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Old 13th Jan 2018, 11:24 am   #41
MrBungle
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Default Re: The Best GDO Ever Made?

I’d be out of the window there as well
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Old 13th Jan 2018, 1:06 pm   #42
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Default Re: The Best GDO Ever Made?

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"the FET gate goes into negative resistance under these conditions" . It was at this point that I got my coat.
If you were to think about negative resistance from a physics point of view it could be a bit of a mind bender I guess...

But in the RF world, it's a much easier concept to understand

If you measure the reflection coefficient of a passive device the magnitude will be less than 1 because it will have resistive loss. This is because the reflected wave back from the device will be smaller than the incident (test) wave. So the mag of the reflection coefficient might be 0.9 for example. A passive device can't reflect more energy than is sent into it.

But if you have a device that has negative resistance it typically means that the reflected wave comes back bigger than the incident (test) wave. That behaviour is explained by the concept of 'negative resistance'. If the reflected wave is smaller than the incident wave it means there is loss in the circuit under test. If the reflected wave is exactly the same size as the incident wave then the circuit under test is lossless so the resistance is zero. But to cater for the case (with some types of active circuit) where the reflected wave comes back 'bigger' than the incident wave we can use the concept of 'negative' resistance to explain this phenomena.

If you have a series LC circuit and ground both ends and somehow induce energy into it (from a dip meter?) and then leave it alone it will oscillate forever at the resonance frequency if there is no resistance in these lossless components.

But we know that there will be resistive loss for real world components. Maybe 1 ohm for a typical LC combo at HF? So the oscillations fade to nothing very quickly. But if you fed one end of the LC circuit into a device (active device?) that had just over 1 ohm of 'negative' resistance then the oscillations would not just stay at the same level, they would grow and grow and grow until the limits of the active device were reached. This is because the active device is able to top up the LC circuit via the (bigger) reflected wave that comes back out of it. So the 1 ohm of real loss in the LC circuit is offset by the >1 ohm of negative resistance in the active circuit.
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Old 13th Jan 2018, 2:36 pm   #43
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Default Re: The Best GDO Ever Made?

Hi Jeremy; I recall now that it was you who did some modelling for Al, and as he did finally get the wideband performance he aspired to, it seems that you got it right.

Can I ask you the direct question, based on your experience, are there factors specific to a FET that could potentially give rise to anomalous behaviour in its role as the oscillator in a GDO? I appreciate, of course that not all FET GDO's have exactly the same circuit or same device, so some may be quite satisfactory while other could have a vulnerability under some particular conditions, as yet, not defined? The few times that my FET-based GDO has given odd readings have been when using it to set up antennas - the resonances being at the wrong frequency and quite notably sharp (narrow) in the dip.

B
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Old 13th Jan 2018, 8:31 pm   #44
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Can I ask you the direct question, based on your experience, are there factors specific to a FET that could potentially give rise to anomalous behaviour in its role as the oscillator in a GDO?
I'm afraid I don't have any experience of tinkering inside FET or valve dip meters. However, I'd expect a valve to be more tolerant of the wide (amplitude?)range of waveforms that can be expected from a multi range oscillator like this. A JFET has the PN junction in the gate to consider. This may cause issues with getting a decent dip at low frequencies if this junction goes into conduction with (larger?) waveforms at low frequencies? A shunt diode can be used at the gate as a form of clamp but I don't know how effective that is in this case. But that is all just a guess and a lot also depends on what is used to meter the dip. A valve meter presumably looks for a dip in grid current. By contrast, I've seen solid state meters that just detect a dip in RF amplitude using a diode detector.

The only dip meter I have here is my old Maplin Precision Gold YN48C dip meter. I can't remember when I bought it but it was probably at their Southend branch in the 1980s. This is a fairly crude device that uses a BJT oscillator and a diode detector for the metering. It probably isn't the worst dip meter ever made but it is probably best described as 'adequate' on most of its ranges if operated with some patience. It isn't always easy to get a dip with it but perseverance and a few different approaches usually gets there in the end. Despite this, I have used it a lot over the years. In recent years I mainly use it as a quick and dirty signal injector although I have used it as a dip meter as well.
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Old 14th Jan 2018, 1:13 am   #45
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Default Re: The Best GDO Ever Made?

Don't forget that when using a GDO while working on antenna's, any signals (particularly strong signals) received by the antenna will also have an effect on the readings.

I would expect this to be more pronounced with FET GDO's.
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Old 14th Jan 2018, 10:49 am   #46
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Default Re: The Best GDO Ever Made?

Did some reading about negative resistance last night and it actually makes a lot of sense. If you have a voltage divider with one resistor as a negative resistance then you have a voltage divider with gain and phase shift (an amplifier). Add a tuned network to this (LC/crystal) with a matching phase shift at the oscillation frequency and it completes the Barkhausen criteria. FET as a transconductance amplifier gives V is proportional to conductance which is the reciprocal of resistance.

This may be completely wrong as it was at 3AM while I was waiting in the hospital with a wife with a swollen up head in a bad mood (had a wisdom tooth out on Thursday and it went a bit wrong)
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Old 14th Jan 2018, 8:59 pm   #47
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Usually an oscillator is best analysed as a negative resistance oscillator when the feedback path isn't obvious. This is generally the case for oscillators up at UHF. Have a look at the JFET circuit below. This can be made to oscillate up near 1GHz with suitable inductor values but the circuit below should oscillate around 500MHz.

Looking back into the drain you should see a negative resistance in series with maybe 1.5 to 2pF across maybe 250-700MHz. So adding the 50nH in the drain makes this resonant up at 500MHz and the negative resistance should be a lot bigger than the loss resistance in the 50nH inductor and the decoupling cap.... so it should oscillate strongly

But you need to work out what is happening inside the JFET and where the feedback is. This then holds the clue as to why it can generate the required negative resistance in series with a small capacitance.

A crude equivalent of the circuit is given in the second image. This is a tank circuit resonant up around 500MHz where the (-20 ohm) negative resistance in pink overcomes the real loss resistance in the inductor. So it should oscillate
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Old 18th Jan 2018, 1:15 am   #48
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Default Re: The Best GDO Ever Made?

Here's an image of my old Maplin YN48C DIP meter and the schematic diagram taken from the manual. It isn't a very good dip meter but it is a bit different because it uses a BJT in the oscillator section.

If anyone wants to analyse the JFET based neg resistance oscillator then it's best to use the typical data from the 2N3819 datasheet.

Cgs 3pF
Cdg 0.7pF
Cds 0.8pF
gm 5mS

There's enough info there to see how the circuit can develop negative resistance via internal feedback. If you build the circuit then best to use a genuine 2N3819 and not a dodgy equivalent or a chinese ebay version. You could try a 2N4416A as that has tighter specs on the device. Don't use anything like a J310 as the Idss is way too high and the device will run at a high current. It's also got higher internal capacitance. It should also oscillate on a spice simulator but care is needed in setting up the simulation and you need to select a valid spice model for the JFET. There are quite a few dodgy spice models out there.
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Old 18th Jan 2018, 7:56 am   #49
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Default Re: The Best GDO Ever Made?

Langrex sell genuine ones for ref if anyone wants any.
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Old 18th Jan 2018, 1:47 pm   #50
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Default Re: The Best GDO Ever Made?

Quote:
Originally Posted by G0HZU_JMR View Post
Here's an image of my old Maplin YN48C DIP meter and the schematic diagram taken from the manual. It isn't a very good dip meter but it is a bit different because it uses a BJT in the oscillator section.
I'm not sure to what extent the use of BJT was too unusual during the sort of era when I think your GDO was made. I don't have any numbers to quote, simply the recollection that there were various models using BJT. Perhaps I should try make contact with the American guy who has a collection of 64 models and see if he has studied their 'evolution'?

Re the 3819's, I used one for my very first GDO, felt it ran out of steam at VHF and then went to a...2N5457 (I think) which seemed better. Given that the 3819 was a very early device, I wonder if some of the even the slightly later options look better when modelled? There's no denying that many well-respected GDO designs do use FETs. I've now sidelined the tunnel diode design (Heathkit) I considered and my "Homage" version of the Model 59 is a work-in-progress.

B
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Old 18th Jan 2018, 9:35 pm   #51
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Default Re: The Best GDO Ever Made?

The 2N3819 is very old for sure. I've been using them since I was a student but they date back way before then.

However, the real point of showing the oscillator circuit was to demonstrate that JFETs (and BJTs) can become oscillators even when the circuit doesn't look like an oscillator at first glance. When this happens, it is often best to analyse the circuit as a 1 port device and look for signs of negative resistance

I chose that topology because it's the easiest to analyse and it's possible to predict the amount of negative resistance and the oscillation frequency based on the datasheet 'typical' figures with just a few simple calculations. No need for a simulator or a VNA. Obviously, a real JFET will have differences in Cdg and gm etc and this will affect the amount of negative resistance and the oscillation frequency a bit. But I'm not sure that's relevant if we are more interested in the mechanism that causes the negative resistance in the first place. The modern ON Semi datasheet for the 2N3819 has s parameter measurements up to 900MHz and the gm holds good up past 500MHz and only really fades badly towards 900MHz. I've not tried to get a 2N3819 to oscillate at 1GHz but I think I could get fairly close...

Other JFET configurations like the (source follower) JFET probes can also generate negative resistance and this is less obvious and it's a bit harder to calculate how much will be generated. The Bob Pease JFET active probe looks to me like it will generate a lot of negative resistance at its input across upper HF and into VHF for example. The Marconi TK 2374 200MHz (JFET) RF probe looks to combat its own negative resistance with a series 100R resistor near the probe tip.
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Old 18th Jan 2018, 11:15 pm   #52
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The modern ON Semi datasheet for the 2N3819 has s parameter measurements up to 900MHz and the gm holds good up past 500MHz and only really fades badly towards 900MHz. I've not tried to get a 2N3819 to oscillate at 1GHz but I think I could get fairly close...
Yes, perhaps one problem that many/all GDO's have to deal with is finding circuits/components that will enable them to operate over a range that's often 100kHz to 300MHz(?). Perhaps that's a requirement where a valve does well? That said, I have an old Taylor workshop sig gen that has one valve VFO for the LF end of the range, and a completely separate VFO for HF/VHF. Of course the Model 59 GDO was designed 'simply' to cover 2-400MHz. Could it be that by not needing (trying) to cover low frequencies, they could focus on doing the more limited job really quite well and that is a significant factor which helped it achieve some acclaim? Later on, of course, they were able to go a step further and supply a completely separate oscillator unit, using a different type of valve, which covered 400-940MHz.

B
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Old 18th Jan 2018, 11:47 pm   #53
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Default Re: The Best GDO Ever Made?

Baz, as well as the 59UHF head you referred to there is a third head unit in the 59 series of GDOs. The 59LF covered from 0.1 to 4.5Mhz. I have not found any pictures of 59LF on the net but I do have a PDF copy of the manual for it. Alas it is too large to post here so if you want a copy PM me an email address.

The last 59LF I saw advertised was in a pretty sorry state but still sold for over $200 so they must be pretty scarce and very desirable.

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Old 19th Jan 2018, 12:44 am   #54
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Baz, as well as the 59UHF head you referred to there is a third head unit in the 59 series of GDOs. The 59LF covered from 0.1 to 4.5Mhz. Al
Hi Al, thanks for that new info! So, the 59 has the 'core' PSU/modulator, but then has up to three different heads which can be connected to that unit. Effectively though, that's 3 different GDO's, whereas I think pretty much all of GDO's made for purely for amateur use tried to cover the widest frequency range in a single box.

I just took a look at the manual for the PMR-10 ("a 59 with bells on"); that only refers to the 2-400MHz head, but I think it's safe to assume that it would have used the same strategy if it been taken up to a greater extent.

B
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Old 19th Jan 2018, 6:29 am   #55
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Default Re: The Best GDO Ever Made?

If you come from a valve background, you are used to reasonably well controlled parameters. The grid voltage for a given anode current is close to the spot value in the data book, and the slope (gm) of the curve is also close to the spot value in the book. Life is easy!

The bipolar transistor comes as a shock. Yeah, the base-emitter voltage is quite dependable, but the current gain (hfe) can vary widely from device-to-device. A 2:1 range is good in this area, 4:1 or more is common. Some devices were sold in graded versions and you still got around 2:1 range. Artful design is needed to make circuits which can tolerate these uncertainties, be sure-fire without selected devices and adjustments to be twiddled.

The FET then comes as a double whammy. Tons of uncertainty in the transconductance (gm) AND the bias voltage can be all over the shop, AND the current in saturation has huge uncertainty, all within one type number of device. Lots of FETs were sold in graded versions. Very early types like the 2N3819 had particularly wide specs. So the artful design needed by bipolar transistors is stepped up a notch.

Because the Junction FET biases like a valve with negative bias needed on the input, and gives transconductance sort of gain, valvey people felt more comfortable with them. This led to a lot of designs which were scaled copies of things which worked nicely with valves, but the designers hadn't understood that the nice solid foundation of tight specs had been jerked out from under them. There's a lot of amateur radio circuits which only work with selected FETs as a result of this. Semiconductors tend to be batchy. Buy a boxful and they may be close-ish. The designer can build several off of his circuit and things look well. Someone else buys FETs, gets a different batch and has trouble.

Professional designers learned to avoid JFETs, and only used them where they really helped.

JFETs are popular in oscillators because they're easy to get going. It's still difficult to make sure fire designs which don't need fiddling in some instances.

Oscillators involve you setting a device up to create negative resistance. Once you have negative resistance, ANY resonance with low-uish loss can oscillate. Oscillators can hop around several resonances and look like they're possessed by demons!

If you don't have a scope or spectrum (spectre?) analyser with enough frequency coverage you may not see what's really happening and just see rather foxy behaviour at lower frequency and DC.

Even mundane FETs work to rather high frequencies, better than period bipolar transistors. This opened the door to spurious oscillations up to much higher frequencies. So the wanted frequency and DC conditions would show sudden jumps and even hysteresis. Just a few centuries earlier, the blame would have been laid on beelzebub and the designer burned at the stake.

Getting a wide-range gate dip oscillator circuit to oscillate on the wanted resonance and not the strays is difficult. The coward's way out is to build a complete oscillator for each band. Transistors are cheap. Tuning capacitors aren't, but you can use values better suited to each band. A butterfly capacitor and a hairpin loop for UHF.

David
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Old 19th Jan 2018, 11:09 am   #56
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Re: The Best GDO Ever Made?
Ref G0HZU - Fully agree with your remark about the Maplin Dip Meter - I have one and ''not very good'' is an understatement. If I can drag you all back to the title of this thread, Dear Readers, which is the best GDO?
In this case, the best for my purpose. I am listener only, and largely on an old 888A on the old amateur bands 10-160m.
I know aerials are not believed to be super critical for reception-only purposes, but I am fascinated with them and quite often change mine for something ''new''.
The Maplin jobby is very difficult to use successfully, in fact I find it to be pretty useless. So, assuming that I can find one, and that money is no object (I wish), what ideally should I buy to help set up my aerials.
All comments gratefully received. pete
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Old 19th Jan 2018, 12:26 pm   #57
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Re: The Best GDO Ever Made?
I am listener only, and largely on an old 888A on the old amateur bands 10-160m. So, assuming that I can find one, and that money is no object (I wish), what ideally should I buy to help set up my aerials.
All comments gratefully received. pete
As you say, for reception, a good match isn't critical and an aerial tuner unit to peak up the received signal is about as much as you can do. For most of us, with small suburban gardens with limited space for aerials, there's no magic solution. When I was still into amateur radio I had a rotatable homebrew mini beam, a 'Cobwebb', an inverted 'V' trapped dipole, and a multi band ground mounted vertical which covered 1.8MHz - 30MHz. The mini beam had a slight edge on the others. They weren't general coverage aerials for short-wave listening, they were for the harmonically related amateur bands.

Verticals can be good for DX due to the low angle of radiation. Can be troublesome for causing TVI - spraying RF into neighbours tellies, but for reception only, that's not an issue. Sandpiper multi-band verticals seem to offer good performance at sensible prices.

At the time that I was active on amateur radio, as most my aerials were home-brew, I had an MFJ Antenna Analyser which shows the resonant frequency and standing wave ratio, but it wouldn't make much sense to buy one just for reception purposes. You did say. 'If money was no object' and we're that so, here's what the current model does and what it costs:

https://www.ebay.co.uk/i/15220074225...D1419548870527

It may seem a lot of money, but people with deep pockets happily spend that much on a wire dipole antenna they could make for a tenth of that.

For general listening (rather than ham bands only) from LW to 30MHz, for my money, the Gary Tempest homebrew amplified loop aerial takes some beating, for lifting the signal level and for low background noise.

As to GDOs, the dial calibration is rarely accurate enough to be sure what frequency the dip is at, so I always checked the dip with a frequency counter loosely coupled to the GDO coil.

Just my thoughts, which I hope are of interest.
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Old 19th Jan 2018, 3:50 pm   #58
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Re: The Best GDO Ever Made?
Ref G0HZU - Fully agree with your remark about the Maplin Dip Meter - I have one and ''not very good'' is an understatement. If I can drag you all back to the title of this thread, Dear Readers, which is the best GDO?
Well, getting back on topic, could you say whether you want to acquire a GDO working & complete, build one from scratch, or might a kit be of interest? I mention the latter because I believe that there has been a kit recently available from a German ham which has been highly rated, but I don't currently have a detailed knowledge of it. Of course there was that kit from Ambit which was very popular (G3), but they are probably now collectors items.

I wonder why the Maplin GDO is so disappointing? Something I note about the Model 59 are the large coils; something around 1" diameter on what looks like a good grade material (polystyrene?) and no ferrite slugs in sight. That style was maintained on the later PMR-10. Comparisons with an HRO coil-pack spring to mind. In contrast, most modern GDO's usually have quite small coils. It's not hard to believe that the coil design of a GDO matters appreciably.

B
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Old 19th Jan 2018, 5:10 pm   #59
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The Ambit kit was from G3WPO. I have one somewhere. Not used it for a while.

I do remember that when I first built it it didn't work too well. Turned out that Ambit had changed the part used for an RF choke. They sent me a replacement choke and that worked much better.
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Old 19th Jan 2018, 6:17 pm   #60
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Way back when I was looking for a dip meter I originally wanted that Ambit/Cirkit meter designed by G3WPO but it wasn't available when I was looking. I think it was being revised in some way (and wasn't available anywhere) so I bought the Maplin meter instead. Maybe it was because of that choke but I think they had changed the design quite a bit when it became available again.

I notice that MFJ make a dip meter (MFJ-201) that looks just like my old Maplin meter and it seems to be priced at nearly 180! There seem to be several meters that look very similar and they all appear to be in an enclosure based on the old Heathkit HD-1250 dip meter design.
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