconnection between the RF strip and the front-panel tuning controls becomes a simple DC-carrying wire rather than anything mechanical.

It also makes preset tuning easier (just a potentiometer for each channel and a single-pole-multi-way switch, as opposed to cams/ratchets/springs and suchlike needed for mechanical-tuning-capacitor preset tuning).

Finally, it means you can dispense with a separate AFC-diode and just feed the AFC voltage in as a supplement to the varicap tuning-line voltage.

The downside is that you need to stabilise the varicap supply rail to guard against drift with power-supply-voltage variations or - in the case of a battery set when the batteries are running down the Class B" audio output stage's varying current loads can amplitude-modulate the power supply with the program-content and if this gets fed back onto the varicap line as audio-frequency voltage-fluctuations you can get some *very* strange effects!

Also, early "varicaps" were somewhat inefficient devices which meant that the potential "Q" of a varicap-tuned circuit was lower than that which could be achieved with a well-designed mechanical L-C circuit. Varicaps made in the last few decades have been much better in this respect.

[Nuvistor]

I am not sure about cheaper, it does save on a costly tuning capacitor.
What it does do is allow more freedom in the placement of the tuning controls, easily designed presets and the possibility of more simple remote control.
Frank
Crossed post.

[lesmw0sec]

A company I did some work for used varicaps for frequency modulation in a radio-mic they produced. Out of interest we tried a 1n914 silicon diode as a replacement. There was no discernible difference to the stability, or linearity of high-deviation modulation between that and the special varicap, which cost about ten times as much.

Guess which diode was used from then on!
Les.

[MrBungle]

My experience with varicaps suggest they are quite temperature sensitive and relatively unstable, even the newer SMD ones. I discovered this when I let the sun shine on my VFO. Even though well shielded it shifted the tuning significantly.

I personally wouldn't use them without a PLL or huff-puff stabilizer now.

Lots of references to the latter here: http://ftp.hanssummers.com/huffpuff/library.html

Edit: with respect to the 1n914, similar thing with the 1n4005 - works quite well although I'd rather not use the word linear although for FM modulation, probably works fine. Tuning, forget it - feels like the tuning knob is connected to the VFO with a frankfurter

[Radio Wrangler]

The latest varicap diodes are still much lower in Q than a good quality mechanical variable capacitor.

They are rather prone to intermodulation and crossmodulation problems when there are very large signals around.

And as already mentioned, their thermal drift can be significant.

For ganged tuning you have to get matched sets because of device variation ove batches.

SMT only means the same die but in a different case, so performance is the same unless you're at such high frequencies the odd stray nanoHenry is a problem.

All that said, they are still good enough for a lot of uses.

By the way, the number of available types is falling rapidly.

David

[MrBungle]

TBH with the SMD ones, I was hoping that the closer contact with a large heat dissipating surface would result in better thermal stability but alas not much difference and I lost 3 in the carpet

[paulsherwin]

All silicon diodes have varicap characteristics to some degree. See http://www.hanssummers.com/varicap/varicaporig.html

[MrBungle]

Well technically any PN junction has a controllable depletion region size so you can do the same with all sorts of silicon. 2n3904's make pretty good varicaps

[stevehertz]

Looks like varicap tuning is not a high end, hifi thing then. More for cheap and cheerful radios.

[Nuvistor]

It's use in domestic equipment started late 60's, early 70's mainly in TV tuners and AFC circuits. In those instances they worked very well, the main failure was not the varicap but the regulated voltage supply usually a TAA550 IC producing 33volts, its feed resistor and the tuning controls. The tuning controls were push button variable potentiometers with switches. The switches were sometime mechanical or electronic.
These components gave many more problems that the varicaps.
Frank

[paulsherwin]

Mainstream hIfi tuners went to frequency displays and varicaps in the mid 80s, though there were a few earlier pioneers - I have a varicap Nikko from 1981. There was initially some resistance, and even today there are people who claim that 'analogue' (traditional capacitor) tuning somehow 'sounds better'.

[G6Tanuki]

Originally Posted by stevehertz

Looks like varicap tuning is not a high end, hifi thing then. More for cheap and cheerful radios.

I'd disagree: anything with frequency-synthesized tuning will use varicaps. And they're de-rigeur in modern "professional" radios like ground-to-air comms stuff as the only sensible way to consistently achieve the required level of front-end selectivity across the entire frequency-range.

Some of us remember the old days of mechanical-capacitor-tuned VHF/UHF TV front-ends, the clunky mechanical implementation of push-button tuning, and the need to continuously fiddle with the tuning during an evening's viewing. The coming of varicap-tuned front-ends was a revelation in both resettability and stability!

[Radio Wrangler]

I have a couple of hifi tuners which are considered to be fairly high-end. Both have varactor tuning.

The Sony ST-S730ES was once their flagship and Sony were pretty good at such things. It has a decent reputation. Internally it's fairly ordinary with varactor tuned RF stages and a synthesiser LO. It's clever bit is using recovered audio to wobble the varactor RF tuning to follow the signal.

The Revox B261 I have is ex-BBC. They were used for off-air quality monitoring, and also for reception at outlying transmitter stations. Either as a way of avoiding landline audio feeds, or as a fallback. To recieve a distant station at a transmitting site is quite demanding. It uses LC filters not ceramic ones. It uses a coax delay line for the discriminator. It doesn't use a stereo decoder chip. Quite a beastie!

There is an informative website:

http://www.fmtunerinfo.com/

Beware that you're entering high-end hifi territory. Don a pith helmet, long boots and shoulder a good blunderbuss.

Some contributors write good sense on reception of distant stations in the presence of nearby ones.

Some contributors write sensibly of constructional quality, of fixing the odd design whoopsie.

But the rankings and shootouts seem heavily weighted by the apparent sound to people who select and change equipment to control the frequency response, rather than twiddling a tone control.

David

[Andrew2]

I use a Huff-Puff stabilised VFO in my Topband transceiver and the varicap element is a standard red LED. It produces enough 'swing' to pull the osc over about 3 KHz, far more than required to stabilise the oscillator.

[Synchrodyne]

It would appear that varicap tuning was widely used in hi-fi FM tuners, even before synthesized tuning became the norm (in the early 1980s?) and certainly afterwards. Sugden started that way with its R21/R51 of circa 1970, and although not a DX device, I understand it was highly regarded and not seen as being disadvantaged within its peer group that used capacitor tuning (e.g. Rogers Ravensbourne 2 FET, Leak Stereofetic and Quad FM3.) Lowther was another early mover, with its Mk 8 and Mk 9 models of the early 1970s. Meridian entered the market at the end of the 1970s with its simple, quality-oriented 104, which had varicap tuning. The Ambit catalogues of the late 1970s included several FM front ends, the most elaborate of which (EF5800 series) had varicap tuning. So the inference is that the circuit designers were able to work around whatever drawbacks came with varicap diode tuning, and deliver the desired levels of performance.

A comparison between varicap and conventional tuning in respect of UHF TV front ends was provided by D.C. Read in Wireless World 1975, page 583ff. This was part of a series of constructional articles on a TV tuner unit designed to deliver high quality baseband video and audio signals, and which ran from 1975 October through 1976 January. The unit in question was designed around the Mullard 1043/05 varicap UHF front end, but was also tried with the capacitor-tuned AT6382-41. The expected perofrmance benefits were enumerated, but in summary it was found that actual benefits were less than expected.

In respect of UHF TV tuners in American practice, varicap tuning arrived at the right time, allowing easier implementation of the FCC dictate that TV receivers must tune the UHF channels as easily as they tuned the VHF channels. But the greater losses associated with the varicap-tuned input bandpass circuits saw some move away from the traditional American design with a passive signal path, to the use of RF amplifiers (initially bipolar, later dual-gate mosfet) and four-gang tuning, in order to restore the loss. Also, it would appear that with varicap VHF tuners, it was not so easy to switch the IF from the UHF tuner into the VHF RF amplifier as if it were a 13th channel, per established practice. Instead it was switched into the VHF mixer stage (bipolar, bipolar cascode or mosfet). This required still more gain in the UHF tuner, sometimes achieved with an IF preamplifier after the mixer (e.g. RCA) and also with an active bipolar mixer fed by a separate oscillator (e.g. Sylvania). So the move to varicaps brought with it consequential and concomitant other changes, as I think happens quite often with a new technology.

I think the adoption of varicap tuning for AM receivers was a little slower in getting started, and needed to await the arrival of devices that provided the necessary >3-to-1 tuning range without being too lossy. But that was done by about the time synthesized tuning arrived. Certainly that was the kind of tuning that was typical when AM stereo arrived. Given that incidental phase-modulation is quite deleterious for AM stereo, one assumes that the varicap devices used and their circuits were adequately “clean”. Electronic tuning also brought with it a shift when it came to AM and FM receiver ICs. RCA, Hitachi and Toko were prominent in this field through the 1970s, with the two Japanese makers essentially building upon what RCA and Motorola had done at the beginning of the decade. But it was Sanyo who emerged as a prominent supplier of such devices that were equipped with the necessary “extras” for electronic tuning; perhaps another consequential change. My mid-1980s Carver TX-11a FM-AM tuner is full of Sanyo ICs.

And varicaps were also used in communications receivers. The JRD NRD-525 (HF) and ICOM R7000 (VHF-UHF) receivers of the late 1980s both had varicap front end tuning. These occupied what might be called the top-end of the consumer equipment bracket, both being highly regarded when they were current.

So I’d say that varicap tuning applications spanned the range from “cheap-and-cheerful” through to the high-performance end when it came to consumer equipment. Perhaps in extremis capacitor or permeability tuning might be make an incrementally better FM tuner in very difficult reception conditions, all else from aerial through to post-decoder filters being state-of-the-art. But just possibly there are those who think that varicaps, like NFB, are the work of the devil and must so prima facie be avoided in hi-fi applications. There is no gainsaying that; debating with those who do not use scientific methods is a losing proposition, as they’ll bring you down to their level and then beat you with their much greater experience.


Cheers,

[Radio Wrangler]

As I have a varactor tuned FM tuner which was for many years used successfully on an FM transmitter site to receive signals from another FM transmitter site, which must qualify as pretty well into extremis, then I have to conclude that the anti-varactor sentiment must be from the satanists

David

[ionburn]

I remember around 30 years ago using a UHF varicap tuner on an old KB dual standard valve portable (strapped to UHF as commonly done in those days). With the addition of voltage regulation to a multi turn pot - one easy replacement for dicky tuning!

It worked a treat, although it would be against the spirit to modify such a set that way now

[ionburn]

Another couple of points I should add is that a search over the internet will provide information from others' experiments with non-defined diodes for varicap use (eg 1n400x series) (see paul's entry earlier).

Also varicap diodes are used over a wide frequency range. I have got some microwave varicaps although have not investigated their use.

[Synchrodyne]

The use of diodes as variable tuning devices in consumer equipment for AFC and AFT purposes rather than main tuning appears to date back to the later 1950s. So at least in this limited form, varicap tuning has had a long history.

In 1958 Westinghouse (USA) introduced a TV AFT system that used “reactance diode” across the oscillator tank circuit. As best I can determine it represented the reintroduction of AFT/AFC to American TV practice. It had been used in the early days, particularly with split sound receivers but evidently had died out. The use of a reactance diode probably helped the re-establishment of AFT, as I doubt whether, with two-valve VHF tuners well-established as the norm, the TV makers would have welcomed the addition of a 3rd valve envelope to accommodate a reactance diode. The Westinghouse AFT system was described in Radio-Electronics for 1958 February, page 56ff.

Notwithstanding this use of diodes for AFT in American TV practice, the reactance valve evidently was still preferred by some for FM applications, hence the subsequent development of triple triode valves such as the 6GY8 that in one envelope did the RF amplifier, autodyne mixer and AFC reactance jobs.

Regarding European TV practice, in Wireless World 1959 September, page 404, it was noted that AFT was universal practice for Band III for German TV receivers, but new for UHF. Provided were details of a Graetz TV AFT circuit that used a germanium reactance diode for VHF and a silicon reactance diode for UHF.

In the UK, in 1958 Murphy introduced AFC, on FM only, for its TV-FM receivers. This used a point-contact diode, and the circuit was described in WW 1958 October, page 441. Murphy was still using the dual-conversion sound channel it had introduced a year earlier (WW 1958 October, page 477), so assuming that it was the main oscillator that was subject to AFC, then the 2nd frequency conversion was within the AFC loop – rather unusual, I’d imagine. Dynatron used a junction diode for AFC in its TV50 chassis, as described in WW 1960 October, page 490. This was applied both for FM and TV, with the AFC bias in the TV case coming from a discriminator in the sound channel. Jason used a varicap diode for AFC in its FMT4 FM tuner, released later in 1960. In that case the AFC bias from the ratio detector was DC-amplified before being applied to the diode. Previously, Jason had used reactance valves for FM AFC, such as in its FMT3 model. (The Jason FMT4 replaced the FMT3, but there was no Jasonkit FMT4, and the Jasonkit FMT3 stayed in place until the end of Jason.)


Cheers,